Climate Change and...

Annotated Bibliography

Climate Variability

Models of Carbon Dynamics

Aalto, T., Ciais, P., Chevillard, A., Moulin, C. (2004). Optimal determination of the parameters controlling biospheric CO2 fluxes over Europe using eddy covariance fluxes and satellite NDVI measurements. Tellus Series B-Chemical and Physical Meteorology 56 (2): 93-104

ABSTRACT: Ecosystem CO2 flux measurements using the eddy covariance method were compared with the biospheric CO2 exchange estimates of a regional scale atmospheric model. The model described the seasonal patterns quite well, but underestimated the amplitude of the fluxes, especially at the northern sites. Two model parameters, photosynthetic efficiency for light use and Q10 for soil respiration, were re-evaluated on a diurnal and seasonal basis using the results from flux measurements. In most cases the photosynthetic efficiency was higher than the earlier estimate. The resulting flux was very sensitive to the value of photosynthetic efficiency, while changes in Q10 did not have a significant effect.

Abrahamson, D. A., Norfleet, M. L., Causarano, H. J., Williams, J. R., Shaw, J. N., Franzluebbers, A. J. (2007). Effectiveness of the soil conditioning index as a carbon management tool in the southeastern USA based on comparison with EPIC. Journal of Soil and Water Conservation 62 (2): 94-102

ABSTRACT: Models are being developed and utilized by scientists and government agencies to quantify the potential for carbon storage in soil. The Environmental Policy Integrated Climate (EPIC) v. 3060 model is a process-based model requiring detailed inputs. The soil conditioning index (SCI) is a simpler tool to predict relative change in soil organic carbon (SOC) using table values for three management components (i.e., organic matter, field operations, and erosion) within the framework of the Revised Universal Soil Loss Equation 2 model. Our objective was to determine whether SOC sequestration from no-tillage cropping systems in the southeastern USA could be simply predicted with SCI compared with detailed simulations using EPIC. Four management systems were evaluated: (1) cotton (Gossypium hirsutum L.) with conventional tillage, (2) cotton with no tillage, (3) corn (Zea mays L.)— cotton rotation with no tillage, and (4) bermudagrass (Cynodon dactylon L.)—corn—cotton rotation with no tillage. All no-tillage systems used wheat (Triticum aestivum L.) as a cover crop. Simulated SOC sequestration with EPIC was 0.46 ± 0.06 Mg ha−1 yr−1 (410 ± 51 lb ac−1 yr−1 ) under the three no-tillage management systems and -0.03 Mg ha−1 yr−1 (-30 lb ac−1 yr−1 ) under conventional tillage. The SCI also predicted a strong difference in SOC between conventional and no tillage. Differences in SOC sequestration among crop rotations were not readily apparent with EPIC but were with SCI. Predictions of SOC sequestration with SCI were comparable to those with EPIC but not necessarily in a linear manner as previously suggested. The SCI appears to be a valuable method for making reasonable, cost-effective estimates of potential changes in SOC with adoption of conservation management in the southeastern USA, although validations under actual field conditions are still needed.

Adler, P. R., Del Grosso, S. J., Parton, W. J. (2007). Life-cycle assessment of net greenhouse-gas flux for bioenergy cropping systems. Ecological Applications 17 (3): 675-691

ABSTRACT: Bioenergy cropping systems could help offset greenhouse gas emissions, but quantifying that offset is complex. Bioenergy crops offset carbon dioxide emissions by converting atmospheric CO2 to organic C in crop biomass and soil, but they also emit nitrous oxide and vary in their effects on soil oxidation of methane. Growing the crops requires energy (e.g., to operate farm machinery, produce inputs such as fertilizer) and so does converting the harvested product to usable fuels (feedstock conversion efficiency). The objective of this study was to quantify all these factors to determine the net effect of several bioenergy cropping systems on greenhouse-gas (GHG) emissions. We used the DAYCENT biogeochemistry model to assess soil GHG fluxes and biomass yields for corn, soybean, alfalfa, hybrid poplar, reed canarygrass, and switchgrass as bioenergy crops in Pennsylvania, USA. DAYCENT results were combined with estimates of fossil fuels used to provide farm inputs and operate agricultural machinery and fossil-fuel offsets from biomass yields to calculate net GHG fluxes for each cropping system considered. Displaced fossil fuel was the largest GHG sink, followed by soil carbon sequestration. N2 O emissions were the largest GHG source. All cropping systems considered provided net GHG sinks, even when soil C was assumed to reach a new steady state and C sequestration in soil was not counted. Hybrid poplar and switchgrass provided the largest net GHG sinks, >200 g CO2 e-C·m-2 ·yr-1 for biomass conversion to ethanol, and >400 g CO2 e-C·m-2 ·yr-1 for biomass gasification for electricity generation. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from corn rotations reduced GHG emissions by 40%, reed canarygrass by 85%, and switchgrass and hybrid poplar by ~115%.

G. I. Ågren, R. Hyvönen, T. Nilsson (2007). Are Swedish forest soils sinks or sources for CO2 -model analyses based on forest inventory data. Biogeochemistry 82 (3): 217-227

ABSTRACT: Forests soils should be neither sinks nor sources of carbon in a long-term perspective. From a Swedish perspective the time since the last glaciation has probably not been long enough to reach a steady state, although changes are currently very slow. In a shorter perspective, climatic and management changes over the past 100 years have probably created imbalances between litter input to soils and organic carbon mineralisation. Using extant data on forest inventories, we applied models to analyse possible changes in the carbon stocks of Swedish forest soils. The models use tree stocks to provide estimates of tree litter production, which are fed to models of litter decomposition and from which carbon stocks are calculated. National soil carbon stocks were estimated to have increased by 3 Tg yr−1 or 12–13 g m−2 yr−1 in the period 1926–2000 and this increase will continue because soil stocks are far from equilibrium with current litter inputs. The figure obtained is likely to be an underestimation because wet sites store more carbon than predicted here and the inhibitory effect of nitrogen deposition on soil carbon mineralisation was neglected. Knowledge about site history prior to the calculation period determines the accuracy of current soil carbon stocks estimates, although changes can be more accurately estimated.

Aires, L. M. I., Pio, C. A., Pereira, J. S. (2008). Carbon dioxide exchange above a Mediterranean C3/C4 grassland during two climatologically contrasting years. Global Change Biology 14 (3): 539-555

ABSTRACT: Eddy-covariance measurements of net ecosystem carbon exchange (NEE) were carried out above a grazed Mediterranean C3/C4 grassland in southern Portugal, during two hydrological years, 2004–2005 and 2005–2006, of contrasting rainfall. Here, we examine the seasonal and interannual variation in NEE and its major components, gross primary production (GPP) and ecosystem respiration (Reco ), in terms of the relevant biophysical controls. The first hydrological year was dry, with total precipitation 45% below the long-term mean (669 mm) and the second was normal, with total precipitation only 12% above the long-term mean. The drought conditions during the winter and early spring of the dry year limited grass production and the leaf area index (LAI) was very low. Hence, during the peak of the growth period, the maximum daily rate of NEE and the light-use and water-use efficiencies were approximately half of those observed in the normal year. In the summer of 2006, the warm-season C4 grass,Cynodon dactylon L., exerted an evident positive effect on NEE by converting the ecosystem into a carbon sink after strong rain events and extending the carbon sequestration for several days, after the end of senescence of the C3 grasses. On an annual basis, the GPP and NEE were 524 and 49 g C m−2 , respectively, for the dry year, and 1261 and −190 g C m−2 for the normal year. Therefore, the grassland was a moderate net source of carbon to the atmosphere, in the dry year, and a considerable net carbon sink, in the normal year. In these 2 years of experiment the total amount of precipitation was the main factor determining the interannual variation in NEE. In terms of relevant controls, GPP and NEE were strongly related to incident photosynthetic photon flux density on short-term time scales. Changes in LAI explained 84% and 77% of the variation found in GPP and NEE, respectively. Variations in Reco were mainly controlled by canopy photosynthesis. After each grazing event, the reduction in LAI affected negatively the NEE.

Allamaras, R., Schomberg, H., Douglas, C., Dao, T. (2000). Soil organic carbon sequestration potential of adopting conservation tillage in U.S. croplands. Journal of Soil and Water Conservation 55 (3): 365-373

ABSTRACT: Spatial prediction and uncertainty assessment of ecological modeling and simulation systems are a difficult task because of system complexities that include multi components, their interaction and variability over space and time. Developing a general methodology and framework of uncertainty assessment for the systems' users has become very important. As the first part of a large study addressing these issues, the focus of this paper is on spatial prediction and uncertainty assessment of topographic factors involved in the Revised Universal Soil Loss Equation (RUSLE). The spatial variability of these topographic factors including slope steepness factor S, slope length factor L, and their combined LS factor were modeled with semivariogram models. Three geostatistical methods, including ordinary kriging, indicator kriging, and sequential indicator simulation, were applied and compared. The predicted value maps of these factors, their error variance or conditional variance maps, and probability maps for the predicted values larger than a given threshold value were derived. The comparison of the geostatistical methods suggests that sequential indicator simulation better than ordinary and indicator kriging.

Andren, Olef, Katter, T. (1997). ICBM - the introductory carbon balance model for exploration of soil carbon balances. Ecological Applications 7 (4): 1226-1236

ABSTRACT: A two-component model was devised, comprising young and old soil C, two decay constants, and parameters for litter input, “humification,” and external influences. Due to the model’s simplicity, the differential equations were solved analytically, and parameter optimizations can be made using generally available nonlinear regression programs. The calibration parameter values were derived from a 35-yr experiment with arable crops on a clay soil in central Sweden. We show how the model can be used for medium-term (30 yr) predictions of the effects of changed inputs, climate, initial pools, litter quality, etc., on soil carbon pools. Equations are provided for calculating steady-state pool sizes as well as model parameters from litter bag or14 C-labeled litter decomposition data. Strategies for model parameterization to different inputs, climatic regions, and soils, as well as the model’s relations to other model families, are briefly discussed.

Andren, Olof, Katterer, T. .., Karlsson, T. .. (2004). ICBM regional model for estimations of dynamics of agricultural soil carbon pools. Nutrient Cycling in Agroecosystems 70 (2): 231-239

ABSTRACT: Swedish arable land covers 3 Mha and its topsoil contains about 300 Mton C. The mineral soils seem to be close to steady-state, but the organic soils (about 10% of total arable land) have been estimated to lose ca. 1 Mton/year. We have devised a conceptual model (ICBMregion), using national agricultural crop yield/manuring statistics and allometric functions to calculate annual C input to the soil together with a five-parameter soil carbon model (ICBMr), calibrated using long-term field data. In Sweden, annual yield statistics are reported for different crops, for each of eight agricultural regions. Present topsoil carbon content and regional distribution of soil types have recently been measured. We use daily weather station data for each region together with crop type (bulked from individual crop data) and soil type to calculate an annual soil climate parameter for each crop/soil type permutation in each region. We use 14 soil types and 9 crop types, which gives 126 parameter sets for each year and region, each representing a fraction of the region's area. For each year, region, crop and soil type, ICBMregion calculates the change in young and old soil carbon per hectare, and sums up the changes to, e.g., national changes. With eight regions, we will have 1008 parameter sets per year, which easily can be handled, and what-if scenarios as well as comparisons between benchmark years are readily made. We will use the model to compare the soil C pools between the IPCC benchmark year 1990 and the present. In principle, we use inverse modelling from the sampled, recent soil C pools to estimate those in 1990. In the calculations, soil climate and yield for each year from 1990 onwards are taken into account. Then we can project soil C balances into the future under different scenarios, e.g., business as usual, land use change or changes in agricultural crops or cultivation practices. Projections of regional climate change are also available, so we can quite easily make projections of soil C dynamics under, e.g., different climate scenarios. We can follow the dynamic effects of carbon sequestration efforts – and estimate their efficiency. The approach is conceptually simple, fairly complete, and can easily be adapted to different needs and availability of data. However, perhaps the greatest advantage is that the results from this comprehensive approach used for, e.g., a 10-year period, can be condensed into a very simple spreadsheet model for calculating effects of management/land use changes on C stocks in agricultural soils.

Anisimov, O. A. (2007). Potential feedback of thawing permafrost to the global climate system through methane emission. Environmental Research Letters 2 (4): 7 p.

ABSTRACT: Large amounts of soil carbon deposited in permafrost may be released due to deeper seasonal thawing under the climatic conditions projected for the future. An increase in the volume of the available organic material together with the higher ground temperatures may lead to enhanced emission of greenhouse gasses. Particular concerns are associated with methane, which has a much stronger greenhouse effect than an equal amount of CO2 . Production of methane is favored in the wetlands, which occupy up to 0.7 million km2 in Russian permafrost regions and have accumulated about 50 Gt of carbon (Gt C). We used the permafrost model and several climatic scenarios to construct projections of the soil temperature and the depth of seasonal thawing. To evaluate the effect of such changes on the volume of the seasonally thawing organic material, we overlaid the permafrost projections on the digitized geographically referenced contours of 59 846 wetlands in the Russian Arctic. Results for the mid-21st century climate indicated up to 50% increase in the volume of organic substrate in the northernmost locations along the Arctic coast and in East Siberia, where wetlands are sparse, and a relatively small increase by 10%–15% in West Siberia, where wetlands occupy 50%–80% of the land. We developed a soil carbon model and used it to estimate the changes in the methane fluxes due to higher soil temperature and increased substrate availability. According to our results, by mid-21st century the annual net flux of methane from Russian permafrost regions may increase by 6–8 Mt, depending on climatic scenario. If other sinks and sources of methane remain unchanged, this may increase the overall content of methane in the atmosphere by approximately 100 Mt, or 0.04 ppm, and lead to 0.012 °C global temperature rise.

Backeus, S., Wikstrom, P., Lamas, T. (2006). Modeling carbon sequestration and timber production in a regional case study. Silva Fennica 40 (4): 615-629

ABSTRACT: Forests make up large ecosystems and by the uptake of carbon dioxide can play an important role in mitigating the greenhouse effect. In this study, mitigation of carbon emissions through carbon uptake and storage in forest biomass and the use of forest biofuel for fossil fuel substitution were considered. The analysis was performed for a 3.2 million hectare region in northern Sweden. The objective was to maximize net present value for harvested timber, biofuel production and carbon sequestration. A carbon price for build-up of carbon storage and for emissions from harvested forest products was introduced to achieve an economic value for carbon sequestration. Forest development was simulated using an optimizing stand-level planning model, and the solution for the whole region was found using linear programming. A range of carbon prices was used to study the effect on harvest levels and carbon sequestration. At a zero carbon price, the mean annual harvest level was 5.4 million m3 , the mean annual carbon sequestration in forest biomass was 1.48 million tonnes and the mean annual replacement of carbon from fossil fuel with forest biofuel was 61,000 tonnes. Increasing the carbon price led to decreasing harvest levels of timber and decreasing harvest levels of forest biofuel. Also, thinning activities decreased more than clear-cut activities when the carbon prices increased. The level of carbon sequestration was governed by the harvest level and the site productivity. This led to varying results for different parts of the region.

Bala, G., Caldeira, K., Mirin, A., Wickett, M., Delire, C. (2005). Multicentury changes to the global climate and carbon cycle: Results from a coupled climate and carbon cycle model. Journal of Climate 18 (21): 4531-4544

ABSTRACT: A coupled climate and carbon (CO2 ) cycle model is used to investigate the global climate and carbon cycle changes out to the year 2300 that would occur if CO2 emissions from all the currently estimated fossil fuel resources were released to the atmosphere. By the year 2300, the global climate warms by about 8 K and atmospheric CO2 reaches 1423 ppmv. The warming is higher than anticipated because the sensitivity to radiative forcing increases as the simulation progresses. In this simulation, the rate of emissions peaks at over 30 Pg C yr-1 early in the twenty-second century. Even at the year 2300, nearly 50% of cumulative emissions remain in the atmosphere. Both soils and living biomass are net carbon sinks throughout the simulation. Despite having relatively low climate sensitivity and strong carbon uptake by the land biosphere, these model projections suggest severe long-term consequences for global climate if all the fossil fuel carbon is ultimately released into the atmosphere.

Baldocchi, D., Black, T., Curtis, P., Falge, E., Fuentes, J., Granier, A., Gu, L., Knohl, A., Pilegaard, K., Schmid, H., Valentini, R., Wilson, K., Wofsy, S., Xu, L., Yamamoto, S. (2005). Predicting the onset of net carbon uptake by deciduous forests with soil temperature and climate data: a synthesis of FLUXNET data. International Journal of Biometeorology 49 (6): 377-387

ABSTRACT: We tested the hypothesis that the date of the onset of net carbon uptake by temperate deciduous forest canopies corresponds with the time when the mean daily soil temperature equals the mean annual air temperature. The hypothesis was tested using over 30 site-years of data from 12 field sites where CO2 exchange is being measured continuously with the eddy covariance method. The sites spanned the geographic range of Europe, North America and Asia and spanned a climate space of 16°C in mean annual temperature. The tested phenology rule was robust and worked well over a 75 day range of the initiation of carbon uptake, starting as early as day 88 near Ione, California to as late as day 147 near Takayama, Japan. Overall, we observed that 64% of variance in the timing when net carbon uptake started was explained by the date when soil temperature matched the mean annual air temperature. We also observed a strong correlation between mean annual air temperature and the day that a deciduous forest starts to be a carbon sink. Consequently we are able to provide a simple phenological rule that can be implemented in regional carbon balance models and be assessed with soil and temperature outputs produced by climate and weather models.

Banfield, G.E., Bhatti, J.S., Jiang, H., Apps, M.J. (2002). Variability in regional scale estimates of carbon stocks in boreal forest ecosystems: results from West-Central Alberta. Forest Ecology and Management 169 (1-2): 15-27

ABSTRACT: Aboveground biomass, forest floor, and soil carbon (C) stocks were estimated for a transitional boreal region in western Alberta using available forest inventory data, model simulation, field observed plot data, and soil polygon (area averaged) information from the Canadian soil organic carbon database (CSOCD). For the three C pools investigated, model simulation provided a regional estimate, while forest inventory, plot, and soil polygon data provided an estimate of the spatial variation. These data were used to examine the variation of the C estimates, in both temporal (e.g. climate change) and spatial (e.g. soil physical characteristics) dimensions. Using the carbon budget model of the Canadian forest sector (CBM-CFS2) the regional average aboveground biomass C was estimated at 43 Mg C ha−2 , similar to the estimate from the 1994 Canadian forest inventory (50 Mg C ha−2 ). Model simulation over the period 1920–1995 elucidated the major role that disturbances (harvest, fire and insects) play in determining the C budget of the region. Decreases in stand replacing disturbances over the period resulted in an accumulation in biomass C.

Regional estimates of forest floor C using aggregated plot data, CSOCD (forested area only) data, and CBM-CFS2 simulations were in close agreement, yielding values of 2.9, 3.4 and 3.3 kg C m−2 , respectively. Regional estimates of total soil C using the three methods were more divergent (14.8, 8.3, and 15.6 kg C m−2 , respectively).

An exponential relationship between clay content and biomass for mature coniferous stand types was found (r2 = 0.68), which is reasonable considering that as a site variable, texture affects tree growth through the modification of nutrient and water availability. The relationship was used to predict the range of potential values for biomass C at maturity across the region. Forest inventories of biomass seldom provide enough data across the range of ages and stand types to develop stand growth curves that capture the variation in growth across the landscape. Consequently, growth dynamics must be inferred from a large area to provide enough biomass-to-age data, which results in a loss in the ability to use it to predict C pools and fluxes at a small scale. Using relationships between site factors (such as soil texture) and biomass C provides a means to modify inventory-based biomass-to-age relationships to assess the variation across the region as well as make predictions at a higher spatial resolution. This is relevant where both spatial extent and a finer scale are required, but site-specific biomass-to-age relationships are unavailable.

Bauer, J., Herbst, M., Huisman, J.A., Weihermuller, L., Vereecken, H. (2008). Sensitivity of simulated soil heterotrophic respiration to temperature and moisture reduction functions. Geoderma 145 (1-2): 17-27

ABSTRACT: In this study, the influence of different soil temperature and moisture reduction functions for scaling decomposition rates of soil organic matter on the prediction of CO2 production and fluxes was analysed. For this purpose, soil temperature and moisture reduction functions of six soil carbon decomposition models (CANDY, CENTURY, DAISY, PATCIS, ROTHC, and SOILCO2) were implemented in the modified SOILCO2-ROTHC model. As a test scenario, a respiration experiment on a silt loam in Columbia (USA) was chosen, which consists of two periods both including soil respiration measurements in a wheat stand and a subsequent bare soil period. Additionally, the dataset contains measured soil temperature, soil moisture as well as CO2 concentrations within the soil profile. The cumulative CO2 fluxes simulated with different temperature reduction functions showed deviations up to 41% (1.77 t C ha−1 ) for the six-month simulation period in 1981. The influence of moisture reduction was smaller with deviations up to 2% (0.10 t C ha−1 ). A combination of corresponding temperature and moisture reduction functions resulted in the highest deviations up to 41% (1.80 t C ha−1 ). Under field conditions the sensitivity towards soil temperature reduction was 6 to 7 times higher compared to soil moisture reduction. The findings of this study show that the choice of soil temperature and soil moisture reduction functions is a crucial factor for a reliable simulation of carbon turnover.

Bernoux, Martial, Cerri, Carlos C., Neill, Christopher, De Moraes, Jener F. L. (1998). The use of stable carbon isotopes for estimating soil organic matter turnover rates. Geoderma 82 (1-3): 43-58.

ABSTRACT: In natural ecosystems, soil organic carbon (C) is derived almost exclusively from the residues of plants growing in situ. In agroecosystems, it has at least two origins: one is the remains from the previous native vegetation, and the other is the remains of the crop and the decomposition of its residues. Where vegetation has changed from plants with the C3 photosynthetic pathway to C4 pathway or vice versa, changes in the natural abundance of13 C in soil organic matter (SOM) over time can be used to identify sources of organic C in soil and to determine the turnover rate of SOM. For example, large areas of C3 tropical forest have been replaced with C4 pasture or cropland. Changes in theδ13 C values of soil organic C in these areas reflect soil organic matter turnover rate, and provide insight regarding the functional role of tropical ecosystems in the global C cycle. This paper illustrates how the stable isotope13 C can be used to estimate SOM turnover rates and the sensitivity of different models and different model parameters, using a chronosequence of forest and pastures of different ages from the Brazilian Amazon. A single-compartment exponential decay model and a two-compartment model in which SOM was divided into stable and labile components yielded similar estimates of soil C turnover time at the surface but divergent estimates at depth. The one-compartment model gave the least variable estimates of model parameters and turnover times and was also relatively insensitive to individual C stocks in single pastures of a particular age. Estimates of soil stable and labile C pools obtained using changes in forest soil δ13 C with depth differed from estimates obtained using the chronosequence. This suggests that upon burning and pasture creation, a portion of the previously stable soil C pool is rendered less stable. Model r2 was a poor criterion for selecting an appropriate soil C turnover model to apply to chronosequence data. In the absence of substantial justification for segregating SOM into different compartments based on lability, modeling should be done with the simplest models possible.

Betts, R.A. (2006). Forcings and feedbacks by land ecosystem changes on climate change. Journal de Physique 139: 119-142

ABSTRACT: Vegetation change is involved in climate change through both forcing and feedback processes. Emissions of CO2 from past net deforestation are estimated to have contributed approximately 0.22 0.51 Wm-2 to the overall 1.46 Wm-2 radiative forcing by anthropogenic increases in CO2 up to the year 2000. Deforestation-induced increases in global mean surface albedo are estimated to exert a radiative forcing of 0 to -0.2 Wm-2 , and dust emissions from land use may exert a radiative forcing of between approximately +0.1 and -0.2 Wm-2 . Changes in the fluxes of latent and sensible heat due to tropical deforestation are simulated to have exerted other local warming effects which cannot be quantified in terms of a Wm-2 radiative forcing, with the potential for remote effects through changes in atmospheric circulation. With tropical deforestation continuing rapidly, radiative forcing by surface albedo change may become less useful as a measure of the forcing of climate change by changes in the physical properties of the land surface. Although net global deforestation is continuing, future scenarios used for climate change prediction suggest that fossil fuel emissions of CO2 may continue to increase at a greater rate than land use emissions and therefore continue to increase in dominance as the main radiative forcing. The CO2 rise may be accelerated by up to 66% by feedbacks arising from global soil carbon loss and forest dieback in Amazonia as a consequence of climate change, and Amazon forest dieback may also exert feedbacks through changes in the local water cycle and increases in dust emissions.

Bolinder, M. A., Vandenbygaart, A. J., Gregorich, E. G., Angers, D. A., Janzen, H. H. (2006). Modelling soil organic carbon stock change for estimating whole-farm greenhouse gas emissions. Canadian Journal of Soil Science 86 (3): 419-429

ABSTRACT: Modelling soil organic carbon (SOC) stock changes in agroecosystems can be performed with different approaches depending on objectives and available data. Our objective in this paper is to describe a scheme for developing a dynamic SOC algorithm for calculating net greenhouse gas emissions from Canadian farms as a function of management and local conditions. Our approach is flexible and emphasizes ease of use and the integration of available knowledge. Using this approach, we assessed the performance of several SOC models having two or more compartments for some common agroecosystems in Canada. Analysis of long-term data for conventional management practices at different sites (n = 36) in Canada, including recent model applications in the literature on some of those data, indicated that the results obtained with two-compartment models, such as the Introductory Carbon Balance Model (ICBM) and Modified Woodruff Model (MWM), yielded results comparable to those of a multi-compartment model (CENTURY). The analysis also showed that a model such as ICBM need stuning to be applied to management and conditions across Canada. Two-compartment models programmable in a simple spreadsheet format, though they may not supplant more complex models in allapplications, offer advantages of simplicity and transparency in whole-farm analyses of greenhouse gas emissions.

Bonan, G. B. (1990). Carbon and nitrogen cycling in North American boreal forests. II. Biogeographic patterns. Canadian Journal of Forest Research 20 (7): 1077-1088

ABSTRACT: A model of carbon and nitrogen cycling developed with ecological relationships from upland boreal forests in interior Alaska was tested with forest structure and forest floor data from several bioclimatic regions of the North American boreal forest. Test forests included black spruce (Piceamariana (Mill.) B.S.P.), white spruce (Piceaglauca (Moench) Voss), white birch (Betulapapyrifera Marsh.), balsam fir (Abiesbalsamea (L.) Mill.), and jack pine (Pinusbanksiana Lamb.) stands located in five different bioclimatic regions. Test comparisons of simulated and actual data included aboveground tree biomass, basal area, density, litter fall, and moss and lichen biomass as well as forest floor biomass, turnover, thickness, nitrogen concentration, and nitrogen mineralization. The model correctly simulated 60 (76%) of the 79 variables tested. Approximately 42% of the incorrectly simulated variables occurred in one forest. The major recurring errors included inaccurate moss and lichen biomass and low moss nitrogen concentrations. These tests indicated that ecological relationships from interior Alaska can be extended to other boreal forest regions and identified the factors controlling vegetation patterns in different bioclimatic regions of the North American boreal forest.

Bondeau, A., Smith, P.C., Zaehle, S., Schaphoff, S., Lucht, W., Cramer, W., Gerten, D., Lotze-Campen, H., Müller, C., Reichstein, M., Smith, B. (2007). Modelling the role of agriculture for the 20th century global terrestrial carbon balance. Global Change Biology 13 (3): 679-706

ABSTRACT: In order to better assess the role of agriculture within the global climate-vegetation system, we present a model of the managed planetary land surface, Lund–Potsdam–Jena managed Land (LPJmL), which simulates biophysical and biogeochemical processes as well as productivity and yield of the most important crops worldwide, using a concept of crop functional types (CFTs). Based on the LPJ-Dynamic Global Vegetation Model, LPJmL simulates the transient changes in carbon and water cycles due to land use, the specific phenology and seasonal CO2 fluxes of agricultural-dominated areas, and the production of crops and grazing land. It uses 13 CFTs (11 arable crops and two managed grass types), with specific parameterizations of phenology connected to leaf area development. Carbon is allocated daily towards four carbon pools, one being the yield-bearing storage organs. Management (irrigation, treatment of residues, intercropping) can be considered in order to capture their effect on productivity, on soil organic carbon and on carbon extracted from the ecosystem. For transient simulations for the 20th century, a global historical land use data set was developed, providing the annual cover fraction of the 13 CFTs, rain-fed and/or irrigated, within 0.5° grid cells for the period 1901–2000, using published data on land use, crop distributions and irrigated areas. Several key results are compared with observations. The simulated spatial distribution of sowing dates for temperate cereals is comparable with the reported crop calendars. The simulated seasonal canopy development agrees better with satellite observations when actual cropland distribution is taken into account. Simulated yields for temperate cereals and maize compare well with FAO statistics. Monthly carbon fluxes measured at three agricultural sites also compare well with simulations. Global simulations indicate a ~24% (respectively ~10%) reduction in global vegetation (respectively soil) carbon due to agriculture, and 6–9 Pg C of yearly harvested biomass in the 1990s. In contrast to simulations of the potential natural vegetation showing the land biosphere to be an increasing carbon sink during the 20th century, LPJmL simulates a net carbon source until the 1970s (due to land use), and a small sink (mostly due to changing climate and CO2 ) after 1970. This is comparable with earlier LPJ simulations using a more simple land use scheme, and within the uncertainty range of estimates in the 1980s and 1990s. The fluxes attributed to land use change compare well with Houghton's estimates on the land use related fluxes until the 1970s, but then they begin to diverge, probably due to the different rates of deforestation considered. The simulated impacts of agriculture on the global water cycle for the 1990s are ~5% (respectively ~20%) reduction in transpiration (respectively interception), and ~44% increase in evaporation. Global runoff, which includes a simple irrigation scheme, is practically not affected.

Bottcher, H., Freibauer, A., Obersteiner, M., Schulze, E.-D. (2008). Uncertainty analysis of climate change mitigation options in the forestry sector using a generic carbon budget model. Ecological Modelling 213 (1): 45-62

ABSTRACT: Industrialized countries agreed on a reduction of greenhouse gas emissions under the Kyoto Protocol. Many countries elected forest management activities and the resulting net balance of carbon emissions and removals of non-CO2 greenhouse gases by forest management in their climate change mitigation measures. In this paper a generic dynamic forestry model (FORMICA) is presented. It has an empirical basis. Several modules trace C pools relevant for the Kyoto Protocol and beyond: biomass, litter, deadwood and soil, and harvested wood products. The model also accounts for the substitution of fossil fuels by wood products and bioenergy.

FORMICA was used to first study the model sensitivity and uncertainty based on data from Thuringia, a federal state of Germany, to determine the major sources of uncertainty in carbon accounting at different levels of carbon pool aggregation (biomass, ecosystem, forestry sector and enhanced forestry sector including the accumulated substitution effect). Rotation length and maximum increment contributed most to uncertainty in biomass. The influence of the latter did not diminish with higher level of pool aggregation. Uncertainty in the enhanced forestry sector was to a smaller degree controlled by product and substitution related parameters. Relative uncertainty decreased with the level of aggregation and comprehensiveness of the carbon budget.

In a second step the model estimated the sink potential of the Thuringian forestry sector. The projected average biomass sink for the period of 2003–2043 of 0.6 t C ha−1 year−1 could be increased by 50% by broadening the perspective to the entire forestry sector, including substitution effects. A simulation of forest conservation on 20% of the forest area increased C fixation. However, even in the biomass C pool the expected C stock changes did not exceed the estimated uncertainty of 40%. A higher level of aggregation (i.e. the inclusion of soil and litter, product pool and substitution effects) decreases relative uncertainty but also diminishes differences between different management options. The analysis demonstrates that the choice of management mitigation options under an accounting scheme should include the impacts on forest products and of substitution effects.

Cao,M., Zhang,Q., Shugart,H. H. (2001). Dynamic responses of African ecosystem carbon cycling to climate change. Climate Research 17 (2): 183-193

ABSTRACT: Global climate change has been modifying ecosystem carbon cycling, which has produced feedbacks on climate by affecting the concentration of atmospheric CO2 . The importance of biospheric CO2 uptake or release to climate change has generated great interest in quantifying the dynamic responses of terrestrial ecosystem carbon cycling to climate change. However, less attention has been given to Africa, although it accounts for about one-fifth of the global net primary production and is one of the regions that have the greatest climate change. Here we use a biogeochemical model to simulate the dynamic variations in the carbon fluxes and stocks of African ecosystems caused by changes in climate and atmospheric CO2 from 1901 and 1995. We estimate that climate change reduces plant production and soil carbon stocks and causes net CO2 release, but the fertilization effect of increasing atmospheric CO2 on photosynthesis reverses the reduction and leads to carbon accumulation in vegetation. Therefore, the combined effect of climate change and increasing atmospheric CO2 causes net CO2 uptake, particularly in central Africa. The mean rate of the carbon sequestration in the period 1981-1995 is calculated to be 0.34 Gt C yr-1 . Nevertheless, Africa is not necessarily a significant carbon sink, because a large part of the carbon sequestration is offset by the carbon release arising from land use changes.

Cao, M. K., Prince, S. D., Li, K. R., Tao, B., Small, J., Shao, X. M. (2003). Response of Terrestrial Carbon Uptake to Climate Interannual Variability in China. Global Change Biology 9 (4): 536-546

ABSTRACT: The interest in national terrestrial ecosystem carbon budgets has been increasing because the Kyoto Protocol has included some terrestrial carbon sinks in a legally binding framework for controlling greenhouse gases emissions. Accurate quantification of the terrestrial carbon sink must account the interannual variations associated with climate variability and change. This study used a process-based biogeochemical model and a remote sensing-based production efficiency model to estimate the variations in net primary production (NPP), soil heterotrophic respiration (HR), and net ecosystem production (NEP) caused by climate variability and atmospheric CO2 increases in China during the period 1981–2000. The results show that China's terrestrial NPP varied between 2.86 and 3.37 Gt C yr−1 with a growth rate of 0.32% year−1 and HR varied between 2.89 and 3.21 Gt C yr−1 with a growth rate of 0.40% year−1 in the period 1981–1998. Whereas the increases in HR were related mainly to warming, the increases in NPP were attributed to increases in precipitation and atmospheric CO2 . Net ecosystem production (NEP) varied between −0.32 and 0.25 Gt C yr−1 with a mean value of 0.07 Gt C yr−1 , leading to carbon accumulation of 0.79 Gt in vegetation and 0.43 Gt in soils during the period. To the interannual variations in NEP changes in NPP contributed more than HR in arid northern China but less in moist southern China. NEP had no a statistically significant trend, but the mean annual NEP for the 1990s was lower than for the 1980s as the increases in NEP in southern China were offset by the decreases in northern China. These estimates indicate that China's terrestrial ecosystems were taking up carbon but the capacity was undermined by the ongoing climate change. The estimated NEP related to climate variation and atmospheric CO2 increases may account for from 40 to 80% to the total terrestrial carbon sink in China.

Cao, M. K., Woodward, F. I. (1998). Net primary and ecosystem production and carbon stocks of terrestrial ecosystems and their responses to climate change. Global Change Biology 4 (2): 185-198

ABSTRACT: Evaluating the role of terrestrial ecosystems in the global carbon cycle requires a detailed understanding of carbon exchange between vegetation, soil, and the atmosphere. Global climatic change may modify the net carbon balance of terrestrial ecosystems, causing feedbacks on atmospheric CO2 and climate. We describe a model for investigating terrestrial carbon exchange and its response to climatic variation based on the processes of plant photosynthesis, carbon allocation, litter production, and soil organic carbon decomposition. The model is used to produce geographical patterns of net primary production (NPP), carbon stocks in vegetation and soils, and the seasonal variations in net ecosystem production (NEP) under both contemporary and future climates. For contemporary climate, the estimated global NPP is 57.0 Gt C y–1 , carbon stocks in vegetation and soils are 640 Gt C and 1358 Gt C, respectively, and NEP varies from –0.5 Gt C in October to 1.6 Gt C in July. For a doubled atmospheric CO2 concentration and the corresponding climate, we predict that global NPP will rise to 69.6 Gt C y–1 , carbon stocks in vegetation and soils will increase by, respectively, 133 Gt C and 160 Gt C, and the seasonal amplitude of NEP will increase by 76%. A doubling of atmospheric CO2 without climate change may enhance NPP by 25% and result in a substantial increase in carbon stocks in vegetation and soils. Climate change without CO2 elevation will reduce the global NPP and soil carbon stocks, but leads to an increase in vegetation carbon because of a forest extension and NPP enhancement in the north. By combining the effects of CO2 doubling, climate change, and the consequent redistribution of vegetation, we predict a strong enhancement in NPP and carbon stocks of terrestrial ecosystems. This study simulates the possible variation in the carbon exchange at equilibrium state. We anticipate to investigate the dynamic responses in the carbon exchange to atmospheric CO2 elevation and climate change in the past and future.

Cao, M. K., Woodward, F. I. (1998). Dynamic responses of terrestrial ecosystem carbon cycling to global climate change. Nature 393 (6682): 249-252

ABSTRACT: Terrestrial ecosystems and the climate system are closely coupled, particularly by cycling of carbon between vegetation, soils and the atmosphere. It has been suggested1,2 that changes in climate and in atmospheric carbon dioxide concentrations have modified the carbon cycle so as to render terrestrial ecosystems as substantial carbon sinks3,4 ; but direct evidence for this is very limited5,6 . Changes in ecosystem carbon stocks caused by shifts between stable climate states have been evaluated7,8 , but the dynamic responses of ecosystem carbon fluxes to transient climate changes are still poorly understood. Here we use a terrestrial biogeochemical model9 , forced by simulations of transient climate change with a general circulation model10 , to quantify the dynamic variations in ecosystem carbon fluxes induced by transient changes in atmospheric CO2 and climate from 1861 to 2070. We predict that these changes increase global net ecosystem production significantly, but that this response will decline as the CO2 fertilization effect becomes saturated and is diminished by changes in climatic factors. Thus terrestrial ecosystem carbon fluxes both respond to and strongly influence the atmospheric CO2 increase and climate change.

Carrera, A., Ares, J., Labraga, J., Thurner, S., Bertiller, M. (2007). Scenarios of future climate and land-management effects on carbon stocks in northern Patagonian shrublands. Environmental Management 40 (6): 944-957

ABSTRACT: We analyzed the possible effects of grazing management and future climate change on carbon (C) stocks in soils of northern Patagonian shrublands. To this aim, we coupled the outputs of three (HadCM3, CSIRO Mk2, and CCSR/NIES) global climate models to the CENTURY (v5.3) model of terrestrial C balance. The CENTURY model was initialized with long-term field data on local biome physiognomy, seasonal phenologic trends, and prevailing land-management systems and was validated with recent sequences of 1-km Normalized Difference Vegetation Index (MODIS-Terra) images and soil C data. In the tested scenarios, the predicted climate changes would result in increased total C in soil organic matter (SOMTC). Maximum SOMTC under changed climate forcing would not differ significantly from that expected under baseline conditions (8 kg m−2 ). A decrease in grazing intensity would result in SOMTC increases of 11% to 12% even if climate changes did not occur. Climate change would account for SOMTC increases of 5% to 6%.

Catovsky, S., Bradford, M. A., Hector, A. (2002). Biodiversity and ecosystem productivity: implications for carbon storage. Oikos 97 (3): 443-448

ABSTRACT: Recent experiments have found that Net Primary Productivity (NPP) can often be a positive saturating function of plant species and functional diversity. These findings raised the possibility that more diverse ecosystems might store more carbon as a result of increased photosynthetic inputs. However, carbon inputs will not only remain in plant biomass, but will be translocated to the soil via root exudation, fine root turnover, and litter fall. Thus, we must consider not just plant productivity (NPP), but also net productivity of the whole ecosystem (NEP), which itself measures net carbon storage. We currently know little about how plant diversity could influence soil processes that return carbon back to the atmosphere, such as heterotrophic respiration and decomposition of organic matter. Nevertheless, it is clear that any effects on such processes could make NPP a poor predictor of whole-ecosystem productivity, and potentially the ability of the ecosystem to store carbon. We examine the range of mechanisms by which plant diversity could influence net ecosystem productivity, incorporating processes involved with carbon uptake (productivity), loss (autotrophic and heterotrophic respiration), and residence time within the system (decomposition rate). Understanding the relationship between plant diversity and ecosystem carbon dynamics must be made a research priority if we wish to provide information relevant to global carbon policy decisions. This goal is entirely feasible if we utilize some basic methods for measuring the major fluxes of carbon into and out of the ecosystem.

Chen, W. J., Chen, J., Cihlar, J. (2000). An integrated terrestrial ecosystem carbon-budget model based on changes in disturbance, climate, and atmospheric chemistry. Ecological Modelling 135 (1): 55-79

ABSTRACT: Disturbances (e.g. fire, insect-induced mortality, and harvest) and management practices (e.g. planting) affect the forest carbon (C) cycle, so do non-disturbance climatic and atmospheric factors (e.g. growing season length and temperature, abiotic decomposition factor, annual precipitation, atmospheric CO2 concentration, and nitrogen (N) deposition). Previous studies investigated the effects of these factors individually or in some combinations, but not their integrated effects at regional and global scales. This study describes an Integrated Terrestrial Ecosystem C-budget model (InTEC), which integrates effects of all these factors on the annual C cycle of a forest region. InTEC is based on the Farquhar's leaf photosynthesis model, the Century C cycle model, the net N mineralization model of Townsend et al. [Ecol. Appl., 6 (1996) 806] and an age–NPP relationship derived from forestry inventory-based age–biomass relationships. To integrate these existing models, which were developed for different purposes and had different spatial and temporal scales, into a coherent mechanistic model, we (1) develop a spatial and temporal up-scaling algorithm to use the instantaneous leaf-level model for a region at annual time step; and then (2) combine the upscaled results with an age–NPP relationship to obtain the annual NPP of a forest region. A historical change approach is then used to describe the regional annual C cycle, which not only improves the accuracy of its historical and present estimates, but also enables us to predict its future responses, both of which are critical in formulating mitigation and adaptation strategies for global changes. Applying InTEC to Canada's forests, we first investigate the impacts of each factor on the C cycle over the short term (i.e. in the year of perturbation) and the long term (i.e. in the years after perturbation). The short-term and long-term effects are determined by changing one of the 10 factors in year 1 since the industrialization while keeping this factor in all other years and all other factors in all years at pre-industrial levels. Integrating all these short-term and long-term effects for the actual historical data of the 10 external forcing factors, we then estimate that the annual mean NBP (=NPP — soil respiration — fire emission — forest praoducts oxidation) of Canada's forests was 40±20 Tg C per year (i.e. a sink) in 1810s, reduced to −131±66 Tg C per year (i.e. a source) in 1870s, increased thereafter to a maximum of 200±100 Tg C per year in 1930s, and decreased again to 57±27 Tg C per year in 1990s. From 1800 to 1998, the aboveground biomass of Canada's forests increased by 19%, while the soil C stock increased by ~2%.

Chen, X.F., Chen, J.M., An, S.Q., Ju, W.M. (2007). Effects of topography on simulated net primary productivity at landscape scale: Carbon sequestration In China's forest ecosystems. Journal of Environmental Management 85 (3): 585-596

ABSTRACT: Local topography significantly affects spatial variations of climatic variables and soil water movement in complex terrain. Therefore, the distribution and productivity of ecosystems are closely linked to topography. Using a coupled terrestrial carbon and hydrological model (BEPS-TerrainLab model), the topographic effects on the net primary productivity (NPP) are analyzed through four modelling experiments for a 5700 km2 area in Baohe River basin, Shaanxi Province, northwest of China. The model was able to capture 81% of the variability in NPP estimated from tree rings, with a mean relative error of 3.1%. The average NPP in 2003 for the study area was 741 g C m−2 yr−1 from a model run including topographic effects on the distributions of climate variables and lateral flow of ground water. Topography has considerable effect on NPP, which peaks near 1350 m above the sea level. An elevation increase of 100 m above this level reduces the average annual NPP by about 25 g C m−2 . The terrain aspect gives rise to a NPP change of 5% for forests located below 1900 m as a result of its influence on incident solar radiation. For the whole study area, a simulation totally excluding topographic effects on the distributions of climatic variables and ground water movement overestimated the average NPP by 5%.

Cheng, W. (1999). Rhizosphere feedbacks in elevated CO2 . Tree Physiology 19 (4-5): 313-320.

ABSTRACT: Understanding rhizosphere processes in relation to increasing atmospheric CO2 concentrations is important for predicting the response of forest ecosystems to environmental changes, because rhizosphere processes are intimately linked with nutrient cycling and soil organic matter decomposition, both of which feedback to tree growth and soil carbon storage. Plants grown in elevated CO2 substantially increase C input to the rhizosphere. Although it is known that elevated CO2 enhances rhizosphere respiration more than it enhances root biomass, the fate and function of this extra carbon input to the rhizosphere in response to elevated CO2 are not clear. Depending on specific plant and soil conditions, the increased carbon input to the rhizosphere can result in an increase, a decrease, or no effect on soil organic matter decomposition and nutrient mineralization. Three mechanisms may account for these inconsistent results: (1) the "preferential substrate utilization" hypothesis; (2) the "priming effect" hypothesis; and (3) the "competition" hypothesis, i.e., competition for mineral nutrients between plants and soil microorganisms. A microbial growth model is developed that quantitatively links the increased rhizosphere input in response to elevated CO2 with soil organic matter decomposition. The model incorporates the three proposed mechanisms, and simulates the complexity of the rhizosphere processes. The model also illustrates mechanistically the interactions among nitrogen availability, substrate quality, and microbial dynamics when the system is exposed to elevated CO2 .

Chojnacky, D.C., Amacher, M.C., Perry, C.H., K.M. Reynolds (2006). A model for monitoring forest floor duff and litter carbon sequestration. USDA Forest Service, Pacific Northwest Research Station

ABSTRACT: The forest floor is an important part of carbon storage, biodiversity, nutrient cycling, and fire fuel hazard. This paper reports on a study of litter and duff layers of the forest floor for eastern U.S. forests. The U. S. Department of Agriculture (USDA) Forest Service, Forest Inventory Analysis (FIA) program currently measures variables related to duff and litter on a subsample of plots covering all U.S. forest lands regardless of ownership. The FIA soils protocol includes duff and litter thickness measurement and sample collection followed by lab measurement of mass and carbon content. We examined these lab data to test a model of duff and litter carbon storage based upon simple measurements of forest floor depth. Duff and litter data were compiled from 1,468 plots sampled in 2001 and 2002 from most states in the eastern U.S. These data were combined with other available FIA data for regression modeling to predict duff and litter carbon from depth of duff and litter layer and several other classification variables (R2 =0.56). Results on duff and litter model predictions show that duff and litter are an important carbon sink in eastern U.S. forests by containing about 50 percent of the forest floor carbon or 10 percent of total forest carbon (excluding mineral soil).

Cui, J. B., Li, C. S., Trettin, C. (2005). Analyzing the ecosystem carbon and hydrologic characteristics of forested wetland using a biogeochemical process model. Global Change Biology 11 (2): 278-289

ABSTRACT: A comprehensive biogeochemical model, Wetland-DNDC, was applied to analyze the carbon and hydrologic characteristics of forested wetland ecosystem at Minnesota (MN) and Florida (FL) sites. The model simulates the flows of carbon, energy, and water in forested wetlands. Modeled carbon dynamics depends on physiological plant factors, the size of plant pools, environmental factors, and the total amount and turnover rates of soil organic matter. The model realistically simulated water level fluctuation, forest production, carbon pools change, and CO2 and CH4 emission under natural variations in different environmental factors at two sites. Analyses were focused on parameters and inputs potentially cause the greatest uncertainty in calculated change in plant and soil C and water levels fluctuation and shows that it was important to obtain accurate input data for initial C content, climatic conditions, and allocation of net primary production to various forested wetland components. The magnitude of the forest responses was dependent not only on the rate of changes in environmental factors, but also on site-specific conditions such as climate and soil. This paper explores the ability of using the biogeochemical process model Wetland-DNDC to estimate the carbon and hydrologic dynamics of forested wetlands and shifts in these dynamics in response to changing environmental conditions.

Dargaville, R., Ciais, P., Baker, D., Rödenbeck, C., Rayner, P. (2006). Estimating high latitude carbon fluxes with inversions of atmospheric CO2. Mitigation and Adaptation Strategies for Global Change 11 (4): 769-782

ABSTRACT: Atmospheric inversions have proven to be useful tools, showing for example the likely existence of a large terrestrial carbon sink in the northern mid-latitudes. However, as we go to smaller spatial scales the uncertainties in the inversions increase rapidly, and the task of finding the distribution of the sink between North America, Europe and Asia has been shown to be very difficult. The uncertainty in the fluxes due to network selection, transport model error and inversion set up tends to be too high for studying either net annual fluxes or interannual variability on spatial scales such as the North American Boreal or Eurasian Boreal regions. We discuss the path forward; to couple together the atmospheric inversions with process based terrestrial carbon models, creating carbon data assimilation systems. Such systems are being developed now and could prove to be very powerful. The multi-disciplinary nature of the data assimilation system requires information from flux towers, soil and above ground biomass inventories, remote sensed fields, atmospheric CO2 concentrations and climate data as well as model development and will need a massive community effort if it will succeed.

Davi, H., Dufrene, E., Francois, C., Le Maire, G., Loustau, D., Bosc, A., Rambal, S., Granier, A., Moors, E. (2006). Sensitivity of water and carbon fluxes to climate changes from 1960 to 2100 in European forest ecosystems. Agricultural and Forest Meteorology 141 (1): 35-56

ABSTRACT: The effects of climate changes on carbon and water fluxes are quantified using a physiologically multi-layer, process-based model containing a carbon allocation model and coupled with a soil model (CASTANEA). The model is first evaluated on four EUROFLUX sites using eddy covariance data, which provide estimates of carbon and water fluxes at the ecosystem scale. It correctly reproduces the diurnal fluxes and the seasonal pattern. Thereafter simulations were conducted on six French forest ecosystems representative of three climatic areas (oceanic, continental and Mediterranean areas) dominated by deciduous species (Fagus sylvatica ,Quercus robur ), coniferous species (Pinus pinaster ,Pinus sylvestris ) or sclerophyllous evergreen species (Quercus ilex ). The model is driven by the results of a meteorological model (ARPEGE) following the B2 scenario of IPCC. From 1960 to 2100, the average temperature increases by 3.1 °C (30%) and the rainfall during summer decreases by 68 mm (-27%). For all the sites, between the two periods, the simulations predict on average a gross primary production (GPP) increase of 513 g(C) m-2 (+38%). This increase is relatively steep until 2020, followed by a slowing down of the GPP rise due to an increase of the effect of water stress. Contrary to GPP, the ecosystem respiration (Reco) raises at a constant rate (350 g(C) m-2 i.e. 31% from 1960 to 2100). The dynamics of the net ecosystem productivity (GPP minus Reco) is the consequence of the effect on both GPP and Reco and differs per site. The ecosystems always remain carbon sinks; however the sink strength globally decreases for coniferous (-8%), increases for sclerophyllous evergreen (+34%) and strongly increases for deciduous forest (+67%) that largely benefits by the lengthening of the foliated period. The separately quantified effects of the main variables (temperature, length of foliated season, CO2 fertilization, drought effect), show that the magnitude of these effects depends on the species and the climatic zone.

de Wit, H. A., Palosuo, T., Hylen, G., Liski, J. (2006). A carbon budget of forest biomass and soils in southeast Norway calculated using a widely applicable method. Forest Ecology and Management 225 (1-3): 15-26

ABSTRACT: Growing stocks of trees in Europe have increased in a magnitude that is significant in terms of carbon (C) sink strength. Estimates of the soil C sink strength that this increased stock of trees may have induced on a regional scale are scarce, uncertain and difficult to compare. This illustrates the need for a widely applicable calculation method. Here, we calculate a C budget of productive forest in southeast Norway based on forest inventory information, biomass expansion factors (BEF), biomass turnover rates and the dynamic soil model Yasso. We estimate a 29% increase (112–145 Tg) of C in biomass between 1971 and 2000, and estimate the associated increase of C in soils (including dead wood) to be 4.5% (181–189 Tg). The C sink strengths in biomass and soils (including dead wood) in 1990 are 0.38 and 0.08 Mg ha−1 yr−1 , respectively. Estimated soil C density is 58 Mg C ha−1 or ca 40% of measured soil C density in Norwegian forest soils. A sensitivity analysis – using uncertainty estimates of model inputs and parameters based on empirical data – shows that the underestimation of the soil C stock can be due to overestimation of decomposition rates of recalcitrant organic matter in the soil model and to including only trees as a source of litter. However, uncertainty in these two factors is shown to have a minimal effect on soil sink estimates. The key uncertainty in the soil sink is the initial value of the soil C stock, i.e. the assumed steady state soil C stock at the start of the time series in 1970. However, this source of uncertainty is reduced in importance for when approaching the end of the data series. This indicates that a longer time series of forest inventory data will decrease the uncertainty in the soil sink estimate due to initialisation of the soil C stock. Other, less significant, sources of uncertainty in estimates of soil stock and sink are BEF for fine roots and turnover rates of fine roots and foliage. The used method for calculation of a forest C budget can be readily applied to other regions for which similar forest resource data are available.

Eliseev, A. V., Mokhov, I. I. (2007). Carbon cycle-climate feedback sensitivity to parameter changes of a zero-dimensional terrestrial carbon cycle scheme in a climate model of intermediate complexity. Theoretical and Applied Climatology 89 (1): 9-24

ABSTRACT: A series of sensitivity runs have been performed with a coupled climate–carbon cycle model. The climatic component consists of the climate model of intermediate complexity IAP RAS CM. The carbon cycle component is formulated as a simple zero-dimensional model. Its terrestrial part includes gross photosynthesis, and plant and soil respirations, depending on temperature viaQ 10 -relationships (Lenton, 2000). Oceanic uptake of anthropogenic carbon is formulated is a bi-linear function of tendencies of atmospheric concentration of CO2 and globally averaged annual mean sea surface temperature. The model is forced by the historical industrial and land use emissions of carbon dioxide for the second half of the 19th and the whole of the 20th centuries, and by the emission scenario SRES A2 for the 21st century. For the standard set of the governing parameters, the model realistically captures the main features of the Earth’s observed carbon cycle. A large number of simulations have been performed, perturbing the governing parameters of the terrestrial carbon cycle model. In addition, the climate part is perturbed, either by zeroing or artificially increasing the climate model sensitivity to the doubling of the atmospheric CO2 concentration. Performing the above mentioned perturbations, it is possible to mimic most of the range found in the C4MIP simulations. In this way, a wide range of the climate–carbon cycle feedback strengths is obtained, differing even in the sign of the feedback. If the performed simulations are subjected to the constraints of a maximum allowed deviation of the simulated atmospheric CO2 concentration (p CO2( ) ) from the observed values and correspondence between simulated and observed terrestrial uptakes, it is possible to narrow the corresponding uncertainty range. Among these constraints, consideringp CO2( ) and uptakes are both important. However, the terrestrial uptakes constrain the simulations more effectively than the oceanic ones. These constraints, while useful, are still unable to rule out both extremely strong positive and modest negative climate–carbon cycle feedback.

Eliseev, A.V., Mokhov, I.I., Karpenko, A.A. (2007). Climate and carbon cycle variations in the 20th and 21st centuries in a model of intermediate complexity. Izvestiya - Atmospheric and Ocean Physics 43 (1): 1-14

ABSTRACT: The climate model of intermediate complexity developed at the Oboukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS CM), has been supplemented by a zero-dimensional carbon cycle model. With the carbon dioxide emissions prescribed for the second half of the 19th century and for the 20th century, the model satisfactorily reproduces characteristics of the carbon cycle over this period. However, with continued anthropogenic CO2 emissions (SRES scenarios A1B, A2, B1, and B2), the climate-carbon cycle feedback in the model leads to an additional atmospheric CO2 increase (in comparison with the case where the influence of climate changes on the carbon exchange between the atmosphere and the underlying surface is disregarded). This additional increase is varied in the range 67–90 ppmv depending on the scenario and is mainly due to the dynamics of soil carbon storage. The climate-carbon cycle feedback parameter varies nonmonotonically with time. Positions of its extremes separate characteristic periods of the change in the intensity of anthropogenic emissions and of climate variations. By the end of the 21st century, depending on the emission scenario, the carbon dioxide concentration is expected to increase to 615–875 ppmv and the global temperature will rise by 2.4–3.4 K relative to the preindustrial value. In the 20th–21st centuries, a general growth of the buildup of carbon dioxide in the atmosphere and ocean and its reduction in terrestrial ecosystems can be expected. In general, by the end of the 21st century, the more aggressive emission scenarios are characterized by a smaller climate-carbon cycle feedback parameter, a lower sensitivity of climate to a single increase in the atmospheric concentration of carbon dioxide, a larger fraction of anthropogenic emissions stored in the atmosphere and the ocean, and a smaller fraction of emissions in terrestrial ecosystems.

Erbrecht, T., Lucht, W. (2006). Impacts of large-scale climatic disturbances on the terrestrial carbon cycle. Carbon Balance and Management 1 (2): 1-7

ABSTRACT: Background: The amount of carbon dioxide in the atmosphere steadily increases as a consequence of anthropogenic emissions but with large interannual variability caused by the terrestrial biosphere. These variations in the CO2 growth rate are caused by large-scale climate anomalies but the relative contributions of vegetation growth and soil decomposition is uncertain. We use a biogeochemical model of the terrestrial biosphere to differentiate the effects of temperature and precipitation on net primary production (NPP) and heterotrophic respiration (Rh) during the two largest anomalies in atmospheric CO2 increase during the last 25 years. One of these, the smallest atmospheric year-to-year increase (largest land carbon uptake) in that period, was caused by global cooling in 1992/93 after the Pinatubo volcanic eruption. The other, the largest atmospheric increase on record (largest land carbon release), was caused by the strong El Niño event of 1997/98.

Results: We find that the LPJ model correctly simulates the magnitude of terrestrial modulation of atmospheric carbon anomalies for these two extreme disturbances. The response of soil respiration to changes in temperature and precipitation explains most of the modelled anomalous CO2 flux.

Conclusion: Observed and modelled NEE anomalies are in good agreement, therefore we suggest that the temporal variability of heterotrophic respiration produced by our model is reasonably realistic. We therefore conclude that during the last 25 years the two largest disturbances of the global carbon cycle were strongly controlled by soil processes rather then the response of vegetation to these large-scale climatic events.

Euskirchen, E. S., Mcguire, A. D., Kicklighter, D. W., Zhuang, Q., Clein, J. S., Dargaville, R. J., Dye, D. G., Kimball, J. S., Mcdonald, K. C., Melillo, J. M., Romanovsky, V. E., Smith, N. V. (2006). Importance of recent shifts in soil thermal dynamics on growing season length, productivity, and carbon sequestration in terrestrial high-latitude ecosystems. Global Change Biology 12 (4): 731-750

ABSTRACT: In terrestrial high-latitude regions, observations indicate recent changes in snow cover, permafrost, and soil freeze–thaw transitions due to climate change. These modifications may result in temporal shifts in the growing season and the associated rates of terrestrial productivity. Changes in productivity will influence the ability of these ecosystems to sequester atmospheric CO2 . We use the terrestrial ecosystem model (TEM), which simulates the soil thermal regime, in addition to terrestrial carbon (C), nitrogen and water dynamics, to explore these issues over the years 1960–2100 in extratropical regions (30–90°N). Our model simulations show decreases in snow cover and permafrost stability from 1960 to 2100. Decreases in snow cover agree well with National Oceanic and Atmospheric Administration satellite observations collected between the years 1972 and 2000, with Pearson rank correlation coefficients between 0.58 and 0.65. Model analyses also indicate a trend towards an earlier thaw date of frozen soils and the onset of the growing season in the spring by approximately 2–4 days from 1988 to 2000. Between 1988 and 2000, satellite records yield a slightly stronger trend in thaw and the onset of the growing season, averaging between 5 and 8 days earlier. In both, the TEM simulations and satellite records, trends in day of freeze in the autumn are weaker, such that overall increases in growing season length are due primarily to earlier thaw. Although regions with the longest snow cover duration displayed the greatest increase in growing season length, these regions maintained smaller increases in productivity and heterotrophic respiration than those regions with shorter duration of snow cover and less of an increase in growing season length. Concurrent with increases in growing season length, we found a reduction in soil C and increases in vegetation C, with greatest losses of soil C occurring in those areas with more vegetation, but simulations also suggest that this trend could reverse in the future. Our results reveal noteworthy changes in snow, permafrost, growing season length, productivity, and net C uptake, indicating that prediction of terrestrial C dynamics from one decade to the next will require that large-scale models adequately take into account the corresponding changes in soil thermal regimes.

Fan, G. Z., Zhang, T. J., Ji, J. J., Li, K. R., Liu, J. Y. (2007). Numerical simulation of the carbon cycle over the Tibetan Plateau, China. Arctic Antarctic and Alpine Research 39 (4): 723-731

ABSTRACT: Significant interaction occurs between ecosystem physiological processes and climate. Studying this interaction is beneficial for understanding dynamics of climate change as well as forecasting future climate change. On the Qinghai-Xizang Plateau, the highest plateau in the world, interaction between ecosystem physiological processes and climate affect mid-levels of the atmosphere, so the study of this interaction has a special significance. We use two models, a carbon cycle model (CCM3) and a land surface model (LSM), to simulate ecosystem carbon cycle characteristics over the Tibetan Plateau and its influence on climate. The CO2 flux varies seasonally with ecosystem physiology processes on the Plateau: fluxes are highest in summer and lowest in winter. The seasonal variation of vegetation net CO2 flux shows that vegetation is an atmospheric carbon sink during most of the year, except in winter. This means that vegetation could weaken the greenhouse effect, which is important in terms of global warming. The land ecosystem is a weak carbon source from October to April, and it is a carbon sink from May to September (especially between June and August). The Tibetan Plateau CO2 fluxes vary spatially. The fluxes are highest over the southwest and southeast boundary areas and the northeast region of the Plateau in summer, and are lowest in the middle and northwest regions in winter. The interaction of CO2 flux and temperature shows that higher temperatures increase vegetation photosynthesis and all respiration. The abrupt increase in land ecosystem physiological processes with increasing temperature indicates that any warming due to increased atmospheric CO2 caused by human activity will be weakened by the land ecosystem over the Tibetan Plateau.

Finzi, A. C., Norby, R. J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W. E., Hoosbeek, M. R., Iversen, C. M., Jackson, R. B., Kubiske, M. E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D. R., Schlesinger, W. H., Ceulemans, R. (2007). Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2 . Proceedings of the National Academy of Sciences 104 (35): 14014-14019

ABSTRACT: Forest ecosystems are important sinks for rising concentrations of atmospheric CO2 . In previous research, we showed that net primary production (NPP) increased by 23 ± 2% when four experimental forests were grown under atmospheric concentrations of CO2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO2 enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO2 at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO2 at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO2 . Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO2 result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO2 .

Fisher, R. A., Williams, M., Da Costa, A. L., Malhi, Y., Da Costa, R. F., Almeida, S., Meir, P. (2007). The response of an Eastern Amazonian rain forest to drought stress: results and modelling analyses from a throughfall exclusion experiment. Global Change Biology 13 (11): 2361-2378

ABSTRACT: Warmer and drier climates over Eastern Amazonia have been predicted as a component of climate change during the next 50-100 years. It remains unclear what effect such changes will have on forest-atmosphere exchange of carbon dioxide (CO2 ) and water, but the cumulative effect is anticipated to produce climatic feedback at both regional and global scales. To allow more detailed study of forest responses to soil drying, a simulated soil drought or 'throughfall exclusion' (TFE) experiment was established at a rain forest site in Eastern Amazonia, Brazil, for which time-series sap flow and soil moisture data were obtained. The experiment excluded 50% of the throughfall from the soil. Sap flow data from the forest plot experiencing normal rainfall showed no limitation of transpiration throughout the two monitored dry seasons. Conversely, data from the TFE showed large dry season declines in transpiration, with tree water use restricted to 20% of that in the control plot at the peak of both dry seasons. The results were examined to evaluate the paradigm that the restriction on transpiration in the dry season was caused by limitation of soil-to-root water transport, driven by low soil water potential and high soil-to-root hydraulic resistance. This paradigm, embedded in the soil-plant-atmosphere (SPA) model and driven using on-site measurements, provided a good explanation (R2 > 0.69) of the magnitude and timing of changes in sap flow and soil moisture. This model-data correspondence represents a substantial improvement compared with other ecosystem models of drought stress tested in Amazonia. Inclusion of deeper rooting should lead to lower sensitivity to drought than the majority of existing models. Modelled annual GPP declined by 13-14% in response to the treatment, compared with estimated declines in transpiration of 30-40%.

J. Garcia-Gonzalo, H. Peltola, E. Briceño-elizondo, S. Kellomäki (2007). Changed thinning regimes may increase carbon stock under climate change: A case study from a Finnish boreal forest. Climatic Change V81 (3): 431-454

ABSTRACT: A physiological growth and yield model was applied for assessing the effects of forest management and climate change on the carbon (C) stocks in a forest management unit located in Finland. The aim was to outline an appropriate management strategy with regard to C stock in the ecosystem (C in trees and C in soil) and C in harvested timber. Simulations covered 100 years using three climate scenarios (current climate, ECHAM4 and HadCM2), five thinning regimes (based on current forest management recommendations for Finland) and one unthinned. Simulations were undertaken with ground true stand inventory data (1451 hectares) representing Scots pine (Pinus sylvestris ), Norway spruce (Picea abies) and silver birch (Betula pendula) stands. Regardless of the climate scenario, it was found that shifting from current practices to thinning regimes that allowed higher stocking of trees resulted in an increase of up to 11% in C in the forest ecosystem. It also increased the C in the timber yield by up to 14%. Compared to current climatic conditions, the mean increase over the thinning regimes in the total C stock in the forest ecosystem due to the climate change was a maximum of 1%; but the mean increase in total C in timber yield over thinning regimes was a maximum of 12%.

Godbold, D.L., Brunner, I. (2007). The platform for European root science, COST action E38: An introduction and overview. Plant Biosystems 141 (3): 390-393

ABSTRACT: Globally, forests cover 4 billion ha or 30% of the Earth's land surface and account for more that 75% of carbon stored in terrestrial ecosystem. However, 20 - 40% of the forest biomass is roots. Roots play a key role in acquisition of water and nutrients from the soil, the transfer of carbon to soil, as well as providing physical stabilisation. In temperate forests of Europe, average biomass of trees is estimated to be ca. 220 t ha-1 , of which 52 t ha-1 are coarse roots and 2.4 t ha-1 are fine roots. Thus, forests and their soils belong to the planets largest reservoirs of carbon. As an outcome of a recently established European platform for scientists working on woody roots, COST action E38, a series of papers has been initiated in order to review the current knowledge on processes in and of roots of woody plants and to identify possible knowledge gaps. These reviews concentrate on aspects of roots as indicators of environmental change, biomass of fine roots, and modelling of course root systems. The reviews of roots as indicators of environmental change cover a number of aspects including, specific root length, the calcium to aluminium ratio, root electrolyte leakage, and ectomycorrhiza community composition.

Grace, P.R., Post, W.M., Hennessy, K. (2006). The potential impact of climate change on Australia's soil organic carbon resources. Carbon Balance and Management 1 (1): 14

ABSTRACT:Soil organic carbon (SOC) represents a significant pool of carbon within the biosphere. Climatic shifts in temperature and precipitation have a major influence on the decomposition and amount of SOC stored within an ecosystem and that released into the atmosphere. We have linked net primary production (NPP) algorithms, which include the impact of enhanced atmospheric CO2 on plant growth, to the SOCRATES terrestrial carbon model to estimate changes in SOC for the Australia continent between the years 1990 and 2100 in response to climate changes generated by the CSIRO Mark 2 Global Circulation Model (GCM).We estimate organic carbon storage in the topsoil (0–10 cm) of the Australian continent in 1990 to be 8.1 Gt. This equates to 19 and 34 Gt in the top 30 and 100 cm of soil, respectively. By the year 2100, under a low emissions scenario, topsoil organic carbon stores of the continent will have increased by 0.6% (49 Mt C). Under a high emissions scenario, the Australian continent becomes a source of CO2 with a net reduction of 6.4% (518 Mt) in topsoil carbon, when compared to no climate change. This is partially offset by the predicted increase in NPP of 20.3%Climate change impacts must be studied holistically, requiring integration of climate, plant, ecosystem and soil sciences. The SOCRATES terrestrial carbon cycling model provides realistic estimates of changes in SOC storage in response to climate change over the next century, and confirms the need for greater consideration of soils in assessing the full impact of climate change and the development of quantifiable mitigation strategies.

Hibbard, K. A., Law, B. E., Reichstein, M., Sulzman, J. (2005). An analysis of soil respiration across northern hemisphere temperate ecosystems.. Biogeochemistry 73 (1): 29-70

ABSTRACT: Over two-thirds of terrestrial carbon is stored belowground and a significant amount of atmospheric CO2 is respired by roots and microbes in soils. For this analysis, soil respiration (Rs) data were assembled from 31 AmeriFlux and CarboEurope sites representing deciduous broadleaf, evergreen needleleaf, grasslands, mixed deciduous/evergreen and woodland/savanna ecosystem types. Lowest to highest rates of soil respiration averaged over the growing season were grassland and woodland/savanna < deciduous broadleaf forests < evergreen needleleaf, mixed deciduous/evergreen forests with growing season soil respiration significantly different between forested and non-forested biomes (p < 0.001). Timing of peak respiration rates during the growing season varied from March/April in grasslands to July–September for all other biomes. Biomes with overall strongest relationship between soil respiration and soil temperature were from the deciduous and mixed forests (R2 ≥ 0.65). Maximum soil respiration was weakly related to maximum fine root biomass (R2 = 0.28) and positively related to the previous years’ annual litterfall (R2 = 0.46). Published rates of annual soil respiration were linearly related to LAI and fine root carbon (R2 = 0.48, 0.47), as well as net primary production (NPP) (R2 = 0.44). At 10 sites, maximum growing season Rs was weakly correlated with annual GPP estimated from eddy covariance towersites (R2 = 0.29; p < 0.05), and annual soil respiration and total growing season Rs were not correlated with annual GPP (p > 0.1). Yet, previous studies indicate correlations on shorter time scales within site (e.g., weekly, monthly). Estimates of annual GPP from the Biome-BGC model were strongly correlated with observed annual estimates of soil respiration for six sites (R2 = 0.84; p < 0.01). Correlations from observations of Rs with NPP, LAI, fine root biomass and litterfall relate above and belowground inputs to labile pools that are available for decomposition. Our results suggest that simple empirical relationships with temperature and/or moisture that may be robust at individual sites may not be adequate to characterize soil CO2 effluxes across space and time, agreeing with other multi-site studies. Information is needed on the timing and phenological controls of substrate availability (e.g., fine roots, LAI) and inputs (e.g., root turnover, litterfall) to improve our ability to accurately quantify the relationships between soil CO2 effluxes and carbon substrate storage.

Ito, A. (2005). Climate-related uncertainties in projections of the twenty-first century terrestrial carbon budget: off-line model experiments using IPCC greenhouse-gas scenarios and AOGCM climate projections. Climate Dynamics 24 (5): 435-448

ABSTRACT: A terrestrial ecosystem model (Sim-CYCLE) was driven by multiple climate projections to investigate uncertainties in predicting the interactions between global environmental change and the terrestrial carbon cycle. Sim-CYCLE has a spatial resolution of 0.5°, and mechanistically evaluates photosynthetic and respiratory CO2 exchange. Six scenarios for atmospheric-CO2 concentrations in the twenty-first century, proposed by the Intergovernmental Panel on Climate Change, were considered. For each scenario, climate projections by a coupled atmosphere–ocean general circulation model (AOGCM) were used to assess the uncertainty due to socio-economic predictions. Under a single CO2 scenario, climate projections with seven AOGCMs were used to investigate the uncertainty stemming from uncertainty in the climate simulations. Increases in global photosynthesis and carbon storage differed considerably among scenarios, ranging from 23 to 37% and from 24 to 81 Pg C, respectively. Among the AOGCM projections, increases ranged from 26 to 33% and from 48 to 289 Pg C, respectively. There were regional heterogeneities in both climatic change and carbon budget response, and different carbon-cycle components often responded differently to a given environmental change. Photosynthetic CO2 fixation was more sensitive to atmospheric CO2 , whereas soil carbon storage was more sensitive to temperature. Consequently, uncertainties in the CO2 scenarios and climatic projections may create additional uncertainties in projecting atmospheric-CO2 concentrations and climates through the interactive feedbacks between the atmosphere and the terrestrial ecosystem.

Ito, A. (2007). Simulated impacts of climate and land-cover change on soil erosion and implication for the carbon cycle, 1901 to 2100. Geophysical Research Letters 34 (L09403): doi:10.1029/2007GL029342

ABSTRACT: The impacts of climatic change and land-cover change on soil carbon displacement by water erosion were investigated using a global ecosystem carbon cycle model (Sim-CYCLE) and an empirical erosion model (RUSLE). Simulations considering the climate and land-cover changes were performed in two phases, from 1901 to 1990 on the basis of historical data, and from 1991 to 2100 using climate projections in the IPCC Forth Assessment Report. During the first phase, total lateral displacement of soil carbon was estimated to be 1.6 ± 0.1 Pg C y−1 with remarkable geographical heterogeneity, and it was gradually intensified in regions where forests were converted into croplands. During the second phase, both projected rainfall and land-use changes affected the erosion regime in many regions. Consequently, the total amount of soil carbon displacement increased by 32–57%, implying an intensified vulnerability to soil loss and further perturbations in the carbon cycle.

Ito, A., Sasai, T. (2006). A comparison of simulation results from two terrestrial carbon cycle models using three climate data sets. Tellus: Series B 58 (5): 513-522

ABSTRACT: This study addressed how different climate data sets influence simulations of the global terrestrial carbon cycle. For the period 1982–2001, we compared the results of simulations based on three climate data sets (NCEP/NCAR, NCEP/DOE AMIP-II and ERA40) employed in meteorological, ecological and biogeochemical studies and two different models (BEAMS and Sim-CYCLE). The models differed in their parameterizations of photosynthetic and phenological processes but used the same surface climate (e.g. shortwave radiation, temperature and precipitation), vegetation, soil and topography data. The three data sets give different climatic conditions, especially for shortwave radiation, in terms of long-term means, linear trends and interannual variability. Consequently, the simulation results for global net primary productivity varied by 16%–43% only from differences in the climate data sets, especially in these regions where the shortwave radiation data differed markedly: differences in the climate data set can strongly influence simulation results. The differences among the climate data set and between the two models resulted in slightly different spatial distribution and interannual variability in the net ecosystem carbon budget. To minimize uncertainty, we should pay attention to the specific climate data used. We recommend developing an accurate standard climate data set for simulation studies.

Jiang, Hong, Apps, Michael J., Peng, Changhui, Zhang, Yanli, Liu, Jinxun (2002). Modelling the influence of harvesting on Chinese boreal forest carbon dynamics. Forest Ecology and Management 169 (1-2): 65-82

ABSTRACT: Chinese boreal forests, geographically distributed in the Daxinganling Mountains of northeastern China, are the most southern part of the global boreal forest biome. The dominant species is larch (Larix gmelinii ) with other major species including birch (Betula platyphylla ), pine (Pinus sylvestris var.mongolica ) and oak (Quercus mongolica ). In this study, the terrestrial ecosystem process model CENTURY 4.0 was used to investigate the influence of different harvest disturbance regimes on the carbon stocks and fluxes of Chinese boreal forest ecosystem relative to a natural disturbance regime. Managed disturbance regime scenarios examined include harvesting intensity (no biomass removal (NBR), conventional harvesting (CH) and whole tree harvesting (WTH)) and rotation length (from 30 to 400 years). Field data were assembled from three forest regions (Xinlin, Tahe and Mohe), representing the northern, middle and southern parts of the Chinese boreal forest, respectively. The results presented in this study indicate that biomass, litter and soil carbon stocks (averaged over a rotation period) can be elevated significantly by suppression of all disturbances (NBR scenario) but are lowest under the most intense harvest scenarios (WTH). Harvest rotation length had a significant influence on carbon stocks (biomass, litter and soil carbon); the lowest simulated carbon stocks were found with the shortest rotations, and relatively higher stocks under longer rotations. Net primary production (NPP) decreased with increasing harvest intensity or decreasing rotation length. Net ecosystem production (NEP) decreased with decreasing harvest intensity or decreasing rotation length. NPP and NEP reach maximum values at rotation lengths of about 200 and 100 years, respectively. Observations and simulated data for ecosystem carbon stocks (biomass, litter and soil carbon) and carbon fluxes (NPP and NEP) in the southern region were slightly higher than those in the mid- and northern regions. The high productivity and biomass of the Chinese boreal forests relative to those of Canada, USA and Russia, are likely due to their southerly location: warm temperature and adequate precipitation create good conditions for forest development and growth. Nevertheless, the long history of forest use by human has resulted in much of the boreal forest in China landscape being in less than a primary state.

Jones, C., McConnell, C., Coleman, K., Cox, P., Falloon, P., Jenkinson, D., Powlson, D. (2005). Global climate change and soil carbon stocks; predictions from two contrasting models for the turnover of organic carbon in soil. Global Change Biology 11 (1): 154-166

ABSTRACT: Enhanced release of CO2 to the atmosphere from soil organic carbon as a result of increased temperatures may lead to a positive feedback between climate change and the carbon cycle, resulting in much higher CO2 levels and accelerated global warming. However, the magnitude of this effect is uncertain and critically dependent on how the decomposition of soil organic C (heterotrophic respiration) responds to changes in climate. Previous studies with the Hadley Centre's coupled climate–carbon cycle general circulation model (GCM) (HadCM3LC) used a simple, single-pool soil carbon model to simulate the response. Here we present results from numerical simulations that use the more sophisticated 'RothC' multipool soil carbon model, driven with the same climate data.

The results show strong similarities in the behaviour of the two models, although RothC tends to simulate slightly smaller changes in global soil carbon stocks for the same forcing. RothC simulates global soil carbon stocks decreasing by 54 Gt C by 2100 in a climate change simulation compared with an 80 Gt C decrease in HadCM3LC. The multipool carbon dynamics of RothC cause it to exhibit a slower magnitude of transient response to both increased organic carbon inputs and changes in climate. We conclude that the projection of a positive feedback between climate and carbon cycle is robust, but the magnitude of the feedback is dependent on the structure of the soil carbon model.

Ju, W., Chen, J. M. (2005). Distribution of soil carbon stocks in Canada's forests and wetlands simulated based on drainage class, topography and remotely sensed vegetation parameters. Hydrological Processes 19 (1): 77-94

ABSTRACT: A quasi-three-dimensional hydrological model was developed and integrated into the integrated terrestrial ecosystem carbon-budget model (InTEC V3·0) to improve the estimation of the carbon (C) dynamics in Canadian forests and wetlands. Climate, soil, digital elevation map, and drainage class data, in conjunction with remotely sensed vegetation parameters, including leaf area index, land cover type, and stand age, are used to drive the model. Soil is divided into three layers, for which temperature and moisture dynamics are simulated. Individual 1 km × 1 km pixels are hydrologically linked with neighbouring pixels through subsurface saturated base-flow, which is simulated using a TOPMODEL-based scheme. Soil C and nitrogen (N) dynamics are simulated using the soil submodel of CENTURY suitably modified for forests and wetlands. The interannual variation in net primary productivity is iteratively computed after integrating the effects of N, climate, stand age and atmospheric CO2 concentration on productivity. Compared with data in the Soil Landscape of Canada, the newly updated InTEC V3·0 can capture 66·6% of spatial variations in soil C and effectively alleviate soil C underestimation in wetland

Ju, W., J.M. Chen, T.A. Black, A.G. Barr, H. McCaughey, N.T. Roulet (2006). Hydrological effects on carbon cycles of Canada's forests and wetlands.. Tellus: Series B 58 (1): 16-30

ABSTRACT: The hydrological cycle has significant effects on the terrestrial carbon (C) balance through its controls on photosynthesis and C decomposition. A detailed representation of the water cycle in terrestrial C cycle models is essential for reliable estimates of C budgets. However, it is challenging to accurately describe the spatial and temporal variations of soil water, especially for regional and global applications. Vertical and horizontal movements of soil water should be included. To constrain the hydrology-related uncertainty in modelling the regional C balance, a three-dimensional hydrological module was incorporated into the Integrated Terrestrial Ecosystem Carbon-budget model (InTEC V3.0). We also added an explicit parameterization of wetlands. The inclusion of the hydrological module considerably improved the model's ability to simulate C content and balances in different ecosystems. Compared with measurements at five flux-tower sites, the model captured 85% and 82% of the variations in volumetric soil moisture content in the 0–10 cm and 10–30 cm depths during the growing season and 84% of the interannual variability in the measured C balance. The simulations showed that lateral subsurface water redistribution is a necessary mechanism for simulating water table depth for both poorly drained forest and peatland sites. Nationally, soil C content and their spatial variability are significantly related to drainage class. Poorly drained areas are important C sinks at the regional scale, however, their soil C content and balances are difficult to model and may have been inadequately represented in previous C cycle models. The InTEC V3.0 model predicted an annual net C uptake by Canada's forests and wetlands for the period 1901–1998 of 111.9 Tg C yr−1 , which is 41.4 Tg C yr−1 larger than our previous estimate (InTEC V2.0). The increase in the net C uptake occurred mainly in poorly drained regions and resulted from the inclusion of a separate wetland parameterization and a detailed hydrologic module with lateral flow in InTEC V3.0.

Karberg, N. J., Pregitzer, K. S., King, J. S., Friend, A. L., Wood, J. R. (2005). Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone. Oecologia 142 (2): 296-306

ABSTRACT: Global emissions of atmospheric CO2 and tropospheric O3 are rising and expected to impact large areas of the Earths forests. While CO2 stimulates net primary production, O3 reduces photosynthesis, altering plant C allocation and reducing ecosystem C storage. The effects of multiple air pollutants can alter belowground C allocation, leading to changes in the partial pressure of CO2 (p CO2 ) in the soil , chemistry of dissolved inorganic carbonate (DIC) and the rate of mineral weathering. As this system represents a linkage between the long- and short-term C cycles and sequestration of atmospheric CO2 , changes in atmospheric chemistry that affect net primary production may alter the fate of C in these ecosystems. To date, little is known about the combined effects of elevated CO2 and O3 on the inorganic C cycle in forest systems. Free air CO2 and O3 enrichment (FACE) technology was used at the Aspen FACE project in Rhinelander, Wisconsin to understand how elevated atmospheric CO2 and O3 interact to alter pCO2 and DIC concentrations in the soil. Ambient and elevated CO2 levels were 360±16 and 542±81 l l–1 , respectively; ambient and elevated O3 levels were 33±14 and 49±24 nl l–1 , respectively. Measured concentrations of soil CO2 and calculated concentrations of DIC increased over the growing season by 14 and 22%, respectively, under elevated atmospheric CO2 and were unaffected by elevated tropospheric O3 . The increased concentration of DIC altered inorganic carbonate chemistry by increasing system total alkalinity by 210%, likely due to enhanced chemical weathering. The study also demonstrated the close coupling between the seasonal13 C of soilp CO2 and DIC, as a mixing model showed that new atmospheric CO2 accounted for approximately 90% of the C leaving the system as DIC. This study illustrates the potential of using stable isotopic techniques and FACE technology to examine long- and short-term ecosystem C sequestration.

Kennedy, M., Anderson, C., O'Hagan, A., Lomas, M., Woodward, I., Gosling, J.P., Heinemeyer, A. (2008). Quantifying uncertainty in the biospheric carbon flux for England and Wales. Journal of the Royal Statistical Society. Series A: Statistics in Society 171 (1): 109-135

SUMMARY: A crucial issue in the current global warming debate is the effect of vegetation and soils on carbon dioxide (CO2 ) concentrations in the atmosphere. Vegetation can extract CO2 through photosynthesis, but respiration, decay of soil organic matter and disturbance effects such as fire return it to the atmosphere. The balance of these processes is the net carbon flux. To estimate the biospheric carbon flux for England and Wales, we address the statistical problem of inference for the sum of multiple outputs from a complex deterministic computer code whose input parameters are uncertain. The code is a process model which simulates the carbon dynamics of vegetation and soils, including the amount of carbon that is stored as a result of photosynthesis and the amount that is returned to the atmosphere through respiration. The aggregation of outputs corresponding to multiple sites and types of vegetation in a region gives an estimate of the total carbon flux for that region over a period of time. Expert prior opinions are elicited for marginal uncertainty about the relevant input parameters and for correlations of inputs between sites. A Gaussian process model is used to build emulators of the multiple code outputs and Bayesian uncertainty analysis is then used to propagate uncertainty in the input parameters through to uncertainty on the aggregated output. Numerical results are presented for England and Wales in the year 2000. It is estimated that vegetation and soils in England and Wales constituted a net sink of 7.55 Mt C (1 Mt C = 1012 g of carbon) in 2000, with standard deviation 0.56 Mt C resulting from the sources of uncertainty that are considered.

Kimball, J. S., Zhao, M., McGuire, A. D., Heinsch, F. A., Clein, J., Calef, M., Jolly, W. M., Kang, S., Euskirchen, S. E., McDonald, K. C., Running, S. W. (2007). Recent Climate-Driven Increases in Vegetation Productivity for the Western Arctic: Evidence of an Acceleration of the Northern Terrestrial Carbon Cycle. Earth Interactions 11 (4): 1-30

ABSTRACT: Northern ecosystems contain much of the global reservoir of terrestrial carbon that is potentially reactive in the context of near-term climate change. Annual variability and recent trends in vegetation productivity across Alaska and northwest Canada were assessed using a satellite remote sensing–based production efficiency model and prognostic simulations of the terrestrial carbon cycle from the Terrestrial Ecosystem Model (TEM) and BIOME–BGC (BioGeoChemical Cycles) model. Evidence of a small, but widespread, positive trend in vegetation gross and net primary production (GPP and NPP) is found for the region from 1982 to 2000, coinciding with summer warming of more than 1.8°C and subsequent relaxation of cold temperature constraints to plant growth. Prognostic model simulation results were generally consistent with the remote sensing record and also indicated that an increase in soil decomposition and plant-available nitrogen with regional warming was partially responsible for the positive productivity response. Despite a positive trend in litter inputs to the soil organic carbon pool, the model results showed evidence of a decline in less labile soil organic carbon, which represents approximately 75% of total carbon storage for the region. These results indicate that the regional carbon cycle may accelerate under a warming climate by increasing the fraction of total carbon storage in vegetation biomass and more rapid turnover of the terrestrial carbon reservoir.

Knorr,W., Prentice,I. C., House,J. I., Holland,E. A. (2005). Long-term sensitivity of soil carbon turnover to warming. Nature 433 (20 January): 298-301

ABSTRACT: The sensitivity of soil carbon to warming is a major uncertainty in projections of carbon dioxide concentration and climate1 . Experimental studies overwhelmingly indicate increased soil organic carbon (SOC) decomposition2, 3, 4, 5, 6, 7, 8 at higher temperatures, resulting in increased carbon dioxide emissions from soils. However, recent findings have been cited as evidence against increased soil carbon emissions in a warmer world9, 10 . In soil warming experiments, the initially increased carbon dioxide efflux returns to pre-warming rates within one to three years10, 11, 12, 13, 14 , and apparent carbon pool turnover times are insensitive to temperature15 . It has already been suggested that the apparent lack of temperature dependence could be an artefact due to neglecting the extreme heterogeneity of soil carbon16 , but no explicit model has yet been presented that can reconcile all the above findings. Here we present a simple three-pool model that partitions SOC into components with different intrinsic turnover rates. Using this model, we show that the results of all the soil-warming experiments are compatible with long-term temperature sensitivity of SOC turnover: they can be explained by rapid depletion of labile SOC combined with the negligible response of non-labile SOC on experimental timescales. Furthermore, we present evidence that non-labile SOC is more sensitive to temperature than labile SOC, implying that the long-term positive feedback of soil decomposition in a warming world may be even stronger than predicted by global models1, 17, 18, 19, 20 .

Kuchment, L.S., Demidov, V.N., Startseva, Z.P. (2006). Coupled modeling of the hydrological and carbon cycles in the soil-vegetation-atmosphere system. Journal of Hydrology 323 (1-4): 4-21

ABSTRACT: A coupled model of the hydrological and carbon cycles in the soil–vegetation–atmosphere system is suggested. The model describes the interception and evaporation of precipitation by canopy, transpiration, vertical transfer of soil moisture, photosynthesis, the interaction between transpiration and photosynthesis, and plant and soil respiration. The validation of this model was carried out using the FIFE measurements from a grassland site in Kansas, the BOREAS measurements from a jack pine forest site in Saskatchewan, and the observations conducted within a deciduous forest in the southeastern United States. The model results show a good agreement with experimental data. The model was shown to adequately describe the influence of soil moisture and atmospheric CO2 concentration on transpiration and net ecosystem CO2 exchange.

Larocque, G. R., Boutin, R., Pare, D., Robitaille, G., Lacerte, V. (2006). Assessing a new soil carbon model to simulate the effect of temperature increase on the soil carbon cycle in three eastern Canadian forest types characterized by different climatic conditions. Canadian Journal of Soil Science 86 (2): 187-202

ABSTRACT: The predictive capacity of process-based models on the carbon (C) cycle in forest ecosystems is limited by the lack of knowledge on the processes involved. Thus, a better understanding of the C cycle may contribute to the development of process-based models that better represent the processes in C cycle models. A new soil C model was developed to predict the effect of an increase in the temperature regime on soil C dynamics and pools in sugar maple (Acer saccharum Marsh.), balsam fir [Abies balsamea (L.) Mill.] and black spruce [Picea mariana (Mill.) B.S.P.] forest types in Eastern Canada. Background information to calibrate the model originated from the experimental sites of the ECOLEAP project as well as from a companion study on laboratory soil incubation. Different types of litter were considered in the model: foliage, twigs, understory species, other fine detritus and fine roots. A cohort approach was used to model litter mineralization over time. The soil organic C in the organic (F and H) and mineral layers (0-20 cm) was partitioned into active, slow and passive pools and the rates of C transfer among the different pools and the amount of CO2 respired were modelled. For each forest type, there was a synchrony of response of the C pools to soil temperature variation. The results of the simulations indicated that steady state conditions were obtained under current temperature conditions. When mean annual soil temperatures were gradually increased, the litter and active and slow C pools decreased substantially, but the passive pools were minimally affected. The increase in soil respiration resulting from a gradual increase in temperature was not pronounced in comparison to changes in mineralization rates. An increase in litter production during the same period could contribute to reducing net C losses.

Lawrence, D., Slater, A. (2008). Incorporating organic soil into a global climate model. Climate Dynamics 30 (2): 145-160

ABSTRACT: Organic matter significantly alters a soil’s thermal and hydraulic properties but is not typically included in land-surface schemes used in global climate models. This omission has consequences for ground thermal and moisture regimes, particularly in the high-latitudes where soil carbon content is generally high. Global soil carbon data is used to build a geographically distributed, profiled soil carbon density dataset for the Community Land Model (CLM). CLM parameterizations for soil thermal and hydraulic properties are modified to accommodate both mineral and organic soil matter. Offline simulations including organic soil are characterized by cooler annual mean soil temperatures (up to ~2.5°C cooler for regions of high soil carbon content). Cooling is strong in summer due to modulation of early and mid-summer soil heat flux. Winter temperatures are slightly warmer as organic soils do not cool as efficiently during fall and winter. High porosity and hydraulic conductivity of organic soil leads to a wetter soil column but with comparatively low surface layer saturation levels and correspondingly low soil evaporation. When CLM is coupled to the Community Atmosphere Model, the reduced latent heat flux drives deeper boundary layers, associated reductions in low cloud fraction, and warmer summer air temperatures in the Arctic. Lastly, the insulative properties of organic soil reduce interannual soil temperature variability, but only marginally. This result suggests that, although the mean soil temperature cooling will delay the simulated date at which frozen soil begins to thaw, organic matter may provide only limited insulation from surface warming.

Lei, X. D., Peng, C. H., Tian, D. L., Sun, J. F. (2007). Meta-analysis and its application in global change research. Chinese Science Bulletin 52 (3): 289-302

ABSTRACT: Meta-analysis is a quantitative synthetic research method that statistically integrates results from individual studies to find common trends and differences. With increasing concern over global change, meta-analysis has been rapidly adopted in global change research. Here, we introduce the methodologies, advantages and disadvantages of meta-analysis, and review its application in global climate change research, including the responses of ecosystems to global warming and rising CO2 and O3 concentrations, the effects of land use and management on climate change and the effects of disturbances on biogeochemistry cycles of ecosystem. Despite limitation and potential misapplication, meta-analysis has been demonstrated to be a much better tool than traditional narrative review in synthesizing results from multiple studies. Several methodological developments for research synthesis have not yet been widely used in global climate change researches such as cumulative meta-analysis and sensitivity analysis. It is necessary to update the results of meta-analysis on a given topic at regular intervals by including newly published studies. Emphasis should be put on multi-factor interaction and long-term experiments. There is great potential to apply meta-analysis to global climate change research in China because research and observation networks have been established (e.g. ChinaFlux and CERN), which create the need for combining these data and results to provide support for governments’ decision making on climate change. It is expected that meta-analysis will be widely adopted in future climate change research.

Lemma, B., Kleja, D. B.n, Olsson, M., Nilsson, I. (2007). Factors controlling soil organic carbon sequestration under exotic tree plantations: A case study using the CO2Fix model in southwestern Ethiopia. Forest Ecology and Management 252 (1-3): 124-131

ABSTRACT: Models are important research tools for predicting the build-up of soil organic carbon (SOC), because they provide an increased insight into factors that are involved in the build-up process. The CO2Fix (v. 3.1) model was used to examine the influence of litter production, litter quality and microclimate on differences in SOC accretion under exotic tree species established on farmland in southwestern Ethiopia. The SOC storage underCupressus lusitanica was larger than that under the other two investigated species (Pinus patula andEucalyptus grandis ). This was mainly because of the higher total litter input and higher proportion of fine woody litter (branches and coarse roots) in theCupressus stand. SOC accretion was greater underPinus than underEucalyptus . However, the total litter input in thePinus andEucalyptus stands was nearly the same. The difference between thePinus andEucalyptus stands was best explained by the fact thatPinus produced more fine woody litter than didEucalyptus . Litter quality and microclimate only accounted for a minor part of the differences in SOC storage in theCupressus ,Pinus andEucalyptus stands. Therefore, the results suggested that total litter input and the proportion of fine woody litter were the main factors that accounted for the inter-specific differences in SOC accretion.

Lenton, T., Williamson, M., Edwards, N., Marsh, R., Price, A., Ridgwell, A., Shepherd, J., Cox, S., The GENIE team (2006). Millennial timescale carbon cycle and climate change in an efficient Earth system model. Climate Dynamics 26 (7-8): 687-711

ABSTRACT: A new Earth system model, GENIE-1, is presented which comprises a 3-D frictional geostrophic ocean, phosphate-restoring marine biogeochemistry, dynamic and thermodynamic sea-ice, land surface physics and carbon cycling, and a seasonal 2-D energy-moisture balance atmosphere. Three sets of model climate parameters are used to explore the robustness of the results and for traceability to earlier work. The model versions have climate sensitivity of 2.8–3.3°C and predict atmospheric CO2 close to present observations. Six idealized total fossil fuel CO2 emissions scenarios are used to explore a range of 1,100–15,000 GtC total emissions and the effect of rate of emissions. Atmospheric CO2 approaches equilibrium in year 3000 at 420–5,660 ppmv, giving 1.5–12.5°C global warming. The ocean is a robust carbon sink of up to 6.5 GtC year−1 . Under ‘business as usual’, the land becomes a carbon source around year 2100 which peaks at up to 2.5 GtC year−1 . Soil carbon is lost globally, boreal vegetation generally increases, whilst under extreme forcing, dieback of some tropical and sub-tropical vegetation occurs. Average ocean surface pH drops by up to 1.15 units. A Greenland ice sheet melt threshold of 2.6°C local warming is only briefly exceeded if total emissions are limited to 1,100 GtC, whilst 15,000 GtC emissions cause complete Greenland melt by year 3000, contributing 7 m to sea level rise. Total sea-level rise, including thermal expansion, is 0.4–10 m in year 3000 and ongoing. The Atlantic meridional overturning circulation shuts down in two out of three model versions, but only under extreme emissions including exotic fossil fuel resources.

Levy, P. E., Friend, A. D., White, A., Cannell, M. G. R. (2004). The influence of land use change on global-scale fluxes of carbon from terrestrial ecosystems. Climatic Change 67 (2-3): 185-209

ABSTRACT: A process-based approach to modelling the effects of land use change and climate change on the carbon balance of terrestrial ecosystems was applied at global scale. Simulations were run both with and without land use change. In the absence of land use change between 1700 and 1990, carbon storage in terrestrial ecosystems was predicted to increase by 145 Pg C. When land use change was represented during this period, terrestrial ecosystems became a net source of 97 Pg C. Land use change was directly responsible for a flux of 222 Pg C, slightly higher but close to estimates from other studies. The model was then run between 1990 and 2100 with a climate simulated by a GCM. Simulations were run with three land use change scenarios: 1. no land use change; 2. land use change specified by the SRES B2 scenario, and; 3. land use change scaled with population change in the B2 scenario. In the first two simulations with no or limited land use change, the net terrestrial carbon sink was substantial (358 and 257 Pg C, respectively). However, with the population-based land-use change scenario, the losses of carbon through land use change were close to the carbon gains through enhanced net ecosystem productivity, resulting in a net sink near zero. Future changes in land use are highly uncertain, but will have a large impact on the future terrestrial carbon balance. This study attempts to provide some bounds on how land use change may affect the carbon sink over the nextcentury.

Li, C. (2007). Quantifying greenhouse gas emissions from soils: Scientific basis and modeling approach. Soil Science & Plant Nutrition 53 (4): 344-352

ABSTRACT: Global climate change is one of the most important issues of contemporary environmental safety. A scientific consensus is forming that the emissions of greenhouse gases, including carbon dioxide, nitrous oxide and methane, from anthropogenic activities may play a key role in elevating the global temperatures. Quantifying soil greenhouse gas emissions is an essential task for understanding the atmospheric impacts of anthropogenic activities in terrestrial ecosystems. In most soils, production or consumption of the three major greenhouse gases is regulated by interactions among soil redox potential, carbon source and electron acceptors. Two classical formulas, the Nernst equation and the Michaelis–Menten equation, describe the microorganism-mediated redox reactions from aspects of thermodynamics and reaction kinetics, respectively. The two equations are functions of a series of environmental factors (e.g. temperature, moisture, pH, Eh) that are regulated by a few ecological drivers, such as climate, soil properties, vegetation and anthropogenic activity. Given the complexity of greenhouse gas production in soils, process-based models are required to interpret, integrate and predict the intricate relationships among the gas emissions, the environmental factors and the ecological drivers. This paper reviews the scientific basis underlying the modeling of greenhouse gas emissions from terrestrial soils. A case study is reported to demonstrate how a biogeochemical model can be used to predict the impacts of alternative management practices on greenhouse gas emissions from rice paddies.

Liao, C., Peng, R., Luo, Y., Zhou, X., Wu, X., Fang, C., Chen, J., Li, B. (2008). Altered ecosystem carbon and nitrogen cycles by plant invasion: a meta-analysis. New Phytologist 177 (3): 706-714

ABSTRACT: • Plant invasion potentially alters ecosystem carbon (C) and nitrogen (N) cycles. However, the overall direction and magnitude of such alterations are poorly quantified.

• Here, 94 experimental studies were synthesized, using a meta-analysis approach, to quantify the changes of 20 variables associated with C and N cycles, including their pools, fluxes, and other related parameters in response to plant invasion.

• Pool variables showed significant changes in invaded ecosystems relative to native ecosystems, ranging from a 5% increase in root carbon stock to a 133% increase in shoot C stock. Flux variables, such as above-ground net primary production and litter decomposition, increased by 50–120% in invaded ecosystems, compared with native ones. Plant N concentration, soil NH4 + and NO3 - concentrations were 40, 30 and 17% higher in invaded than in native ecosystems, respectively. Increases in plant production and soil N availability indicate that there was positive feedback between plant invasion and C and N cycles in invaded ecosystems.

• Invasions by woody and N-fixing plants tended to have greater impacts on C and N cycles than those by herbaceous and nonN-fixing plants, respectively. The responses to plant invasion are not different among forests, grasslands, and wetlands. All of these changes suggest that plant invasion profoundly influences ecosystem processes.

Liu, J., Liu, S., Loveland, T. R. (2006). Temporal evolution of carbon budgets of the Appalachian forests in the U.S. from 1972 to 2000. Forest Ecology and Management 222 (1-3): 191-201

ABSTRACT: Estimating dynamic terrestrial ecosystem carbon (C) sources and sinks over large areas is difficult. The scaling of C sources and sinks from the field level to the regional level has been challenging due to the variations of climate, soil, vegetation, and disturbances. As part of an effort to estimate the spatial, temporal, and sectional dimensions of the United States C sources and sinks (the U.S. Carbon Trends Project), this study estimated the forest ecosystem C sequestration of the Appalachian region (186,000 km2 ) for the period of 1972–2000 using the General Ensemble Biogeochemical Modeling System (GEMS) that has a strong capability of assimilating land use and land cover change (LUCC) data. On 82 sampling blocks in the Appalachian region, GEMS used sequential 60 m resolution land cover change maps to capture forest stand-replacing events and used forest inventory data to estimate non-stand-replacing changes. GEMS also used Monte Carlo approaches to deal with spatial scaling issues such as initialization of forest age and soil properties. Ensemble simulations were performed to incorporate the uncertainties of input data. Simulated results show that from 1972 to 2000 the net primary productivity (NPP), net ecosystem productivity (NEP), and net biome productivity (NBP) averaged 6.2 Mg C ha−1 y−1 (±1.1), 2.2 Mg C ha−1 y−1 (±0.6), and 1.8 Mg C ha−1 y−1 (±0.6), respectively. The inter-annual variability was driven mostly by climate. Detailed C budgets for the year 2000 were also calculated. Within a total 148,000 km2 forested area, average forest ecosystem C density was estimated to be 186 Mg C ha−1 (±20), of which 98 Mg C ha−1 (±12) was in biomass and 88 Mg C ha−1 (±13) was in litter and soil. The total simulated C stock of the Appalachian forests was estimated to be 2751 Tg C (±296), including 1454 Tg C (±178) in living biomass and 1297 Tg C (±192) in litter and soil. The total net C sequestration (i.e. NBP) of the forest ecosystem in 2000 was estimated to be 19.5 Tg C y−1 (±6.8).

Marion, G.M., Schlesinger, W.H., Fonteyn, P.J. (1985). Caldep: A regional model for soil CaCO3 (Caliche) deposition in southwestern deserts. Soil Science 139 (5): 468-479

ABSTRACT: Our objective was to develop and validate a regional model for CaCO3 deposition in desert soils of the southwestern United States. There were five major components in the simulation model: a stochastic precipitation model, an evapotranspiration model, chemical thermodynamic relationships, soil parameterization, and a soil water and CaCO3 flux model.

For the present climate, a cold-dry Pleistocene climate, and a cool-wet (summer) Pleistocene climate, the model predicted a shallower depth for the calcic horizon than was found in field soils. However, the model was compatible with field soils if one assumed that most pedogenic carbonate formed during a cool-wet (winter) Pleistocene climate. The model was highly sensitive to the frequency of extreme precipitation events and to soil water-holding capacity. The biotic factor played an important role in CaCO3 deposition through its control of soil CO2 concentrations and evapotranspiration rates. The range in predicted CaCO3 deposition rates agreed with the rates for most field studies (1 to 5 g/m2 /yr); also, the model predicted an increasing rate of CaCO3 deposition with increasing precipitation, which agreed with field studies. The model is a valuable research tool for evaluating the role of state factors on soil CaCO3 deposition.

Markewitz, D. (2006). Fossil fuel carbon emissions from silviculture: Impacts on net carbon sequestration in forests. Forest Ecology and Management 236 (2-3): 153-161

ABSTRACT: Increasing concentrations of CO2 in the atmosphere have increased the value of sequestration and storage of C in forests. To maximize the value of this forest function, land managers require accounting systems to track the C stored in forests and in wood and fiber products. Accounting frameworks and data for quantifying C in forests and in wood and fiber products are generally available. In contrast, C emitted from fossil fuels utilized for silvicultural activities such as site preparation or fertilization, which are designed to increase C sequestration, have not been accounted for. The fossil fuel C emissions associated with silvicultural activities must be systematically evaluated to ensure that a net positive C balance results from activities ranging from planting to harvesting. The necessary data for evaluation are compiled from existing information. Utilizing the data, total C emissions from silvicultural activities for an intensive fiber farming operation of southern pine on a 25-year rotation is estimated to be <3 Mg C ha−1 . Increased C sequestration in soil or wood and fiber products in response to silvicultural treatments is simulated for 100 years to compare to the fossil fuel C emissions from silvicultural activities. The comparison demonstrates that the expected gains in C accumulation in soils of 16 Mg ha−1 over 100 years or gains due to increased harvest for paper products, also 16 Mg ha−1 , could each individually be largely balanced by silvicultural C emissions. On the other hand, C storage in wood products due to accelerated growth of trees to a saw log category might exceed the incurred C emissions by 3-fold (i.e., 35 Mg ha−1 ). If the combined C sequestration benefits from soil C accumulation, increased C storage in paper products, and storage in saw timber products could be captured these would outweigh the fossil fuel C emissions due to increased silvicultural activities.

Neilson, E.T., MacLean, D.A., Meng, F.-R., Arp, P.A. (2007). Spatial distribution of carbon in natural and managed stands in an industrial forest in New Brunswick, Canada. Forest Ecology and Management 253 (1-3): 148-160

ABSTRACT: Industrial forest could be managed to enhance carbon (C) sequestration together with other ecological and socio-economic objectives. However, this requires quantifying C dynamics of all major forest types within the management area, over the whole forest rotation. We used data from permanent sample plots and temporary forest development survey plots to generate volume yield curves and used the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to estimate C yield and dynamics over a rotation for major forest types in northern New Brunswick, Canada. We compared C yields of natural versus managed and hardwood versus softwood forest under different silviculture treatments over the entire rotation. Carbon in 40–80-year-old and > 80-year-old tolerant hardwood stands averaged about 115 and 130–142 t ha−1, respectively, while softwood spruce (Picea sp.)–balsam fir (Abies balsamea (L.) Mill.) 40–80 and > 80 years old averaged 90 and 88–94 t C ha−1 . Among 10 stand types, total C ranged from 50 to 109 t ha−1 at age 50 years, 92–138 t ha−1 at age 100, and 79–145 t ha−1 at age 150 years. C in most stand types declined from age 100 to 150 years, except for eastern white cedar (Thuja occidentalis L.), sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton). At age 100 years, planted softwood stands had 94–135 t ha−1 , versus 92–117 t ha−1 for natural softwoods and 127–138 t ha−1 for natural hardwoods. Planted white spruce (Picea glauca (Moench) Voss) and natural sugar maple and yellow birch sequestered the most C. The total C (above and belowground biomass and deadwood, excluding soil carbon) on the 428,000 ha test landbase was 35 million tonnes, or an average of 82 t ha−1 .

Niklaus, P.A., Falloon, P. (2006). Estimating soil carbon sequestration under elevated CO2 by combining carbon isotope labelling with soil carbon cycle modelling. Global Change Biology 12 (10): 1909-1921

ABSTRACT: Elevated CO2 concentrations generally stimulate grassland productivity, but herbaceous plants have only a limited capacity to sequester extra carbon (C) in biomass. However, increased primary productivity under elevated CO2 could result in increased transfer of C into soils where it could be stored for prolonged periods and exercise a negative feedback on the rise in atmospheric CO2 .

Measuring soil C sequestration directly is notoriously difficult for a number of methodological reasons. Here, we present a method that combines C isotope labelling with soil C cycle modelling to partition net soil sequestration into changes in new C fixed over the experimental duration (Cnew ) and pre-experimental C (Cold ). This partitioning is advantageous because the Cnew accumulates whereas Cold is lost in the course of time (ΔCnew >0 whereas ΔCold <0).We applied this method to calcareous grassland exposed to 600 μLCO2 L−1 for 6 years. The CO2 used for atmospheric enrichment was depleted in13 C relative to the background atmosphere, and this distinct isotopic signature was used to quantify net soil Cnew fluxes under elevated CO2 . Using13 C/12 C mass balance and inverse modelling, the Rothamsted model 'RothC' predicted gross soil Cnew inputs under elevated CO2 and the decomposition of Cold . The modelled soil C pools and fluxes were in good agreement with experimental data. C isotope data indicated a net sequestration of ≈90 g Cnew m−2 yr−1 in elevated CO2 . Accounting for Cold -losses, this figure was reduced to ≈30 g C m−2 yr−1 at elevated CO2 ; the elevated CO2 -effect on net C sequestration was in the range of ≈10 g C m−2 yr−1 . A sensitivity and error analysis suggests that the modelled data are relatively robust. However, elevated CO2 -specific mechanisms may necessitate a separate parameterization at ambient and elevated CO2 ; these include increased soil moisture due to reduced leaf conductance, soil disaggregation as a consequence of increased soil moisture, and priming effects. These effects could accelerate decomposition of Cold in elevated CO2 so that the CO2 enrichment effect may be zero or even negative. Overall, our findings suggest that the C sequestration potential of this grassland under elevated CO2 is rather limited.

Niyogi, D., Xue, Y. K. (2006). Soil moisture regulates the biological response of elevated atmospheric CO2 concentrations in a coupled atmosphere biosphere model. Global and Planetary Change 54 (1-2): 94-108

ABSTRACT: Terrestrial biosphere models/land surface models are routinely used to study the effects of CO2 doubling and climate change. The objective of this study is to show that the biological response associated with CO2 doubling is important, and that the effects intrinsically depend on the soil moisture state. Therefore, using a coupled biosphere–atmosphere model, we tested the hypothesis that the biological effects of CO2 changes in biosphere models are significantly coupled to the hydrological feedback via soil moisture availability in a terrestrial biosphere/land surface model. The results from a 15-day simulation of a photosynthesis-based land surface model, dynamically coupled to an atmospheric boundary layer and surface energy balance scheme, were analyzed to test the hypothesis. The objective was to analyze the biological effects of CO2 doubling under high as well as limiting soil moisture conditions for prescribed changes to the vegetation/land use type. The approach was to analyze the results from a coupled land surface-atmosphere model obtained by changing the biome type for each run. Sensitivity for all of the nine global vegetation type changes, as defined through the Simple Biosphere Model ver. 2 (SiB2) land cover classification, were analyzed for evapotranspiration and net carbon assimilation. The results indicated that: (i) the soil moisture (and its interaction with CO2 ) has a direct (first-order) effect on the biological effects of CO2 changes and the terrestrial ecosystem response; (ii) the biological impacts associated with CO2 changes in a biospheric model should be interpreted in consideration of the soil moisture status; and droughts or high soil moisture availability can enhance or completely balance or even reverse the effects associated with CO2 changes; (iii) for each vegetation type, the model results indicated a different response to soil moisture and CO2 changes; and resolving the direct and indirect effects explicitly, both C3 and C4 vegetation, appeared to be significantly affected by the biological effects of CO2 changes, and (iv) the explicit coupling between soil moisture/hydrological state and the CO2 changes need to be explicitly considered in projecting climate change impacts. The study results also indicated that feedback pathways can be efficiently determined by dissociating the direct and the interactive effects of CO2 impacts.

Ogle, S. M., Breidt, F. Jay, Easter, M., Williams, S., Paustian, K. (2007). An empirically based approach for estimating uncertainty associated with modelling carbon sequestration in soils. Ecological Modelling 205 (3-4): 453-463

ABSTRACT: Simulation modelling is used to estimate C sequestration associated with agricultural management for purposes of greenhouse gas mitigation. Models are not completely accurate or precise estimators of C pools, however, due to insufficient knowledge and imperfect conceptualizations about ecosystem processes, leading to uncertainty in the results. It can be difficult to quantify the uncertainty using traditional error propagation techniques, such as Monte Carlo Analyses, because of the structural complexity of simulation models. Empirically based methods provide an alternative to the error propagation techniques, and our objective was to apply this alternative approach. Specifically, we developed a linear mixed-effect model to quantify both bias and variance in modeled soil C stocks that were estimated using the Century ecosystem simulation model. The statistical analysis was based on measurements from 47 agricultural experiments.

A significant relationship was found between model results and measurements although there were biases and imprecision in the modeled estimates. Century under-estimated soil C stocks for several management practices, including organic amendments, no-till adoption, and inclusion of hay or pasture in rotation with annual crops. Century also over-estimated the impact of N fertilization on soil C stocks. For lands set-aside from agricultural production, Century under-estimated soil C stocks on low carbon soils and over-estimated the stocks on high carbon soils. Using an empirically based approach allows for simulation model results to be adjusted for biases as well as quantify the variance associated with modeled estimates, according to the measured “reality” of management impacts from a network of experimental sites.

Ogle, S. M., Breidt, F. J., Paustian, K. (2006). Bias and variance in model results associated with spatial scaling of measurements for parameterization in regional assessments. Global Change Biology 12 (3): 516-523

ABSTRACT: Models are central to global change analyses, but they are often parameterized using data that represent only a portion of heterogeneity in a region. This creates uncertainty in the results and constrains the reliability of model inferences. Our objective was to evaluate the uncertainty associated with differential scaling of parameterization data to model soil organic carbon stock changes as a function of US agricultural land use and management. Specifically, we compared analyses in which model parameters were derived from field experimental data that were scaled to the entire US vs. the same data scaled to climate regions within the country. We evaluated the effect of differential scaling on both bias and variance in model results.

Model results had less variance by scaling data to the entire country because of a larger sample size for deriving individual parameter values, although there was a relatively large bias associated with this parameterization, estimated at 2.7 Tg C yr−1 . Even with the large bias, resulting confidence intervals from the two parameterizations had considerable overlap for the estimated national rate of SOC change (i.e. 77% overlap in those intervals). Consequently, the results were relatively similar when focusing on the uncertainty rather than solely on the mean estimate. In contrast, large biases created less overlap in confidence intervals for the change rates within individual climate regions, compared with the national estimates. For example, the overlap in resulting intervals from the two parameterizations was only 32% for the warm temperate moist region, with a corresponding bias of 3.1 Tg C yr−1 .

These findings demonstrate that there is a greater risk of making erroneous inferences because of large biases if models are parameterized with broader scale information, such as an entire country, and then used to address impacts at a finer spatial scale, such as sub-regions within a country. In addition, the study demonstrates a trade-off between variance and bias in model results that depends on the scaling of data for model parameterization.

Patra, P. K., Ishizawa, M., Maksyutov, S., Nakazawa, T., Inoue, G. (2005). Role of biomass burning and climate anomalies for land-atmosphere carbon fluxes based on inverse modeling of atmospheric CO2 . Global Biogeochemical Cycles 19 (3): doi:10.1029/2004GB002258

ABSTRACT: A Time-dependent inverse (TDI) model is used to estimate carbon dioxide (CO2 ) fluxes for 64 regions of the globe from atmospheric measurements in the period January 1994 to December 2001. The global land anomalies agree fairly well with earlier results. Large variability in CO2 fluxes are recorded from the land regions, which are typically controlled by the available water for photosynthesis, and air temperature and soil moisture dependent heterotrophic respiration. For example, the anomalous CO2 emissions during the 1997/1998 El Niño period are estimated to be about 1.27 ± 0.22, 2.06 ± 0.37, and 1.17 ± 0.20 Pg-C yr−1 from tropical regions in Asia, South America, and Africa, respectively. The CO2 flux anomalies for boreal Asia region are estimated to be 0.83 ± 0.19 and 0.45 ± 0.14 Pg-C yr−1 of CO2 during 1996 and 1998, respectively. Comparison of inversion results with biogeochemical model simulations provide strong evidence that biomass burning (natural and anthropogenic) constitutes the major component in land-atmosphere carbon flux anomalies. The net biosphere-atmosphere carbon exchanges based on the biogeochemical model used in this study are generally lower than those estimated from TDI model results, by about 1.0 Pg-C yr−1 for the periods and regions of intense fire. The correlation and principal component analyses suggest that changes in meteorology (i.e., rainfall and air temperature) associated with the El Niño Southern Oscillation are the most dominant controlling factors of CO2 flux anomaly in the tropics, followed by the Indian Ocean Dipole Oscillation. Our results indicate that the Arctic and North Atlantic Oscillations are closely linked with CO2 flux variability in the temperate and high-latitude regions.

Peltoniemi, M., Thürig, E., Ogle, S., Palosuo, T., Schrumpf, M., Wutzler, T., Butterbach-Bahl, K., Chertov, O., Komarov, A., Mikhailov, A., Gärdenäs, A., Perry, C., Liski, J., Smith, P., Mäkipää, R. (2007). Models in country scale carbon accounting of forest soils. Silva Fennica 41 (3): 575-602

ABSTRACT: Countries need to assess changes in the carbon stocks of forest soils as a part of national greenhouse gas (GHG) inventories under the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol (KP). Since measuring these changes is expensive, it is likely that many countries will use alternative methods to prepare these estimates. We reviewed seven well-known soil carbon models from the point of view of preparing country-scale soil C change estimates. We first introduced the models and explained how they incorporated the most important input variables. Second, we evaluated their applicability at regional scale considering commonly available data sources. Third, we compiled references to data that exist for evaluation of model performance in forest soils. A range of process-based soil carbon models differing in input data requirements exist, allowing some flexibility to forest soil C accounting. Simple models may be the only reasonable option to estimate soil C changes if available resources are limited. More complex models may be used as integral parts of sophisticated inventories assimilating several data sources. Currently, measurement data for model evaluation are common for agricultural soils, but less data have been collected in forest soils. Definitions of model and measured soil pools often differ, ancillary model inputs require scaling of data, and soil C measurements are uncertain. These issues complicate the preparation of model estimates and their evaluation with empirical data, at large scale. Assessment of uncertainties that accounts for the effect of model choice is important part of inventories estimating large-scale soil C changes. Joint development of models and large-scale soil measurement campaigns could reduce the inconsistencies between models and empirical data, and eventually also the uncertainties of model predictions

Peñuelas, J., Prieto, P., Beier, C., Cesaraccio, C., De Angelis, P., De Dato, G., Emmett, B. A., Estiarte, M., Garadnai, J., Gorissen, A., Láng, E. K., Kröel-Dulay, G., Llorens, L., Pellizzaro, G., Riis-Nielsen, T., Schmidt, I. K., Sirca, C., Sowerby, A., Spano, D., Tietema, A. (2007). Response of plant species richness and primary productivity in shrublands along a north–south gradient in Europe to seven years of experimental warming and drought: reductions in primary productivity in the heat and drought year of 2003. Global Change Biology 13 (12): 2563-2581

ABSTRACT: We used a nonintrusive field experiment carried out at six sites – Wales (UK), Denmark (DK), the Netherlands (NL), Hungary (HU), Sardinia (Italy – IT), and Catalonia (Spain – SP) – along a climatic and latitudinal gradient to examine the response of plant species richness and primary productivity to warming and drought in shrubland ecosystems. The warming treatment raised the plot daily temperature by ca. 1 °C, while the drought treatment led to a reduction in soil moisture at the peak of the growing season that ranged from 26% at the SP site to 82% in the NL site. During the 7 years the experiment lasted (1999–2005), we used the pin-point method to measure the species composition of plant communities and plant biomass, litterfall, and shoot growth of the dominant plant species at each site. A significantly lower increase in the number of species pin-pointed per transect was found in the drought plots at the SP site, where the plant community was still in a process of recovering from a forest fire in 1994. No changes in species richness were found at the other sites, which were at a more mature and stable state of succession and, thus less liable to recruitment of new species. The relationship between annual biomass accumulation and temperature of the growing season was positive at the coldest site and negative at the warmest site. The warming treatment tended to increase the aboveground net primary productivity (ANPP) at the northern sites. The relationship between annual biomass accumulation and soil moisture during the growing season was not significant at the wettest sites, but was positive at the driest sites. The drought treatment tended to reduce the ANPP in the NL, HU, IT, and SP sites. The responses to warming were very strongly related to the Gaussen aridity index (stronger responses the lower the aridity), whereas the responses to drought were not. Changes in the annual aboveground biomass accumulation, litterfall, and, thus, the ANPP, mirrored the interannual variation in climate conditions: the most outstanding change was a decrease in biomass accumulation and an increase in litterfall at most sites during the abnormally hot year of 2003. Species richness also tended to decrease in 2003 at all sites except the cold and wet UK site. Species-specific responses to warming were found in shoot growth: at the SP site,Globularia alypum was not affected, while the other dominant species,Erica multiflora , grew 30% more; at the UK site,Calluna vulgaris tended to grow more in the warming plots, whileEmpetrum nigrum tended to grow less. Drought treatment decreased plant growth in several studied species, although there were some species such asPinus halepensis at the SP site orC. vulgaris at the UK site that were not affected. The magnitude of responses to warming and drought thus depended greatly on the differences between sites, years, and species and these multiple plant responses may be expected to have consequences at ecosystem and community level. Decreases in biodiversity and the increase inE. multiflora growth at the SP site as a response to warming challenge the assumption that sensitivity to warming may be less well developed at more southerly latitudes; likewise, the fact that one of the studied shrublands presented negative ANPP as a response to the 2003 heat wave also challenges the hypothesis that future climate warming will lead to an enhancement of plant growth and carbon sequestration in temperate ecosystems. Extreme events may thus change the general trend of increased productivity in response to warming in the colder sites.

Petrescu, A. M. R., Van Huissteden, J., Jackowicz-Korczynski, M., Yurova, A., Christensen, T. R., Crill, P. M., Bäckstrand, K., Maximov, T. C. (2008). Modelling CH4 emissions from Arctic wetlands: effects of hydrological parameterization. Biogeosciences 5 (1): 111-121

ABSTRACT: This study compares the CH4 fluxes from two arctic wetland sites of different annual temperatures during 2004 to 2006. The PEATLAND-VU model was used to simulate the emissions. The CH4 module of PEATLAND-VU is based on the Walter-Heimann model. The first site is located in northeast Siberia, Indigirka lowlands, Kytalyk reserve (70° N, 147° E) in a continuous permafrost region with mean annual temperatures of −14.3°C. The other site is Stordalen mire in the eastern part of Lake Torneträsk (68° N, 19° E) ten kilometres east of Abisko, northern Sweden. It is located in a discontinuous permafrost region. Stordalen has a sub arctic climate with a mean annual temperature of −0.7°C. Model input consisted of observed temperature, precipitation and snow cover data.

In all cases, modelled CH4 emissions show a direct correlation between variations in water table and soil temperature variations. The differences in CH4 emissions between the two sites are caused by different climate, hydrology, soil physical properties, vegetation type and NPP.

For Kytalyk the simulated CH4 fluxes show similar trends during the growing season, having average values for 2004 to 2006 between 1.29–2.09 mg CH4 m−2 hr−1 . At Stordalen the simulated fluxes show a slightly lower average value for the same years (3.52 mg CH4 m−2 hr−1 ) than the observed 4.7 mg CH4 m−2 hr−1 . The effect of the longer growing season at Stordalen is simulated correctly.

Our study shows that modelling of arctic CH4 fluxes is improved by adding a relatively simple hydrological model that simulates the water table position from generic weather data. Our results support the generalization in literature that CH4 fluxes in northern wetland are regulated more tightly by water table than temperature. Furthermore, parameter uncertainty at site level in wetland CH4 process models is an important factor in large scale modelling of CH4 fluxes.

Piao, S. L., Friedlingstein, P., Ciais, P., Zhou, L. M., Chen, A. P. (2006). Effect of climate and CO2 changes on the greening of the Northern Hemisphere over the past two decades. Geophysical Research Letters 33 (L23402): doi:10.1029/2006GL028205

ABSTRACT: Study of the effect of current climate changes on vegetation growth, and their spatial patterns improves our understanding of the interactions between terrestrial ecosystems and climatic systems. This paper explores the spatial patterns of vegetation growth responding to climate variability over Northern Hemisphere (>25°N) from 1980 to 2000 using a mechanistic terrestrial carbon model. The results indicate that changes in climate and atmospheric CO2 likely function as dominant controllers for the greening trend during the study period. At the continental scale, atmospheric CO2 , temperature, and precipitation account for 49%, 31%, and 13% of the increase in growing season LAI, respectively, but their relative role is not constant across the study area. The increase in vegetation activity in most of Siberia is associated with warming, while that in central North America is primarily explained by the precipitation change. The model simulation also suggests that the regression slope of LAI to temperature increases with soil moisture, but decreases with temperature. This implies that the contribution of rising temperature to the current enhanced greening trend will weaken or even disappear under continued global warming. We also find that the effects of both vegetation precipitation use efficiency and atmospheric CO2 fertilization on the greening trend increase as soil moisture becomes limiting.

Post, J., Hattermann, F. F., Krysanova, V., Suckow, F. (2008). Parameter and input data uncertainty estimation for the assessment of long-term soil organic carbon dynamics. Environmental Modelling & Software 23 (2): 125-138

ABSTRACT: The use of integrated soil organic matter (SOM) models to assess SOM dynamics under climate change, land use change and different land management practices require a quantification of uncertainties and key sensitive factors related to the respective modelling framework. Most uncertainty studies hereby focus on model parameter uncertainty, neglecting other sources like input data derived uncertainties, and spatial and temporal properties of uncertainty. Sources of uncertainties assessed in this study stem from uncertainties in model parameterisation and from uncertainties in model input data (climate, soil data, and land management assumptions). Thereby, Monte Carlo based global sensitivity and uncertainty analysis using a latin hypercube stratified sampling technique was applied to derive plot scale (focusing on temporal propagation) and river basin scale propagation of uncertainty for long-term soil organic carbon (SOC) dynamics. The model used is the eco-hydrological river basin model SWIM (Soil and Water Integrated Model), which has been extended by a process-based multi-compartment model for SOM turnover. Results obtained by this study can be transferred and used in other simulation models of this kind. Uncertainties resulting from all input factors used (model parameters + model input data) show a coefficient of variation between 5.1 and 6.7% and accounted for ± 0.065 to ± 0.3% soil carbon content (0.06–0.15 t C ha−1 yr−1 ). Parameter derived uncertainty contributed most to overall uncertainty. Concerning input data contributions, uncertainties stemming from soil and climate input data variations are striking. At the river basin scale, cropland and forest ecosystems, loess and gleyic soils possess the highest degree of uncertainty. Quantified magnitudes of uncertainty stemming from the examined sources vary temporally and spatially due to specific natural settings (e.g. climate, land use and soil properties) and deliver useful information for interpreting simulation results on long-term soil organic carbon dynamics under environmental change. Derived from this analysis, key sensitive model parameters and interactions between them were identified: the mineralization rate coefficient, the carbon use efficiency parameter (synthesis coefficient) along with parameters determining the soil temperature influence on SOM turnover (mainly Q10 value) and the soil input related data (soil bulk density and initial soil C content) introduced the highest degree of model uncertainty. The here gained information can be transferred to other process-based SOM turnover models to consider stronger most crucial parameters introducing highest uncertainty contribution to soil C storage assessment under changing environmental conditions.

Potter, C.S., Klooster, S.A. (1997). Global model estimates of carbon and nitrogen storage in litter and soil pools: response to changes in vegetation quality and biomass allocation. Tellus: Series B 49 (1): 1-17

ABSTRACT: Changes in plant production, structure, and tissue composition are primary drivers for terrestrial biogeochemistry under future environmental conditions. Consequently, there is a need for process-oriented assessment of the potential global importance of vegetation controls over extended periods of C and N sequestration in terrestrial ecosystems. In this study, plant litter quality (lignin content) and carbon allocation to woody tissues are used as surrogates for testing the hypothetical effects of vegetation change on C and N cycles. We tested the CASA (Carnegie-Ames-Stanford approach) biosphere model, which uses global gridded (1°) satellite imagery on a monthly time interval to simulate seasonal patterns in net ecosystem carbon balance and near steady-state C/N storage in detritus and soils. Under contemporary "reference" settings, combined organic matter storage (litter plus surface soil to c. 30 cm depth) for C and N is estimated highest in tropical and boreal forest ecosystem zones, and in cultivated ecosystems. The worldwide C:N ratio (by weight) for standing litter plus surface soil organic matter (SOM) is estimated at 23. About 14% of the projected global pool of 1327 Pg (1015 g) soil C resides in "modern" form, in the sense that this proportion is in near-steady state exchange with plant production and decomposition on time scales of several decades. Likewise, about 12% of the projected global pool of 104 Pg soil N is in modern form. Sensitivity tests treated litter quality and allocation effects independently from other direct effects of changes in climate, atmospheric CO2 levels, and primary production. For forested ecosystems, the model predicts that a hypothetical 50% decrease in litter lignin concentration would result in a long-term net loss of about 10% C from surface litter and soil organic matter pools. A 50% decrease in C allocation to woody tissues would invoke approximately the same net loss of C as a 50% decrease in litter lignin. With respect to nitrogen, the 50% downward adjustment in litter allocation to woody tissues may increase both the estimated net N mineralization rates and SLOW N pool by approximately 9% on a global basis. This pattern is consistent with an overall increase in N available for cycling, which is affected by the fraction of relatively N-poor to N-rich litter inputs. For comparison to the effects of these surrogate changes in vegetation tissue composition, model response to a globally uniform increase in surface air temperature of 1 °C is a net loss of 5% C from litter and SOM pools.

Potter, C. S., Randerson, J. T., Field, C. B., Matson, P. A., Vitousek, P. M., Mooney, H. A., Klooster, S. A. (1993). Terrestrial ecosystem production: a process model based on global satellite and surface data. Global Biogeochemical Cycles 7 (4): 811-841

ABSTRACT: This paper presents a modeling approach aimed at seasonal resolution of global climatic and edaphic controls on patterns of terrestrial ecosystem production and soil microbial respiration. We use satellite imagery (Advanced Very High Resolution Radiometer and International Satellite Cloud Climatology Project solar radiation), along with historical climate (monthly temperature and precipitation) and soil attributes (texture, C and N contents) from global (1°) data sets as model inputs. The Carnegie-Ames-Stanford approach (CASA) Biosphere model runs on a monthly time interval to simulate seasonal patterns in net plant carbon fixation, biomass and nutrient allocation, litterfall, soil nitrogen mineralization, and microbial CO2 production. The model estimate of global terrestrial net primary production is 48 Pg C yr−1 with a maximum light use efficiency of 0.39 g C MJ−1PAR. Over 70% of terrestrial net production takes place between 30° N and 30° S latitude. Steady state pools of standing litter represent global storage of around 174 Pg C (94 and 80 Pg C in nonwoody and woody pools, respectively), whereas the pool of soil C in the top 0.3 m that is turning over on decadal time scales comprises 300 Pg C. Seasonal variations in atmospheric CO2 concentrations from three stations in the Geophysical Monitoring for Climate Change Flask Sampling Network correlate significantly with estimated net ecosystem production values averaged over 50°-80° N, 10°-30° N, and 0°- 10° N.

Rayner, P. J., Scholze, M., Knorr, W., Kaminski, T., Giering, R., Widmann, H. (2005). Two decades of terrestrial carbon fluxes from a carbon cycle data assimilation system (CCDAS). Global Biogeochemical Cycles 19 (2)

ABSTRACT: This paper presents the space-time distribution of terrestrial carbon fluxes for the period 1979–1999 generated by a terrestrial carbon cycle data assimilation system (CCDAS). CCDAS is based around the Biosphere Energy Transfer Hydrology model. We assimilate satellite observations of photosynthetically active radiation and atmospheric CO2 concentration observations in a two-step process. The control variables for the assimilation are the parameters of the carbon cycle model. The optimized model produces a moderate fit to the seasonal cycle of atmospheric CO2 concentration and a good fit to its interannual variability. Long-term mean fluxes show large uptakes over the northern midlatitudes and uptakes over tropical continents partly offsetting the prescribed efflux due to land use change. Interannual variability is dominated by the tropics. On interannual timescales the controlling process is net primary productivity (NPP) while for decadal changes the main driver is changes in soil respiration. An adjoint sensitivity analysis reveals that uncertainty in long-term storage efficiency of soil carbon is the largest contributor to uncertainty in net flux.

Reich, P. B., Hobbie, S. E., Lee, T., Ellsworth, D. S., West, J. B., Tilman, D., Knops, J. M. H., Naeem, S., Trost, J. (2006). Nitrogen limitation constrains sustainability of ecosystem response to CO2 . Nature 440 (7086): 922-925

ABSTRACT: Enhanced plant biomass accumulation in response to elevated atmospheric CO2 concentration could dampen the future rate of increase in CO2 levels and associated climate warming. However, it is unknown whether CO2 -induced stimulation of plant growth and biomass accumulation will be sustained or whether limited nitrogen (N) availability constrains greater plant growth in a CO2-enriched world1, 2, 3, 4, 5, 6, 7, 8, 9 . Here we show, after a six-year field study of perennial grassland species grown under ambient and elevated levels of CO2 and N, that low availability of N progressively suppresses the positive response of plant biomass to elevated CO2 . Initially, the stimulation of total plant biomass by elevated CO2 was no greater at enriched than at ambient N supply. After four to six years, however, elevated CO2 stimulated plant biomass much less under ambient than enriched N supply. This response was consistent with the temporally divergent effects of elevated CO2 on soil and plant N dynamics at differing levels of N supply. Our results indicate that variability in availability of soil N and deposition of atmospheric N are both likely to influence the response of plant biomass accumulation to elevated atmospheric CO2 . Given that limitations to productivity resulting from the insufficient availability of N are widespread in both unmanaged and managed vegetation5, 7, 8, 9 , soil N supply is probably an important constraint on global terrestrial responses to elevated CO2 .

Robinson, S. D. (2006). Carbon accumulation in peatlands, southwestern Northwest Territories, Canada. Canadian Journal of Soil Science 86 (2): 305-319

ABSTRACT: Northern peatlands have stored significant quantities of carbon (C) since the early Holocene at rates that vary among peatland types. Pollen concentration dating was used to provide estimates of true C accumulation and sequestration efficiency in different peatland systems in the discontinuous permafrost zone near Fort Simpson, Northwest Territories, Canada. The catotelm portions of bog, permafrost-affected peat plateau, andSphagnum -dominated cores were interpreted to conform to Clymo’s (1984) model of C accumulation, while peat deposited under conditions with high water tables (rich fen and collapse fen) did not. The model assumes a consistent surface production, yet production in fens is thought to be highly sensitive to water table changes and may have contributed to poor model fits. Decay rates measured over the past 1200 yr range from 0.0015 to 0.0004 yr-1 . True C accumulation rates (range 7.0 in peat plateau to 18.6 g C m-2 yr-1 in bog) and sequestration efficiencies (range 0.24 in peat plateau to 0.67 in poor fen) by 1200 yr BP were low in comparison with other North American sites. Decay rates measured over 1200 yr were significantly greater than that measured over the entire life span of the peatland (0.00033 yr-1), suggesting that a catotelm true C accumulation model incorporating a decreasing rate of decay would be more applicable.

Sitch, S., Smith, B., Prentice, I. C., Arneth, A., Bondeau, A., Cramer, W., Kaplan, J. O., Levis, S., Lucht, W., Sykes, M. T., Thonicke, K., Venevsky, S. (2003). Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic global vegetation model. Global Change Biology 9 (2): 161-185

ABSTRACT: The Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ) combines process-based, large-scale representations of terrestrial vegetation dynamics and land-atmosphere carbon and water exchanges in a modular framework. Features include feedback through canopy conductance between photosynthesis and transpiration and interactive coupling between these 'fast' processes and other ecosystem processes including resource competition, tissue turnover, population dynamics, soil organic matter and litter dynamics and fire disturbance. Ten plants functional types (PFTs) are differentiated by physiological, morphological, phenological, bioclimatic and fire-response attributes. Resource competition and differential responses to fire between PFTs influence their relative fractional cover from year to year. Photosynthesis, evapotranspiration and soil water dynamics are modelled on a daily time step, while vegetation structure and PFT population densities are updated annually.

Simulations have been made over the industrial period both for specific sites where field measurements were available for model evaluation, and globally on a 0.5°° × 0.5°° grid. Modelled vegetation patterns are consistent with observations, including remotely sensed vegetation structure and phenology. Seasonal cycles of net ecosystem exchange and soil moisture compare well with local measurements. Global carbon exchange fields used as input to an atmospheric tracer transport model (TM2) provided a good fit to observed seasonal cycles of CO2 concentration at all latitudes. Simulated inter-annual variability of the global terrestrial carbon balance is in phase with and comparable in amplitude to observed variability in the growth rate of atmospheric CO2. Global terrestrial carbon and water cycle parameters (pool sizes and fluxes) lie within their accepted ranges. The model is being used to study past, present and future terrestrial ecosystem dynamics, biochemical and biophysical interactions between ecosystems and the atmosphere, and as a component of coupled Earth system models.

Tan, Z. X., Liu, S. G., Johnston, C. A., Loveland, T. R., Tieszen, L. L., Liu, J. X., Kurtz, R. (2005). Soil organic carbon dynamics as related to land use history in the northwestern Great Plains. Global Biogeochemical Cycles 19 (GB3011): doi:10.1029/2005GB002536

ABSTRACT: Strategies for mitigating the global greenhouse effect must account for soil organic carbon (SOC) dynamics at both spatial and temporal scales, which is usually challenging owing to limitations in data and approach. This study was conducted to characterize the SOC dynamics associated with land use change history in the northwestern Great Plains ecoregion. A sampling framework (40 sample blocks of 10 × 10 km2 randomly located in the ecoregion) and the General Ensemble Biogeochemical Modeling System (GEMS) were used to quantify the spatial and temporal variability in the SOC stock from 1972 to 2001. Results indicate that C source and sink areas coexisted within the ecoregion, and the SOC stock in the upper 20-cm depth increased by 3.93 Mg ha−1 over the 29 years. About 17.5% of the area was evaluated as a C source at 122 kg C ha−1 yr−1 . The spatial variability of SOC stock was attributed to the dynamics of both slow and passive fractions, while the temporal variation depended on the slow fraction only. The SOC change at the block scale was positively related to either grassland proportion or negatively related to cropland proportion. We concluded that the slow C pool determined whether soils behaved as sources or sinks of atmospheric CO2 , but the strength depended on antecedent SOC contents, land cover type, and land use change history in the ecoregion.

Tate, K. R., Scott, N. A., Parshotam, A., Brown, L., Wilde, R. H., Giltrap, D. J., Trustrum, N. A., Gomez, B., Ross, D. J. (2000). A multi-scale analysis of a terrestrial carbon budget: Is New Zealand a source or sink of carbon?. Agriculture, Ecosystems & Environment 82 (1-3): 229-246

ABSTRACT: Interest in national carbon (C) budgets has increased following the signing of the Kyoto Protocol as countries begin to develop source/sink C inventories. In this study, specific-site measurements, regional databases, satellite observations, and models were used to test the hypothesis that New Zealand’s terrestrial ecosystems are C neutral because C uptake by planted forests and scrub is roughly balanced by C losses from indigenous forests and soils.

Net ecosystem C balance was estimated from the difference between net primary production (NPP) and heterotrophic soil respiration. The productivity portion of the CASA model and NOAA–AVHRR imagery were used to estimate national NPP (128±14 Mt C per year). Main sources of uncertainty were the coarse spatial scale (1×1 km2 grid cells), and the general lack of information on photosynthetically active radiation, light-use efficiency, and below-ground C allocation for the major vegetation types: indigenous and exotic forests, scrub, and grasslands (improved, unimproved and tussock). Total soil CO2 -C production predicted from an Arrhenius-type function coupled to climate and land-cover data was 380±30 MtC per year, suggesting that New Zealand’s terrestrial ecosystems may be either (a) a net source of atmospheric CO2 or (b) roughly in C balance if ca. 252 Mt CO2 -C per year (66%) can be attributed to roots. Soil moisture limitations on respiration were small, reducing the national value to 365±28 MtC per year. Differences between NPP and heterotrophic soil respiration were −29 Mt C per year for improved pastures, −8 Mt C per year for indigenous forests, and +4 Mt C per year for planted forests; the large negative value for improved grasslands may be due to under-estimation of NPP and root respiration. Soil C losses to coastal waters, as estimated from a consideration of all the major erosion processes, were ca. 3–11 Mt C per year.

These national-scale estimates of ecosystem C balance were in general agreement with those based on plot-scale data for some major ecosystems including planted forests (4 Mt C per year vs 3.7 Mt C per year, respectively) and indigenous forest (−8 Mt C per year vs ca. −2.8 Mt C per year, respectively). Poor agreement for forest regenerating after land abandonment (−17 Mt C per year vs +3 Mt C per year) was probably due to an underestimate of NPP at the national scale.

Overall, the results suggest that New Zealand is a net C source, despite the fact that some ecosystems are accumulating C. For some land-use types, using the balance between NPP and soil respiration at the national scale to estimate the net ecosystem C balance may be too coarse, and studies of land-use changes at finer spatial scales are needed to reduce uncertainties in national-scale C balance estimates.

Tate, K. R., Wilde, R. H., Giltrap, D. J., Baisden, W. T., Saggar, S., Trustrum, N. A., Scott, N. A., Barton, J. R. (2005). Soil organic carbon stocks and flows in New Zealand: System development, measurement and modelling. Canadian Journal of Soil Science 85 (4): 481-489

ABSTRACT: An IPCC-based Carbon Monitoring System (CMS) was developed to monitor soil organic C stocks and flows to assist New Zealand to achieve its CO2 emissions reduction target under the Kyoto Protocol. Geo-referenced soil C data from 1158 sites (0.3 m depth) were used to assign steady-state soil C stocks to various combinations of soil class, climate, and land use. Overall, CMS soil C stock estimates are consistent with detailed, stratified soil C measurements at specific sites and over larger regions. Soil C changes accompanying land-use changes were quantified using a national set of land-use effects (LUEs). These were derived using a General Linear Model to include the effects of numeric predictors (e.g., slope angle). Major uncertainties a rise from estimates of changes in the areas involved, the assumption that soil C is at steady state for all land-cover types, and lack of soil C data for some LUEs. Total national soil organic C stocks estimated using the LUEs for 0–0.1, 0.1–0.3, and 0.3–1 m depths were 1300 ± 20, 1590 ± 30, and 1750 ± 70 Tg, respectively. Most soil C is stored in grazing lands (1480 ± 60 Tg to 0.3 m depth), which appear to be at or near steady state; their conversion to exotic forests and shrubland contributed most to the predicted national soil C loss of 0.6 ± 0.2 Tg C yr-1 during 1990–2000. Predicted and measured soil C changes for the grazing-forestry conversion agreed closely. Other uncertainties in our current soil CMS include: spatially integrated annual changes in soil C for the major land-use changes, lack of soil C change estimates below 0.3 m, C losses from erosion, the contribution of agricultural management of organic soils, and a possible interaction between land use and our soil-climate classification. Our approach could be adapted for use by other countries with land-use-change issues that differ from those in the IPCC default methodology. Key words: Soil organic carbon, land-use change, stocks, flows, measurement, modelling, IPCC

Tipping, E., Smith, E., Bryant, C., Adamson, J. (2007). The organic carbon dynamics of a moorland catchment in N. W. England. Biogeochemistry

ABSTRACT: The carbon cycle was quantified in the catchment of Doe House Gill, which drains high-relief moorland, with thin organic-rich soils (leptosols and podzols) 10–25 cm deep, in northern England. The soil C pool of 8,300 g m-2 is due mainly to humic acid and older humin. If steady state is assumed, and a single soil C pool, the average14 C content of the whole soil (93% modern) yields a mean carbon residence time of 800 years, although this varied from 300 to 1,600 years in the four samples studied. Stream water fluxes of dissolved and particulate organic carbon (DOC, POC) were 2.5 and 0.4 g m−2 a−1 respectively in 2002–2003, lower than values for some other upland streams in the UK. The C pool, flux, and isotope data were used, with the assumption of steady state, to calibrate DyDOC, a model that simulates the soil carbon cycle, including the generation and transport of DOC. According to DyDOC, the litter pool (ca. 100 gC m−2 ) turns over quickly, and most (>90%) of the litter carbon is rapidly mineralised. The soil is calculated to gain only 16 gC m−2 a−1 , and to lose the same amount, about 80% as CO2 and 20% as DOC. From the DO14 C content of 107.5% modern (due to “bomb carbon”) the model could be calibrated by assuming all DOC to come directly from litter, but DOC is more likely a mixture, derived from more than one soil C pool. The seasonal variability exhibited by stream water DOC concentration (maximum in September, minimum in January) is attributed mainly to variations in rainfall and evapotranspiration, rather than in the metabolic production rate of “potential DOC”. The model predicts that, for a Q10 of 2, the total soil organic C pool would decrease by about 5% if subjected to warming over 200 years. DyDOC predicts higher DOC fluxes in response to increased litter inputs or warming, and can simulate changes in DOC flux due to variations in sorption to soil solids, that might occur due to acidification and its reversal.

Trumbore, S.E., Bubier, J.L., Harden, J.W., Crill, P.M. (1999). Carbon cycling in boreal wetlands: a comparison of three approaches. Journal of Geophysical Research Volume 104 (D22): 27,673-27,682

ABSTRACT: Three independent methods were used to measure net ecosystem production (NEP) in four wetlands near Thompson, Manitoba, Canada. The first method calculated NEP by subtracting heterotrophic respiration from net primary productivity, using both measurements and estimates derived from the literature. The second method used radiocarbon data from cores to derive long-term NEP averaged over the past several decades. The third method used direct measurement of NEP combined with a model to fill in for days with no data. The three methods, with their independently derived uncertainties, all show the same magnitude and pattern of NEP variation across four different wetland types. However, direct measurement yielded distinctly lower estimates of NEP in the most productive sites. Highest NEP (31 – 180 gC m−2 yr−1 ) was observed in the two wetlands with the highest proportion of sedge vegetation. A bog collapse scar and a nutrient-rich fen had NEP values not significantly different from zero. The maximum NEP at sites with intermediate nutrient status is due to slower overall decomposition and is likely associated with greater allocation of production below ground by sedges. The three methods for estimating NEP differ in the effort required, the sources of error, and in the timescale over which they apply. Used in combination, they allow estimation of parameters such as below- ground production and the contribution of heterotrophic decomposition to total soil respiration. Using the radiocarbon method, we also derived estimates of the rate of N accumulation in the four wetland types.

Van Huissteden, J., Van Den Bos, R., Alvarez, I. M. (2006). Modelling the effect of water-table management on CO2 and CH4 fluxes from peat soils. Netherlands Journal of Geosciences 85 (1): 3-18

ABSTRACT: Drainage of peatlands for agriculture causes an increase of CO2 flux from peat decomposition, contributing to national CO2 emission. The reverse process, i.e. for re-creation of wetlands, reduces the CO2 flux, but increases the CH4 flux. We developed a process model (PEATLAND) to simulate these fluxes from peat soils subject to different water-table management scenarios. The model combines primary production, aerobic decomposition of soil organic matter (including the soil-parent material, peat), CH4 formation, oxidation, and transport. Model input requires specification of water table and air temperature data sets, vegetation parameters such as primary production and parameters related to gas transport, and basic soil physical data. Validation using closed flux-chamber measurements of CO2 and CH4 from five different sites in the western Netherlands shows that seasonal changes in fluxes of CO2 and CH4 are correctly modelled. However, the CO2 submodel underestimates peat decomposition when peat decomposition rates obtained from laboratory incubation experiments are used as input. Field decomposition rates are considerably higher. This is attributed to enhancement of decomposition by the addition of easily decomposable material from root exudation (‘priming effect’). Model experiments indicate that 1) drainage increases the CO2 production from peat decomposition strongly; 2) restoring a high water table may decrease the total greenhouse gas flux by a small amount although the CH4 flux increases strongly; 3) a warmer climate may cause higher greenhouse gas fluxes from peat soils resulting in a positive feedback to climate warming, and 4) high vegetation productivity in fen meadows may stimulate peat decomposition by the priming effect.

Vetter, M., Wirth, C., Bottcher, H., Churkina, G., Schulze, E. D., Wutzler, T., Weber, G. (2005). Partitioning direct and indirect human-induced effects on carbon sequestration of managed coniferous forests using model simulations and forest inventories. Global Change Biology 11 (5): 810-827

ABSTRACT: Temperate forest ecosystems have recently been identified as an important net sink in the global carbon budget. The factors responsible for the strength of the sinks and their permanence, however, are less evident. In this paper, we quantify the present carbon sequestration in Thuringian managed coniferous forests. We quantify the effects of indirect human-induced environmental changes (increasing temperature, increasing atmospheric CO2 concentration and nitrogen fertilization), during the last century using BIOME-BGC, as well as the legacy effect of the current age-class distribution (forest inventories and BIOME-BGC). We focused on coniferous forests because these forests represent a large area of central European forests and detailed forest inventories were available.

The model indicates that environmental changes induced an increase in biomass C accumulation for all age classes during the last 20 years (1982–2001). Young and old stands had the highest changes in the biomass C accumulation during this period. During the last century mature stands (older than 80 years) turned from being almost carbon neutral to carbon sinks. In high elevations nitrogen deposition explained most of the increase of net ecosystem production (NEP) of forests. CO2 fertilization was the main factor increasing NEP of forests in the middle and low elevations.

According to the model, at present, total biomass C accumulation in coniferous forests of Thuringia was estimated at 1.51 t C ha−1 yr−1 with an averaged annual NEP of 1.42 t C ha−1 yr−1 and total net biome production of 1.03 t C ha−1 yr−1 (accounting for harvest). The annual averaged biomass carbon balance (BCB: biomass accumulation rate-harvest) was 1.12 t C ha−1 yr−1 (not including soil respiration), and was close to BCB from forest inventories (1.15 t C ha−1 yr−1 ). Indirect human impact resulted in 33% increase in modeled biomass carbon accumulation in coniferous forests in Thuringia during the last century. From the forest inventory data we estimated the legacy effect of the age-class distribution to account for 17% of the inventory-based sink. Isolating the environmental change effects showed that these effects can be large in a long-term, managed conifer forest.

Vuichard, N., Soussana, J.-F., Ciais, P., Viovy, N., Ammann, C., Calanca, P., Clifton-Brown, J., Fuhrer, J., Jones, M., Martin, C. (2007). Estimating the greenhouse gas fluxes of European grasslands with a process-based model: 1. Model evaluation from in situ measurements. Global Biogeochemical Cycles 21 (GB1004): doi:10.1029/2005GB002611

ABSTRACT: We improved a process-oriented biogeochemical model of carbon and nitrogen cycling in grasslands and tested it against in situ measurements of biomass and CO2 and CH4 fluxes at five European grassland sites. The new version of the model (PASIM) calculates the growth and senescence of aboveground vegetation biomass accounting for sporadic removals when the grassland is cut and for continuous removals when it is grazed. Limitations induced by high leaf area index (LAI), soil water deficits and aging of leaves are also included. We added to this a simple empirical formulation to account for the detrimental impact on vegetation of trampling and excreta by grazing animals. Finally, a more realistic methane emission module than is currently used was introduced on the basis of the quality of the animals' diet. Evaluation of this improved version of PASIM is performed at (1) Laqueuille, France, on grassland continuously grazed by cattle with two plots of intensive and extensive grazing intensities, (2) Oensingen, Switzerland, on cut grassland with two fertilized and nonfertilized plots, and (3) Carlow, Ireland, on grassland that is both cut and grazed by cattle during the growing season. In addition, we compared the modeled animal CH4 emissions with in situ measurements on cattle for two grazing intensities at the grazed grassland site of Laqueuille. Altogether, when all improvements to the PASIM model are included, we found that the new parameterizations resulted into a better fit to the observed seasonal cycle of biomass and of measured CO2 and CH4 fluxes. However, the large uncertainties in measurements of biomass and LAI make simulation of biomass dynamics difficult to make. Also simulations for cut grassland are better than for grazed swards. This work paves the way for simulating greenhouse gas fluxes over grasslands in a spatially explicit manner, in order to quantify and understand the past, present and future role of grasslands in the greenhouse gas budget of the European continent.

Harmon, M.E., Harmon, J.M., Ferrell, W.K., Brooks, D. (1996). Modeling carbon stores in Oregon and Washington forest products: 1900-1992. Climatic Change 33 (4): 521-550

ABSTRACT: A new model, FORPROD, for estimating the carbon stored in forest products, considers both the manufacture of the raw logs into products and the fate of the products during use and disposal. Data for historical patterns of harvest, manufacturing efficiencies, and product use and disposal were used for estimating the accumulation of carbon in Oregon and Washington forest products from 1900 to 1992. Pools examined were long- and short-term structures, paper supplies, mulch, open dumps, and landfills. The analysis indicated that of the 1,692 Tg of carbon harvested during the selected period, only 396 Tg, or 23%, is currently stored. Long-term structures and landfills contain the largest fraction of that store, holding 74% and 20%, respectively. Landfills currently have the highest rates of accumulation, but total landfill stores are relatively low because they have been used only in the last 40 years. Most carbon release has occurred during manufacturing, 45% to 60% lost to the atmosphere, depending upon the year. Sensitivity analyses of the effects of recycling, landfill decomposition, and replacement rates of long-term structures indicate that changing these parameters by a factor of two changes the estimated fraction of total carbon stored less than 2%.

Smithwick, E. A. H., M.E. Harmon, J.B. Domingo (2007). Changing temporal patterns of forest carbon stores and net ecosystem carbon balance: the stand to landscape transformation. Landscape Ecology 22 (1): 77-94

ABSTRACT: Short- and long-term patterns of net ecosystem carbon balance (NECB) for small, relatively uniform forest stands have been examined in detail, but the same is not true for landscapes, especially those with heterogeneous disturbance histories. In this paper, we explore the effect of two contrasting types of disturbances (i.e., fire and tree harvest) on landscape level NECB by using an ecosystem process model that explicitly accounts for changes in carbon (C) stores as a function of disturbance regimes. The latter were defined by the average disturbance interval, the regularity of the disturbance interval (i.e., random, based on a Poisson frequency distribution, or regular), the amount of C removed by the disturbance (i.e., severity), and the relative abundance of stands in the landscape with unique disturbance histories. We used the model to create over 300 hypothetical landscapes, each with a different disturbance regime, by simulating up to 200 unique stand histories and averaging their total C stores. Mean NECB and its year-to-year variability was computed by calculating the difference in mean total C stores from one year to the next. Results indicated that landscape C stores were higher for random than for regular disturbance intervals, and increased as the mean disturbance interval increased and as the disturbance severity decreased. For example, C storage was reduced by 58% when the fire interval was shortened from 250 years to 100 years. Average landscape NECB was not significantly different than zero for any of the simulated landscapes. Year-to-year variability in landscape NECB, however, was related to the landscape disturbance regime; increasing with disturbance severity and frequency, and higher for random versus regular disturbance intervals. We conclude that landscape C stores of forest systems can be predicted using the concept of disturbance regimes, a result that may be a useful for adjusting estimates of C storage to broad scales that are solely based on physiological processes.

Smithwick, E. A. H., M.G. Ryan, D.M. Kashian, W.H. Romme, D.B. Tinker, M.G. Turner (2008). Modeling the effects of fire and climate change on carbon and nitrogen storage in lodgepole pine (Pinus contorta ) stands. Global Change Biology 15 (3): 535-548

ABSTRACT: The interaction between disturbance and climate change and resultant effects on ecosystem carbon (C) and nitrogen (N) fluxes are poorly understood. Here, we model (using CENTURY version 4.5) how climate change may affect C and N fluxes among mature and regenerating lodgepole pine (Pinus contorta var.latifolia Engelm . ex S. Wats.) stands that vary in postfire tree density following stand-replacing fire. Both young (postfire) and mature stands had elevated forest production and net N mineralization under future climate scenarios relative to current climate. Forest production increased 25% [Hadley (HAD)] to 36% [Canadian Climate Center (CCC)], compared with 2% under current climate, among stands that varied in stand age and postfire density. Net N mineralization increased under both climate scenarios, e.g., +19% to 37% (HAD) and +11% to 23% (CCC), with greatest increases for young stands with sparse tree regeneration. By 2100, total ecosystem carbon (live+dead+soils) in mature stands was higher than prefire levels, e.g., +16% to 19% (HAD) and +24% to 28% (CCC). For stands regenerating following fire in 1988, total C storage was 0–9% higher under the CCC climate model, but 5–6% lower under the HAD model and 20–37% lower under the Control. These patterns, which reflect variation in stand age, postfire tree density, and climate model, suggest that although there were strong positive responses of lodgepole pine productivity to future changes in climate, C flux over the next century will reflect complex relationships between climate, age structure, and disturbance-recovery patterns of the landscape.

Harmon, M. E. (2009). Effects of partial harvest on the carbon stores in Douglas-fir/Western hemlock forests: a simulation study. Ecosystems 12 (5): 777-791

ABSTRACT: The STANDCARB 2.0 model was used to examine the effects of partial harvest of trees within stands on forest-related carbon (C) stores in a typical Pacific NorthwestPseudotsuga /Tsuga forest. For harvest rotation intervals of 20 to 250 years the effect of completely dispersed (that is, a checkerboard) versus completely aggregated cutting patterns (that is, single blocks) was compared. The simulations indicated that forests with frequent, but partial removal of live trees can store as much C as those with complete tree harvest on less frequent intervals. Stores in forest products generally declined as the fraction of live trees harvested declined and as the interval between harvests increased. Although the proportion of total system stores in forest products increased as the frequency of harvests and proportion of trees removed increased, this did not offset the reduction in forest C stores these treatments caused. Spatial arrangement of harvest influenced tree species composition profoundly; however, the effects of aggregated versus dispersed cutting patterns on C stores were relatively small compared to the other treatments. This study indicates that there are multiple methods to increase C stores in the forest sector including either increasing the time between harvests or reducing the fraction of trees harvested during each harvest.

J.T. Randerson, F.M. Hoffman, P.E. Thornton, N.M. Mahowald, K. Lindsay, Y. Lee, C.D. Nevison, S.C. Doney, G. Bonan, R. Stöckli, C. Covey, S.W. Running, I.Y. Fung (2009). Systematic assessment of terrestrial biogeochemistry in coupled climate–carbon models. Global Change Biology 15 (10): 2462-2484

ABSTRACT: With representation of the global carbon cycle becoming increasingly complex in climate models, it is important to develop ways to quantitatively evaluate model performance against in situ and remote sensing observations. Here we present a systematic framework, the Carbon-LAnd Model Intercomparison Project (C-LAMP), for assessing terrestrial biogeochemistry models coupled to climate models using observations that span a wide range of temporal and spatial scales. As an example of the value of such comparisons, we used this framework to evaluate two biogeochemistry models that are integrated within the Community Climate System Model (CCSM) – Carnegie-Ames-Stanford Approach' (CASA') and carbon–nitrogen (CN). Both models underestimated the magnitude of net carbon uptake during the growing season in temperate and boreal forest ecosystems, based on comparison with atmospheric CO2 measurements and eddy covariance measurements of net ecosystem exchange. Comparison with MODerate Resolution Imaging Spectroradiometer (MODIS) measurements show that this low bias in model fluxes was caused, at least in part, by 1–3 month delays in the timing of maximum leaf area. In the tropics, the models overestimated carbon storage in woody biomass based on comparison with datasets from the Amazon. Reducing this model bias will probably weaken the sensitivity of terrestrial carbon fluxes to both atmospheric CO2 and climate. Global carbon sinks during the 1990s differed by a factor of two (2.4 Pg C yr−1 for CASA' vs. 1.2 Pg C yr−1 for CN), with fluxes from both models compatible with the atmospheric budget given uncertainties in other terms. The models captured some of the timing of interannual global terrestrial carbon exchange during 1988–2004 based on comparison with atmospheric inversion results from TRANSCOM (r=0.66 for CASA' and r=0.73 for CN). Adding (CASA') or improving (CN) the representation of deforestation fires may further increase agreement with the atmospheric record. Information from C-LAMP has enhanced model performance within CCSM and serves as a benchmark for future development. We propose that an open source, community-wide platform for model-data intercomparison is needed to speed model development and to strengthen ties between modeling and measurement communities. Important next steps include the design and analysis of land use change simulations (in both uncoupled and coupled modes), and the entrainment of additional ecological and earth system observations. Model results from C-LAMP are publicly available on the Earth System Grid.

M. Easter, K. Paustian, K. Killian, S. Williams, T. Feng, R. Al-Adamat, N.H. Batjes, M. Bernoux, T. Bhattacharyya, C.C. Cerri, C.E.P. Cerri, K. Coleman, P. Falloon, C. Feller, P. Gicheru, P. Kamoni, E. Milne, D.K. Pal, D.S. Powlson, Z. Rawajfih, M. Sessay, S. Wokabi (2007). The GEFSOC soil carbon modelling system: A tool for conducting regional-scale soil carbon inventories and assessing the impacts of land use change on soil carbon. 122 (1): 13-25

ABSTRACT: The GEFSOC soil carbon modelling system was built to provide interdisciplinary teams of scientists, natural resource managers and policy analysts (who have the appropriate computing skills) with the necessary tools to conduct regional-scale soil carbon (C) inventories. It allows users to assess the effects of land use change on soil organic C (SOC) stocks, soil fertility and the potential for soil C sequestration. The tool was developed in conjunction with case-studies of land use and management impacts on SOC in Brazil, Jordan, Kenya and India, which represent a diversity of land use and land management patterns and are countries where sustaining soil organic matter and fertility for food security is an on-going problem. The tool was designed to run using two common desktop computers, connected via a local area network. It utilizes open-source software that is freely available. All new software and user interfaces developed for the tool are available in an open source environment allowing users to examine system details, suggest improvements or write additional modules to interface with the system. The tool incorporates three widely used models for estimating soil C dynamics: (1) the Century ecosystem model; (2) the RothC soil C decomposition model; and (3) the Intergovernmental Panel on Climate Change (IPCC) method for assessing soil C at regional scales. The tool interacts with a Soil and Terrain Digital Database (SOTER) built for the specific country or region the user intends to model. A demonstration of the tool and results from an assessment of land use change in a sample region of North America are presented.

R. A. Houghton, F. Hall, S. J. Goetz (2009). Importance of biomass in the global carbon cycle. Journal of Geophysical Research 114 (G00E03): doi:10.1029/2009JG000935

ABSTRACT: Our knowledge of the distribution and amount of terrestrial biomass is based almost entirely on ground measurements over an extremely small, and possibly biased sample, with many regions still unmeasured. Our understanding of changes in terrestrial biomass is even more rudimentary, although changes in land use, largely tropical deforestation, are estimated to have reduced biomass, globally. At the same time, however, the global carbon balance requires that terrestrial carbon storage has increased, albeit the exact magnitude, location, and causes of this residual terrestrial sink are still not well quantified. A satellite mission capable of measuring aboveground woody biomass could help reduce these uncertainties by delivering three products. First, a global map of aboveground woody biomass density would halve the uncertainty of estimated carbon emissions from land use change. Second, an annual, global map of natural disturbances could define the unknown but potentially large proportion of the residual terrestrial sink attributable to biomass recovery from such disturbances. Third, direct measurement of changes in aboveground biomass density (without classification of land cover or carbon modeling) would indicate the magnitude and distribution of at least the largest carbon sources (from deforestation and degradation) and sinks (from woody growth). The information would increase our understanding of the carbon cycle, including better information on the magnitude, location, and mechanisms responsible for terrestrial sources and sinks of carbon. This paper lays out the accuracy, spatial resolution, and coverage required for a satellite mission that would generate these products.

Muller, C., Eickhout, B., Zaehle, S., Bondeau, A., Cramer, W., Lucht, W. (2007). Effects of changes in CO2 , climate, and land use on the carbon balance of the land biosphere during the 21st century. Journal of Geophysical Research-Biogeosciences 112 (G2): 2032

ABSTRACT: We studied the effects of climate and land-use change on the global terrestrial carbon cycle for the 21st century. Using the process-based land biosphere model (LPJmL), we mechanistically simulated carbon dynamics for natural and managed lands (agriculture and forestry) and for land-use change processes. We ran LPJmL with twelve different dynamic land-use patterns and corresponding climate and atmospheric CO2 projections. These input data were supplied from the IMAGE 2.2 implementations of the IPCC-SRES storylines for the A2, B1, and B2 scenarios. Each of these SRES scenarios was implemented under four different assumptions on spatial climate patterns in IMAGE 2.2, resulting in twelve different Earth System projections. Our selection of SRES scenarios comprises deforestation and afforestation scenarios, bounding a broad range of possible land-use change. Projected land-use change under different socio-economic scenarios has profound effects on the terrestrial carbon balance: While climate change and CO2 fertilization cause an additional terrestrial carbon uptake of 105-225 PgC, land-use change causes terrestrial carbon losses of up to 445 PgC by 2100, dominating the terrestrial carbon balance under the A2 and B2 scenarios. Our results imply that the potential positive feedback of the terrestrial biosphere on anthropogenic climate change will be strongly affected by land-use change. Spatiotemporally explicit projections of land-use change and the effects of land management on terrestrial carbon dynamics need additional attention in future research.

Raupach, M. R., Rayner, P. J., Barrett, D. J., Defries, R. S., Heimann, M., Ojima, D. S., Quegan, S., Schmullius, C. C. (2005). Model-data synthesis in terrestrial carbon observation: methods, data requirements and data uncertainty specifications. Global Change Biology 11 (3): 378-397

ABSTRACT: Systematic, operational, long-term observations of the terrestrial carbon cycle (including its interactions with water, energy and nutrient cycles and ecosystem dynamics) are important for the prediction and management of climate, water resources, food resources, biodiversity and desertification. To contribute to these goals, a terrestrial carbon observing system requires the synthesis of several kinds of observation into terrestrial biosphere models encompassing the coupled cycles of carbon, water, energy and nutrients. Relevant observations include atmospheric composition (concentrations of CO2 and other gases); remote sensing; flux and process measurements from intensive study sites; in situ vegetation and soil monitoring; weather, climate and hydrological data; and contemporary and historical data on land use, land use change and disturbance (grazing, harvest, clearing, fire). A review of model-data synthesis tools for terrestrial carbon observation identifies 'nonsequential' and 'sequential' approaches as major categories, differing according to whether data are treated all at once or sequentially. The structure underlying both approaches is reviewed, highlighting several basic commonalities in formalism and data requirements. An essential commonality is that for all model-data synthesis problems, both nonsequential and sequential, data uncertainties are as important as data values themselves and have a comparable role in determining the outcome. Given the importance of data uncertainties, there is an urgent need for soundly based uncertainty characterizations for the main kinds of data used in terrestrial carbon observation. The first requirement is a specification of the main properties of the error covariance matrix. As a step towards this goal, semi-quantitative estimates are made of the main properties of the error covariance matrix for four kinds of data essential for terrestrial carbon observation: remote sensing of land surface properties, atmospheric composition measurements, direct flux measurements, and measurements of carbon stores.

Silver, W. L. (1998). The potential effects of elevated CO2 and climate change on tropical forests soils and biogeochemical cycling. Climatic Change 39 (2-3): 337-361

ABSTRACT: Tropical forests are responsible for a large proportion of the global terrestrial C flux annually for natural ecosystems. Increased atmospheric CO2 and changes in climate are likely to affect the distribution of C pools in the tropics and the rate of cycling through vegetation and soils. In this paper, I review the literature on the pools and fluxes of carbon in tropical forests, and the relationship of these to nutrient cycling and climate. Tropical moist and humid forests have the highest rates of annual net primary productivity and the greatest carbon flux from soil respiration globally. Tropical dry forests have lower rates of carbon circulation, but may have greater soil organic carbon storage, especially at depths below 1 meter. Data from tropical elevation gradients were used to examine the sensitivity of biogeochemical cycling to incremental changes in temperature and rainfall. These data show significant positive correlations of litterfall N concentrations with temperature and decomposition rates. Increased atmospheric CO2 and changes in climate are expected to alter carbon and nutrient allocation patterns and storage in tropical forest. Modeling and experimental studies suggest that even a small increase in temperature and CO2 concentrations results in more rapid decomposition rates, and a large initial CO2 efflux from moist tropical soils. Soil P limitation or reductions in C:N and C:P ratios of litterfall could eventually limit the size of this flux. Increased frequency of fires in dry forest and hurricanes in moist and humid forests are expected to reduce the ecosystem carbon storage capacity over longer time periods

D. Bachelet, R. P. Neilson, J. M. Lenihan, R. J. Drapek (2004). Regional differences in the carbon source-sink potential of natural vegetation in the U.S.A.. Environmental Management 33 (Supplement 1): S23-S43

ABSTRACT: We simulated the variability in natural ecosystem carbon storage under historical conditions (1895–1994) in six regions of the conterminous USA as delineated for the USGCRP National Assessment (2001). The largest simulated variations in carbon fluxes occurred in the Midwest, where large fire events (1937, 1988) decreased vegetation biomass and soil carbon pools. The Southeast showed decadal-type trends and alternated between a carbon source (1920s, 1940s, 1970s) and a sink (1910s, 1930s, 1950s) in response to climate variations. The drought of the 1930s was most obvious in the creation of a large carbon source in the Midwest and the Great Plains, depleting soil carbon reserves. The Northeast shows the smallest amplitudes in the variation of its carbon stocks. Western regions release large annual carbon fluxes from their naturally fire-prone grassland- and shrubland-dominated areas, which respond quickly to chronic fire disturbance, thus reducing temporal variations in carbon stocks. However, their carbon dynamics reflect the impacts of prolonged drought periods as well as regional increases in rainfall from ocean-atmosphere climate regime shifts, most evident in the 1970s. Projections into the future by using the warm CGCM1 climate scenario show the Northeast becoming mostly a carbon source, the Southeast becoming the largest carbon source in the 21st century, and the two western-most regions becoming carbon sinks in the second half of the 21st century. Similar if more moderate trends are observed by using the more moderately warm HADCM2SUL scenario.

J. R. Barker, G. A. Baumgardner, D. P. Turner, J. J. Lee (1995). Potential carbon benefits of the conservation reserve program in the United States. Journal of Biogeography 22 (4/5): 743-751

ABSTRACT: Three scenarios of the Conservation Reserve Program (CRP) were simulated to project carbon (C) pools and fluxes of associated grassland and forestland for the years 1986-2035; and to evaluate the potential to offset greenhouse gas emissions through C sequestration. The approach was to link land-area enrolments with grassland and forestland C densities to simulate C pools and fluxes over 50 years. The CRP began in 1986 and by 1996 consisted of 16.2× 106 ha cropland converted to 14.7× 106 ha grassland and of 1.5× 106 ha forestland. The CRP1 simulated the likely outcome of the CRP as contracts expire in 1996 with the anticipated return of 8.7× 106 ha grassland and of 0.4× 106 ha forestland to crop production. The CRP2 assumed that the CRP continues with no land returning to crop production. The CRP3 was an expansion of the CRP2 to include afforestation of 4×106 ha new land. Average net annual C gains for the years 1996-2005 were <1, 12, and 16 TgC yr-1 for CRP1, CRP2, and CRP3, respectively. Afforestation of marginal cropland as simulated under CRP3 could provide approximately 15% of the C offset needed to attain the Climate Change Action Plan of reducing greenhouse gas emmissions to their 1990 level by the year 2000 within the United States.

P. M. Cox, R. A. Betts, A. Betts, C. D. Jones, S. A. Spall, I. J. Totterdell (2002). Modelling vegetation and the carbon cycle as interactive elements of the climate system. International Geophysics 83: 259-279

ABSTRACT: The climate system and the global carbon cycle are tightly coupled. Atmospheric carbon in the form of the radiatively active gases, carbon dioxide and methane, plays a significant role in the natural greenhouse effect. The continued increase in the atmospheric concentrations of these gases, due to human emissions, is predicted to lead to significant climatic change over the next 100 years. The best estimates suggest that more than half of the current anthropogenic emissions of carbon dioxide are being absorbed by the ocean and by land ecosystems (Schimel et al., 1995). In both cases the processes involved are sensitive to the climatic conditions. Temperature affects the solubility of carbon dioxide in sea water and the rate of terrestrial and oceanic biological processes. In addition, vegetation is known to respond directly to increased atmospheric CO2 through increased photosynthesis and reduced transpiration (Sellers et al., 1996a; Field et al., 1995), and may also change its structure and distribution in response to any associated climate change (Betts et al., 1997). Thus there is great potential for the biosphere to produce a feedback on the climatic change due to given human emissions.

Despite this, simulations carried out with General Circulation Models (GCMs) have generally neglected the coupling between the climate and the biosphere. Indeed, vegetation distributions have been static and atmospheric concentrations of CO2 have been prescribed based on results from simple carbon cycle models, which neglect the effects of climate change (Enting et al., 1994). This chapter describes the inclusion of vegetation and the carbon cycle as interactive elements in a GCM. The coupled climate-carbon cycle model is able to reproduce key aspects of the observations, including the global distribution of vegetation types, seasonal and zonal variations in ocean primary production, and the interannual variability in atmospheric CO2 . A transient simulation carried out with this model suggests that previously-neglected climate-carbon cycle feedbacks could significantly accelerate atmospheric CO2 rise and climate change over the twenty-first century.

B. E. Law, D. Turner, J. Campbell, O. J. Sun, S. Van Tuyl, W. D. Ritts, W.B. Cohen (2004). Disturbance and climate effects on carbon stocks and fluxes across Western Oregon, USA. Global Change Biology 10 (9): 1429-1444

ABSTRACT: We used a spatially nested hierarchy of field and remote-sensing observations and a process model, Biome-BGC, to produce a carbon budget for the forested region of Oregon, and to determine the relative influence of differences in climate and disturbance among the ecoregions on carbon stocks and fluxes. The simulations suggest that annual net uptake (net ecosystem production (NEP)) for the whole forested region (8.2 million hectares) was 13.8 Tg C (168 g C m−2 yr−1 ), with the highest mean uptake in the Coast Range ecoregion (226 g C m−2 yr−1 ), and the lowest mean NEP in the East Cascades (EC) ecoregion (88 g C m−2 yr−1 ). Carbon stocks totaled 2765 Tg C (33 700 g C m−2 ), with wide variability among ecoregions in the mean stock and in the partitioning above- and belowground. The flux of carbon from the land to the atmosphere that is driven by wildfire was relatively low during the late 1990s (~0.1 Tg C yr−1 ), however, wildfires in 2002 generated a much larger C source (~4.1 Tg C). Annual harvest removals from the study area over the period 1995–2000 were ~5.5 Tg C yr−1 . The removals were disproportionately from the Coast Range, which is heavily managed for timber production (approximately 50% of all of Oregon's forest land has been managed for timber in the past 5 years). The estimate for the annual increase in C stored in long-lived forest products and land fills was 1.4 Tg C yr−1 . Net biome production (NBP) on the land, the net effect of NEP, harvest removals, and wildfire emissions indicates that the study area was a sink (8.2 Tg C yr−1 ). NBP of the study area, which is the more heavily forested half of the state, compensated for ~52% of Oregon's fossil carbon dioxide emissions of 15.6 Tg C yr−1 in 2000. The Biscuit Fire in 2002 reduced NBP dramatically, exacerbating net emissions that year. The regional total reflects the strong east–west gradient in potential productivity associated with the climatic gradient, and a disturbance regime that has been dominated in recent decades by commercial forestry.

Luo, Y. (2007). Terrestrial carbon-cycle feedback to climate warming. Annual Review of Ecology, Evolution, and Systematics 38: 683-712

ABSTRACT: The coupled carbon-climate models reported in the literature all demonstrate a positive feedback between terrestrial carbon cycles and climate warming. A primary mechanism underlying the modeled positive feedback is the kinetic sensitivity of photosynthesis and respiration to temperature. Field experiments, however, suggest much richer mechanisms driving ecosystem responses to climate warming, including extended growing seasons, enhanced nutrient availability, shifted species composition, and altered ecosystem-water dynamics. The diverse mechanisms likely define more possibilities of carbon-climate feedbacks than projected by the kinetics-based models. Nonetheless, experimental results are so variable that we have not generated the necessary insights on ecosystem responses to effectively improve global models. To constrain model projections of carbon-climate feedbacks, we need more empirical data from whole ecosystem warming experiments across a wide range of biomes, particularly in tropic regions, and closer interactions between models and experiments.

Matebr, R.J., A.C. Hirst (1999). Climate change feedback on the future oceanic CO2 uptake. Tellus Series B Chemical and Physical Meteorology 51 (3): 722-733

ABSTRACT: Output from a coupled atmosphere–ocean model forced by the IS92a greenhouse gas scenario was used to investigate the feedback between climate change and the oceanic uptake of CO2 . To improve the climate simulation, we used Gent and co-workers eddy parameterization in the ocean and a prognostic equation for export production from the upper ocean. For the period of 1850 to 2100, the change in the oceanic uptake of CO2 with climate was separated into 3 feedbacks. (i) Climate change warmed the sea-surface temperature which increased the partial pressure of CO2 in the surface ocean and reduced the accumulated ocean uptake by 48 Gt C. (ii) Climate change reduced meridional overturning and convective mixing and increased density stratification in high latitudes which slowed the transport of anthropogenic CO2 into the ocean interior and reduced the cumulative ocean CO2 uptake by 41 Gt C. (iii) Climate change altered "natural" cycling of carbon in the ocean which increased the cumulative ocean CO2 uptake by 33 Gt C. The change in natural carbon cycling with climate change was dominated by 2 opposing factors. First, the supply of nutrients to the upper ocean decreased which reduced the export of organic matter (by 15% by year 2100) and produced a net CO2 flux out of the ocean. However, associated with the reduced nutrient supply was the reduction in the supply of dissolved inorganic carbon to the upper ocean, which produced net CO2 flux into the ocean. For our model, the latter effect dominated. By the year 2100, the combinations of these 3 climate change feedbacks resulted in a decrease in the cumulative oceanic CO2 uptake of 56 Gt C or 14% of the 402 Gt C of oceanic CO2 uptake predicted by a run with no climate change. Our total reduction in oceanic CO2 uptake with climate change for the 1850 to 2100 period was similar to the 58 Gt C reduction in oceanic CO2 uptake predicted by Sarmiento and Le Quéré. However, our consistency with this previous estimate is misleading. By including the Gent and co-workers eddy parameterization in the ocean, we reduced the positive feedback between climate change and the oceanic uptake of CO2 from 169 to 89 Gt C (80 Gt C change). This reduction reflects a decrease in both sea surface warming and anthropogenic forcing feedbacks. By using a prognostic parameterization of export production, we reduced the negative feedback response of the natural carbon cycle to climate change from 111 to 33 Gt C (78 Gt C). These 2 large offsetting changes in the ocean response to climate change produced only a net change of 2 Gt C. This resulted in a net reduction in oceanic uptake of 2 Gt C from the previous study.

I. Y. Fung, S. C. Doney, K. Lindsay, J. John (2005). Evolution of carbon sinks in a changing climate. Proceedings of the National Academy of Sciences 102 (32): 11201-11206

ABSTRACT: Climate change is expected to influence the capacities of the land and oceans to act as repositories for anthropogenic CO2 and hence provide a feedback to climate change. A series of experiments with the National Center for Atmospheric Research–Climate System Model 1 coupled carbon–climate model shows that carbon sink strengths vary with the rate of fossil fuel emissions, so that carbon storage capacities of the land and oceans decrease and climate warming accelerates with faster CO2 emissions. Furthermore, there is a positive feedback between the carbon and climate systems, so that climate warming acts to increase the airborne fraction of anthropogenic CO2 and amplify the climate change itself. Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses. Analysis of our results in the context of comparable models suggests that destabilization of the tropical land sink is qualitatively robust, although its degree is uncertain.

J. C. Orr, V. J. Fabry, O. Aumont, L. Bopp, S. C. Doney, R. A. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R. M. Key, K. Lindsay, E. Maier-Reimer, R. Matear, P. Monfray, A. Mouchet, R. G. Najjar, G. Plattner, K. B. Rodgers, C. L. Sabine, J. L. Sarmiento, R. Schlitzer, R. D. Slater, I. J. Totterdell, M. Weirig, Y. Yamanaka, A. Yool (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437 (29 September): 681-686

ABSTRACT: Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a 'business-as-usual' scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

L. Cao, K. Caldeira, A. K. Jain (2007). Effects of carbon dioxide and climate change on ocean acidification and carbonate mineral saturation. Geophysical Research Letters 34 (L05607): doi:10.1029/2006GL028605

ABSTRACT: We use an earth system model of intermediate complexity to show how consideration of climate change affects predicted changes in ocean pH and calcium carbonate saturation state. Our results indicate that consideration of climate change produces second-order modifications to ocean chemistry predictions made with constant climate; these modifications occur primarily as a result of changes in sea surface temperature, and climate-induced changes in dissolved inorganic carbon concentrations. Under a CO2 emission scenario derived from the WRE1000 CO2 stabilization concentration pathway and a constant climate, we predict a 0.47 unit reduction in surface ocean pH relative to a pre-industrial value of 8.17, and a reduction in the degree of saturation with respect to aragonite from a pre-industrial value of 3.34 to 1.39 by year 2500. With the same CO2 emissions but the consideration of climate change under a climate sensitivity of 2.5°C the reduction in projected global mean surface pH is about 0.48 and the saturation state of aragonite decreases to 1.50. With a climate sensitivity of 4.5°C, these values are 0.51 and 1.62, respectively. Our study therefore suggests that future changes in ocean acidification caused by emissions of CO2 to the atmosphere are largely independent of the amounts of climate change.

D. Hope, M.F. Billett, R. MIlne, T.A.W. Brown (1997). Exports of organic carbon in British rivers. 11 (3): 325-344

ABSTRACT: This study provides the first detailed estimate of riverine organic carbon fluxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Purification Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean flows to estimate fluxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7·7-103·5 kg ha-1 year-1 , with a median flux of 31·9 kg ha-1 year-1 in the 85 rivers for which data were available. There was a trend for DOC fluxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC fluxes in regions where no organic carbon data were available. Calculated and predicted fluxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0·68±0·07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0·20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two-thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to affect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change.

D. Justić, N. N. Rabalais, R. E.Turner (1997). Impacts of climate change on net productivity of coastal waters: implications for carbon budgets and hypoxia. Climate Research 8: 225-237

ABSTRACT: General circulation models predict that freshwater discharge from the Mississippi River (USA) to the coastal ocean would increase 20% if atmospheric CO2 concentration doubles. Here we use a coupled physical-biological 2-box model to investigate the potential impacts of increased freshwater and nutrient inputs on the production and decay of organic matter in the coastal waters of the northern Gulf of Mexico. Model results for a doubled CO2 climate indicate that the annual net productivity of the upper water column (NP, 0 to 10 m) is likely to increase by 65 g C m-2 yr-1 , relative to a 1985-1992 average (122 g C m-2 yr-1 ). Interestingly, this projected increase is of the same magnitude as the one that has occurred slnce the 1940s due to the introduction of anthropogenlc nutrients. An increase in annual NP of 32 g C m-2 yr-1 was observed during the Great Mississippi River Flood of 1993, thus indicating the general validity of a doubled CO2 scenario. The total oxygen uptake in the lower water column (10 to 20 m), in contrast, is likely to remain at its present value of about 200 g O2 m-2 yr-1 . Thus, carbon export and burial, rather than in situ respiration, are likely to be the dominant processes balancing coastal carbon budgets, leading perhaps to an expanded extent of the hypoxic zone.

D.A. Semenov (2008). Biological contribution to global climate dynamics on a “geological” timescale. Ecological Modelling 212 (1-2): 171-177

ABSTRACT: A model consisting of two blocks (equation) is proposed for the analytical study of the “biosphere—climate” system over a great period of time. The first equation describes the balance of carbon dioxide in the atmosphere and represents the biological block of the model. The second equation is the equation of energy balance or the physical block of the system. The model is based on the most general concepts of living matter and the evolution process. A possible interpretation of some events and phenomena in the Earth history is given in terms of the model.

M. Berthelot, P. Friedlingstein, P. Ciais, J.-L. Dufresne, P. Monfray (2005). How uncertainties in future climate change predictions translate into future terrestrial carbon fluxes. Global Change Biology 11 (6): 959-970

ABSTRACT: We forced a global terrestrial carbon cycle model by climate fields of 14 ocean and atmosphere general circulation models (OAGCMs) to simulate the response of terrestrial carbon pools and fluxes to climate change over the next century. These models participated in the second phase of the Coupled Model Intercomparison Project (CMIP2), where a 1% per year increase of atmospheric CO2 was prescribed. We obtain a reduction in net land uptake because of climate change ranging between 1.4 and 5.7 Gt C yr−1 at the time of atmospheric CO2 doubling. Such a reduction in terrestrial carbon sinks is largely dominated by the response of tropical ecosystems, where soil water stress occurs. The uncertainty in the simulated land carbon cycle response is the consequence of discrepancies in land temperature and precipitation changes simulated by the OAGCMs. We use a statistical approach to assess the coherence of the land carbon fluxes response to climate change. The biospheric carbon fluxes and pools changes have a coherent response in the tropics, in the Mediterranean region and in high latitudes of the Northern Hemisphere. This is because of a good coherence of soil water content change in the first two regions and of temperature change in the high latitudes of the Northern Hemisphere.

Then we evaluate the carbon uptake uncertainties to the assumptions on plant productivity sensitivity to atmospheric CO2 and on decomposition rate sensitivity to temperature. We show that these uncertainties are on the same order of magnitude than the uncertainty because of climate change. Finally, we find that the OAGCMs having the largest climate sensitivities to CO2 are the ones with the largest soil drying in the tropics, and therefore with the largest reduction of carbon uptake.

Yoshikawa, C., M. Kawamiya, T. Kato, Y. Yamanaka, T. Matsuno (2008). Geographical distribution of the feedback between future climate change and the carbon cycle. Journal of Geophysical Research 113

ABSTRACT: We examined climate-carbon cycle feedback by performing a global warming experiment using MIROC-based coupled climate-carbon cycle model. The model showed that by the end of the 21st century, warming leads to a further increase in carbon dioxide (CO2 ) level of 123 ppm by volume (ppmv). This positive feedback can mostly be attributed to land-based soil-carbon dynamics. On a regional scale, Siberia experienced intense positive feedback, because the acceleration of microbial respiration due to warming causes a decrease in the soil carbon level. Amazonia also had positive feedback resulting from accelerated microbial respiration. On the other hand, some regions, such as western and central North America and South Australia, experienced negative feedback, because enhanced litterfall surpassed the increased respiration in soil carbon. The oceanic contribution to the feedback was much weaker than the land contribution on global scale, but the positive feedback in the northern North Atlantic was as strong as those in Amazonia and Siberia in our model. In the northern North Atlantic, the weakening of winter mixing caused a reduction of CO2 absorption at the surface. Moreover, weakening of the formation of North Atlantic Deep Water caused reduced CO2 subduction to the deep water. Understanding such regional-scale differences may help to explain disparities in coupled climate-carbon cycle model results.

Miyama, T. (2009). Estimating allowable carbon emission for CO2 concentration stabilization using a GCM-based Earth system model. Geophysical Research Letters 36 (L19709): doi:10.1029/2009GL039678

ABSTRACT: For atmospheric CO2 concentration stabilization, we projected allowable carbon emission with an Earth system model based on a general circulation model. Our calculations on centennial timescale in various scenarios reveal how saturation with respect to CO2 and climate-carbon cycle feedback reduce natural carbon uptake, and hence allowable emission. In 450 ppm stabilization scenario, for example, climate-carbon cycle feedback reduces the accumulative allowable carbon emission until year 2300 from 1248 to 980 Pg C. The Emission at the year 2050 is about the half of the year 2000 level for the SP450 scenario. Terrestrial carbon cycle is especially susceptible to climate-carbon cycle feedback, and is a significant source of projection uncertainty. Our model responds nonlinearly to CO2 and climate, suggesting process-based models are indispensable tool for future climate-carbon cycle projections.

Ji, J. (2008). Prediction of carbon exchanges between China terrestrial ecosystem and atmosphere in 21st century. Science in China Series D Earth Sciences 51 (6)

ABSTRACT: The projected changes in carbon exchange between China terrestrial ecosystem and the atmosphere and vegetation and soil carbon storage during the 21st century were investigated using an atmosphere-vegetation interaction model (AVIM2). The results show that in the coming 100 a, for SRES B2 scenario and constant atmospheric CO2 concentration, the net primary productivity (NPP) of terrestrial ecosystem in China will be decreased slowly, and vegetation and soil carbon storage as well as net ecosystem productivity (NEP) will also be decreased. The carbon sink for China terrestrial ecosystem in the beginning of the 20th century will become totally a carbon source by the year of 2020, while for B2 scenario and changing atmospheric CO2 concentration, NPP for China will increase continuously from 2.94 Gt C · a−1 by the end of the 20th century to 3.99 Gt C · a−1 by the end of the 21st century, and vegetation and soil carbon storage will increase to 110.3 Gt C. NEP in China will keep rising during the first and middle periods of the 21st century, and reach the peak around 2050s, then will decrease gradually and approach to zero by the end of the 21st century.

O’Neill, B. C., N.B. Melnikov (2008). Learning about parameter and structural uncertainty in carbon cycle models. Climatic Change 89 (1-2): 23-44

ABSTRACT: Uncertainty in the response of the global carbon cycle to anthropogenic emissions plays a key role in assessments of potential future climate change and response strategies. We investigate how fast this uncertainty might change as additional data on the global carbon budget becomes available over the twenty-first century. Using a simple global carbon cycle model and focusing on both parameter and structural uncertainty in the terrestrial sink, we find that additional global data leads to substantial learning (i.e., changes in uncertainty) under some conditions but not others. If the model structure is assumed known and only parameter uncertainty is considered, learning is rather limited if observational errors in the data or the magnitude of unexplained natural variability are not reduced. Learning about parameter values can be substantial, however, when errors in data or unexplained variability are reduced. We also find that, on the one hand, uncertainty in the model structure has a much bigger impact on uncertainty in projections of future atmospheric composition than does parameter uncertainty. But on the other, it is also possible to learn more about the model structure than the parameter values, even from global budget data that does not improve over time in terms of its associated errors. As an example, we illustrate how one standard model structure, if incorrect, could become inconsistent with global budget data within 40 years. The rate of learning in this analysis is affected by the choice of a relatively simple carbon cycle model, the use of observations only of global emissions and atmospheric concentration, and the assumption of perfect autocorrelation in observational errors and variability. Future work could usefully improve the approach in each of these areas.

R. G. Zepp, Erickson, D.J., III, N. D. Paul, B. Sulzberger (2007). Interactive effects of solar UV radiation and climate change on biogeochemical cycling. Photochemical & Photobiological Sciences 6 (3): 286-300

ABSTRACT: This report assesses research on the interactions of UV radiation (280–400 nm) and global climate change with global biogeochemical cycles at the Earth's surface. The effects of UV-B (280–315 nm), which are dependent on the stratospheric ozone layer, on biogeochemical cycles are often linked to concurrent exposure to UV-A radiation (315–400 nm), which is influenced by global climate change. These interactions involving UV radiation (the combination of UV-B and UV-A) are central to the prediction and evaluation of future Earth environmental conditions. There is increasing evidence that elevated UV-B radiation has significant effects on the terrestrial biosphere with implications for the cycling of carbon, nitrogen and other elements. The cycling of carbon and inorganic nutrients such as nitrogen can be affected by UV-B-mediated changes in communities of soil organisms, probably due to the effects of UV-B radiation on plant root exudation and/or the chemistry of dead plant material falling to the soil. In arid environments direct photodegradation can play a major role in the decay of plant litter, and UV-B radiation is responsible for a significant part of this photodegradation. UV-B radiation strongly influences aquatic carbon, nitrogen, sulfur and metals cycling that affect a wide range of life processes. UV-B radiation changes the biological availability of dissolved organic matter to microorganisms, and accelerates its transformation into dissolved inorganic carbon and nitrogen, including carbon dioxide and ammonium. The coloured part of dissolved organic matter (CDOM) controls the penetration of UV radiation into water bodies, but CDOM is also photodegraded by solar UV radiation. Changes in CDOM influence the penetration of UV radiation into water bodies with major consequences for aquatic biogeochemical processes. Changes in aquatic primary productivity and decomposition due to climate-related changes in circulation and nutrient supply occur concurrently with exposure to increased UV-B radiation, and have synergistic effects on the penetration of light into aquatic ecosystems. Future changes in climate will enhance stratification of lakes and the ocean, which will intensify photodegradation of CDOM by UV radiation. The resultant increase in the transparency of water bodies may increase UV-B effects on aquatic biogeochemistry in the surface layer. Changing solar UV radiation and climate also interact to influence exchanges of trace gases, such as halocarbons (e.g., methyl bromide) which influence ozone depletion, and sulfur gases (e.g., dimethylsulfide) that oxidize to produce sulfate aerosols that cool the marine atmosphere. UV radiation affects the biological availability of iron, copper and other trace metals in aquatic environments thus potentially affecting metal toxicity and the growth of phytoplankton and other microorganisms that are involved in carbon and nitrogen cycling. Future changes in ecosystem distribution due to alterations in the physical and chemical climate interact with ozone-modulated changes in UV-B radiation. These interactions between the effects of climate change and UV-B radiation on biogeochemical cycles in terrestrial and aquatic systems may partially offset the beneficial effects of an ozone recovery.

P. D. Falloon, P. Smith, J. U. Smith, J. Szabó, K. Coleman, S. Marshall (1998). Regional estimates of carbon sequestration potential: linking the Rothamsted Carbon Model to GIS databases. Biology and Fertility of Soils 27 (3): 236-241

ABSTRACT: Soil organic matter (SOM) represents a major pool of carbon within the biosphere. It is estimated at about 1400 Pg globally, which is roughly twice that in atmospheric CO2 . The soil can act as both a source and a sink for carbon and nutrients. Changes in agricultural land use and climate can lead to changes in the amount of carbon held in soils, thus, affecting the fluxes of CO2 to and from the atmosphere. Some agricultural management practices will lead to a net sequestration of carbon in the soil. Regional estimates of the carbon sequestration potential of these practices are crucial if policy makers are to plan future land uses to reduce national CO2 emissions. In Europe, carbon sequestration potential has previously been estimated using data from the Global Change and Terrestrial Ecosystems Soil Organic Matter Network (GCTE SOMNET). Linear relationships between management practices and yearly changes in soil organic carbon were developed and used to estimate changes in the total carbon stock of European soils. To refine these semi-quantitative estimates, the local soil type, meteorological conditions and land use must also be taken into account. To this end, we have modified the Rothamsted Carbon Model, so that it can be used in a predictive manner, with SOMNET data. The data is then adjusted for local conditions using Geographical Information Systems databases. In this paper, we describe how these developments can be used to estimate carbon sequestration at the regional level using a dynamic simulation model linked to spatially explicit data. Some calculations of the potential effects of afforestation on soil carbon stocks in Central Hungary provide a simple example of the system in use.

D. V. Spracklen, L. J. Mickley, J. A. Logan, R. C. Hudman, R. Yevich, M. D. Flannigan, A. L. Westerling (2009). Impacts of climate change from 2000 to 2050 on wildfire activity and carbonaceous aerosol concentrations in the western United States. Journal of Geophysical Research 114 (D20301): doi:10.1029/2008JD010966

ABSTRACT: We investigate the impact of climate change on wildfire activity and carbonaceous aerosol concentrations in the western United States. We regress observed area burned onto observed meteorological fields and fire indices from the Canadian Fire Weather Index system and find that May–October mean temperature and fuel moisture explain 24–57% of the variance in annual area burned in this region. Applying meteorological fields calculated by a general circulation model (GCM) to our regression model, we show that increases in temperature cause annual mean area burned in the western United States to increase by 54% by the 2050s relative to the present day. Changes in area burned are ecosystem dependent, with the forests of the Pacific Northwest and Rocky Mountains experiencing the greatest increases of 78 and 175%, respectively. Increased area burned results in near doubling of wildfire carbonaceous aerosol emissions by midcentury. Using a chemical transport model driven by meteorology from the same GCM, we calculate that climate change will increase summertime organic carbon (OC) aerosol concentrations over the western United States by 40% and elemental carbon (EC) concentrations by 20% from 2000 to 2050. Most of this increase (75% for OC and 95% for EC) is caused by larger wildfire emissions with the rest caused by changes in meteorology and for OC by increased monoterpene emissions in a warmer climate. Such an increase in carbonaceous aerosol would have important consequences for western U.S. air quality and visibility.

M. Hurteau, M. North (2009). Fuel treatment effects on tree-based forest carbon storage and emissions under modeled wildfire scenarios. Frontiers in Ecology and the Environment 7 (8): 409-414

ABSTRACT: Forests are viewed as a potential sink for carbon (C) that might otherwise contribute to climate change. It is unclear, however, how to manage forests with frequent fire regimes to maximize C storage while reducing C emissions from prescribed burns or wildfire. We modeled the effects of eight different fuel treatments on tree-based C storage and release over a century, with and without wildfire. Model runs show that, after a century of growth without wildfire, the control stored the most C. However, when wildfire was included in the model, the control had the largest total C emission and largest reduction in live-tree-based C stocks. In model runs including wildfire, the final amount of tree-based C sequestered was most affected by the stand structure initially produced by the different fuel treatments. In wildfire-prone forests, tree-based C stocks were best protected by fuel treatments that produced a low-density stand structure dominated by large, fire-resistant pines.

Wang, Y.P., J. Polglase (1995). Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics. Plant, Cell & Environment 18 (10): 1226-1244

ABSTRACT: Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO2 and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (Rh ) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO2 concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO2 concentration but to decrease with warming. As CO2 concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased Rh , particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO2 and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year−1 for boreal forests and 0–23% year−1 in the tundra between 1960 and 1990.

S. W. Running, R. R. Nemani (1991). Regional hydrologic and carbon balance responses of forests resulting from potential climate change. Climatic Change 19 (4): 349-368

ABSTRACT: The projected response of coniferous forests to a climatic change scenario of doubled atmospheric CO2 , air temperature of +4 °C, and +10% precipitation was studied using a computer simulation model of forest ecosystem processes. A topographically complex forested region of Montana was simulated to study regional climate change induced forest responses. In general, increases of 10–20% in LAI, and 20–30% in evapotranspiration (ET) and photosynthesis (PSN) were projected. Snowpack duration decreased by 19–69 days depending on location, and growing season length increased proportionally. However, hydrologic outflow, primarily fed by snowmelt in this region, was projected to decrease by as much as 30%, which could virtually dry up rivers and irrigation water in the future. To understand the simulated forest responses, and explore the extent to which these results might apply continentally, seasonal hydrologic partitioning between outflow and ET, PSN, respiration, and net primary production (NPP) were simulated for two contrasting climates of Jacksonville, Florida (hot, wet) and Missoula, Montana (cold, dry). Three forest responses were studied sequentially from; climate change alone, addition of CO2 induced tree physiological responses of -30% stomatal conductance and +30% photosynthetic rates, and finally with a reequilibration of forest leaf area index (LAI), derived by a hydrologic equilibrium theory. NPP was projected to increase 88%, and ET 10%, in Missoula, MT, yet decrease 5% and 16% respectively for Jacksonville, FL, emphasizing the contrasting forest responses possible with future climatic change.

Xiao, J., Zhuang, Q., Baldocchi, D. D., Law, B. E., Richardson, A. D, Chen, J., 36 co-authors, (2008). Estimation of net ecosystem carbon exchange for the conterminous United States by combining MODIS and AmeriFlux data. Agricultural and Forest Meteorology 148 (11): 1827-1847

ABSTRACT: Eddy covariance flux towers provide continuous measurements of net ecosystem carbon exchange (NEE) for a wide range of climate and biome types. However, these measurements only represent the carbon fluxes at the scale of the tower footprint. To quantify the net exchange of carbon dioxide between the terrestrial biosphere and the atmosphere for regions or continents, flux tower measurements need to be extrapolated to these large areas. Here we used remotely sensed data from the Moderate Resolution Imaging Spectrometer (MODIS) instrument on board the National Aeronautics and Space Administration's (NASA) Terra satellite to scale up AmeriFlux NEE measurements to the continental scale. We first combined MODIS and AmeriFlux data for representative U.S. ecosystems to develop a predictive NEE model using a modified regression tree approach. The predictive model was trained and validated using eddy flux NEE data over the periods 2000–2004 and 2005–2006, respectively. We found that the model predicted NEE well (r = 0.73, p < 0.001). We then applied the model to the continental scale and estimated NEE for each 1 km × 1 km cell across the conterminous U.S. for each 8-day interval in 2005 using spatially explicit MODIS data. The model generally captured the expected spatial and seasonal patterns of NEE as determined from measurements and the literature. Our study demonstrated that our empirical approach is effective for scaling up eddy flux NEE measurements to the continental scale and producing wall-to-wall NEE estimates across multiple biomes. Our estimates may provide an independent dataset from simulations with biogeochemical models and inverse modeling approaches for examining the spatiotemporal patterns of NEE and constraining terrestrial carbon budgets over large areas.

bottom right