Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
R.J. Alig, O. Krankina, A. Yost, J. Kuzminykh (2006). Forest carbon dynamics in the Pacific Northwest (USA) and the St. Petersburg region of Russia: comparisons and policy implications. Climate Change 79 (3-4): 335-360
ABSTRACT: Forests of the United States and Russia can play a positive role in reducing the extent of global warming caused by greenhouse gases, especially carbon dioxide. To determine the extent of carbon sequestration, physical, ecological, economic, and social issues need to be considered, including different forest management objectives across major forest ownership groups. Private timberlands in the U.S. Pacific Northwest are relatively young, well stocked, and sequestering carbon at relatively high rates. Forests in northwestern Russia are generally less productive than those in the Northwestern U.S. but cover extensive areas. A large increase in carbon storage per hectare in live tree biomass is projected on National Forest timberlands in the U.S. Pacific Northwest for all selected scenarios, with an increase of between 157–175 Mg by 2050 and a near doubling of 1970s levels. On private timberlands in the Pacific Northwest, average carbon in live tree biomass per hectare has been declining historically but began to level off near 65 Mg in 2000; projected levels by 2050 are roughly what they were in 1970 at approximately 80 Mg. In the St. Petersburg region, average carbon stores were similar to those on private lands in the Pacific Northwest: 57 Mg per hectare in 2000 and ranging from 40 to 64 Mg by 2050. Although the projected futures reflect a broad range of policy options, larger differences in projected carbon stores result from the starting conditions determined by ownership, regional environmental conditions, and past changes in forest management. However, an important change of forest management objective, such as the end of all timber harvest on National Forests in the Pacific Northwest or complete elimination of mature timber in the St. Petersburg region, can lead to substantial change in carbon stores over the next 50 years.
ABSTRACT: Disturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1-y-old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, Northwest Territories; a 1-y-old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10-y-old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (Rn), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO2 fluxes were much reduced, but night-time CO2 fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 g C m−2 day−1 , while the mature site was a sink of about −3.8 g C m−2 day−1 . The one-year-old burn had lower Rn, H and LE than the mature jackpine forest, and had a continuous CO2 efflux of about 0.8 g C m-2 day−1 compared to the mature forest sink of − 0.5 g C m−2 day−1 . The carbon source was likely caused by decomposition of fire-killed vegetation. The 10-y-old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO2 fluxes were slightly lower at the 10-y-old site, there was no significant difference between the daily integrals (−1.3 g C m−2 day−1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.
Amiro, B.D., Orchansky, A.L., Barr, A.G., Black, T.A., Chambers, S.D., Chapin III, F.S., Goulden, M.L., Litvak, M., Liu, H.P., McCaughey, J.H. (2006). The effect of post-fire stand age on the boreal forest energy balance. Agricultural and Forest Meteorology 140 (1-4): 41-50
ABSTRACT: Fire in the boreal forest renews forest stands and changes the ecosystem properties. The successional stage of the vegetation determines the radiative budget, energy balance partitioning, evapotranspiration and carbon dioxide flux. Here, we synthesize energy balance measurements from across the western boreal zone of North America as a function of stand age following fire. The data are from 22 sites in Alaska, Saskatchewan and Manitoba collected between 1998 and 2004 for a 150-year forest chronosequence. The summertime albedo immediately after a fire is about 0.05, increasing to about 0.12 for a period of about 30 years and then averaging about 0.08 for mature coniferous forests. A mature deciduous (aspen) forest has a higher summer albedo of about 0.16. Wintertime albedo decreases from a high of 0.7 for 5- to 30-year-old forests to about 0.2 for mature forests (deciduous and coniferous). Summer net radiation normalized to incoming solar radiation is lower in successional forests than in more mature forests by about 10%, except for the first 1–3 years after fire. This reduction in net radiative forcing is about 12–24 W m−2 as a daily average in summer (July). The summertime daily Bowen ratio exceeds 2 immediately after the fire, decreasing to about 0.5 for 15-year-old forests, with a wide range of 0.3–2 for mature forests depending on the forest type and soil water status. The magnitude of these changes is relatively large and may affect local, regional and perhaps global climates. Although fire has always determined stand renewal in these forests, increased future area burned could further alter the radiation balance and energy partitioning, causing a cooling feedback to counteract possible warming from carbon dioxide released by boreal fires.
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.
Balshi, M. S., McGuire, A. D., Duffy, P., Flannigan, M., Walsh, J., Melillo, J. (2009). Assessing the response of area burned to changing climate in western boreal North America using a Multivariate Adaptive Regression Splines (MARS) approach. Global Change Biology 15 (3): 578-600
ABSTRACT: Fire is a common disturbance in the North American boreal forest that influences ecosystem structure and function. The temporal and spatial dynamics of fire are likely to be altered as climate continues to change. In this study, we ask the question: how will area burned in boreal North America by wildfire respond to future changes in climate? To evaluate this question, we developed temporally and spatially explicit relationships between air temperature and fuel moisture codes derived from the Canadian Fire Weather Index System to estimate annual area burned at 2.5° (latitude × longitude) resolution using a Multivariate Adaptive Regression Spline (MARS) approach across Alaska and Canada. Burned area was substantially more predictable in the western portion of boreal North America than in eastern Canada. Burned area was also not very predictable in areas of substantial topographic relief and in areas along the transition between boreal forest and tundra. At the scale of Alaska and western Canada, the empirical fire models explain on the order of 82% of the variation in annual area burned for the period 1960–2002. July temperature was the most frequently occurring predictor across all models, but the fuel moisture codes for the months June through August (as a group) entered the models as the most important predictors of annual area burned. To predict changes in the temporal and spatial dynamics of fire under future climate, the empirical fire models used output from the Canadian Climate Center CGCM2 global climate model to predict annual area burned through the year 2100 across Alaska and western Canada. Relative to 1991–2000, the results suggest that average area burned per decade will double by 2041–2050 and will increase on the order of 3.5–5.5 times by the last decade of the 21st century. To improve the ability to better predict wildfire across Alaska and Canada, future research should focus on incorporating additional effects of long-term and successional vegetation changes on area burned to account more fully for interactions among fire, climate, and vegetation dynamics.
Balshi, M. S., Mcguire, A. D., Zhuang, Q., Melillo, J., Kicklighter, D. W., Kasischke, E., Wirth, C., Flannigan, M., Harden, J., Clein, J. S., Burnside, T. J., Mcallister, J., Kurz, W. A., Apps, M., Shvidenko, A. (2007). The role of historical fire disturbance in the carbon dynamics of the pan-boreal region: A process-based analysis. Journal of Geophysical Research-Biogeosciences 112 (G02029): doi:10.1029/2006JG000380
ABSTRACT: Wildfire is a common occurrence in ecosystems of northern high latitudes, and changes in the fire regime of this region have consequences for carbon feedbacks to the climate system. To improve our understanding of how wildfire influences carbon dynamics of this region, we used the process-based Terrestrial Ecosystem Model to simulate fire emissions and changes in carbon storage north of 45°N from the start of spatially explicit historically recorded fire records in the twentieth century through 2002, and evaluated the role of fire in the carbon dynamics of the region within the context of ecosystem responses to changes in atmospheric CO2 concentration and climate. Our analysis indicates that fire plays an important role in interannual and decadal scale variation of source/sink relationships of northern terrestrial ecosystems and also suggests that atmospheric CO2 may be important to consider in addition to changes in climate and fire disturbance. There are substantial uncertainties in the effects of fire on carbon storage in our simulations. These uncertainties are associated with sparse fire data for northern Eurasia, uncertainty in estimating carbon consumption, and difficulty in verifying assumptions about the representation of fires that occurred prior to the start of the historical fire record. To improve the ability to better predict how fire will influence carbon storage of this region in the future, new analyses of the retrospective role of fire in the carbon dynamics of northern high latitudes should address these uncertainties.
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.
Bergeron, O., Margolis, H. A., Coursolle, C., Giasson, M. (2008). How does forest harvest influence carbon dioxide fluxes of black spruce ecosystems in eastern North America?. Agricultural and Forest Meteorology 148 (4): 537-548
ABSTRACT: Forest harvest is a major disturbance in the boreal forest of eastern North America and should significantly impact biosphere–atmosphere interactions at the regional scale. During a cooler, wetter year (2004) and a warmer, drier year (2005), we compared carbon dioxide (CO2 ) fluxes over a mature black spruce stand (EOBS) with a site that was harvested in 2000 (HBS00) and that had similar soil parent material, site fertility, climate, and pre-harvest species composition. During the study period, EOBS was C neutral (0–6 g C m−2 year−1 ), while HBS00 was a fairly strong C source (119–167 g C m−2 year−1 ). Averaged over both years, gross ecosystem productivity (GEP) was 101% higher at EOBS compared to HBS00 (636 g C m−2 year−1 versus 316 g C m−2 year−1 ), while ecosystem respiration (R) was 37% higher at EOBS (633 g Cm−2 year−1 versus 462 g C m−2 year−1 ). The mean between-site difference in annual NEP was six times greater than was the mean between-year difference, thus suggesting that the C budget of boreal black spruce forests is much more affected by developmental stage (i.e., stand age) than by between-year climate variability. Relative to the mature site, the harvested site had a more dynamic structure due to plant regrowth that induced greater between- and within-year variability in the response of GEP and R to environmental conditions over the 2-year study period. For example, maximum photosynthetic capacity was stable between years at EOBS (12.5μmol m−2 s−1 ), whereas it increased from 4.7 to 7.3μmol m−2 s−1 from 2004 to 2005 at HBS00. The photosynthetically active growing season started about a week later and finished a week earlier at HBS00 relative to EOBS. The earlier snowmelt at the harvested site did not promote an earlier start of the growing season at this site compared to the mature site. Although environmental conditions in spring have a significant influence on the annual C budget of mature sites, this does not seem to be the case for disturbed sites where mid-summer conditions are more important to the annual C balance.
Bhatti, J. S., Apps, M. J., Jiang, H. (2002). Influence of nutrients, disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada. Plant And SoilPlant Soil 242 (1): 1-14
ABSTRACT: The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes–-from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by age-class structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.
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.
ABSTRACT: Changes in climate, atmospheric carbon dioxide concentration and fire regimes have been occurring for decades in the global boreal forest1, 2, 3 , with future climate change likely to increase fire frequency4 —the primary disturbance agent in most boreal forests3, 5. Previous attempts to assess quantitatively the effect of changing environmental conditions on the net boreal forest carbon balance have not taken into account the competition between different vegetation types on a large scale6, 7, 8, 9 . Here we use a process model with three competing vascular and non-vascular vegetation types to examine the effects of climate, carbon dioxide concentrations and fire disturbance on net biome production, net primary production and vegetation dominance in 100 Mha of Canadian boreal forest. We find that the carbon balance of this region was driven by changes in fire disturbance from 1948 to 2005. Climate changes affected the variability, but not the mean, of the landscape carbon balance, with precipitation exerting a more significant effect than temperature. We show that more frequent and larger fires in the late twentieth century resulted in deciduous trees and mosses increasing production at the expense of coniferous trees. Our model did not however exhibit the increases in total forest net primary production that have been inferred from satellite data1, 10 . We find that poor soil drainage decreased the variability of the landscape carbon balance, which suggests that increased climate and hydrological changes have the potential to affect disproportionately the carbon dynamics of these areas. Overall, we conclude that direct ecophysiological changes resulting from global climate change have not yet been felt in this large boreal region. Variations in the landscape carbon balance and vegetation dominance have so far been driven largely by increases in fire frequency.
Botting, R. S., Fredeen, A. L. (2006). Net ecosystem CO2 exchange for moss and lichen dominated forest floors of old-growth sub-boreal spruce forests in central British Columbia, Canada. Forest Ecology and Management 235 (1-3): 240-251
ABSTRACT: This study used instantaneous chamber-based CO2 exchange measurements (2004) in conjunction with a seasonal record of microclimate (2003) to model growing season forest floor net ecosystem CO2 exchange (ffNEE) for terrestrial bryophyte and lichen communities in sub-boreal forests in central British Columbia, Canada. Multiple regression models using microclimate variables described between 35 and 53% of the variation in ffNEE for moss or lichen dominated forest floor at an ambient CO2 concentration. Light and moss or lichen moisture and temperature were all important variables in describing ffNEE from moss and lichen dominated forest floor patches while substrate temperature was the most important variable explaining ffNEE from bare litter + soil and wood. Moss dominated forest floor had relatively invariant mean diel ffNEE across the 3-month growing season while lichen dominated wood had low summer ffNEE which increased in September. Over a 3-month growing season in 2003, moss dominated forest floor had a total ffNEE of -33.8 g C m-2 and lichen dominated wood had a total ffNEE of -42.9 g C m-2 . When ffNEE values from the moss, lichen, bare wood, and bare litter + soil components of the forest floor community were summed over the 3-month period, the old-growth sub-boreal spruce forest floor had a net CO2 exchange of -31.6 g C m-2 , representing a loss of this amount of carbon over the growing season. The moss dominated, but not lichen dominated, forest floor appeared limited by ambient forest floor CO2 levels (430m mol CO2 mol-1) and exhibited increased photosynthesis at elevated CO2 (700m mol CO2 mol-1 ).
ABSTRACT: During the spring and summer of 1994 we monitored soil-atmosphere exchanges of methane and carbon dioxide at upland sites in the Canadian boreal forest near the northern study area (NSA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). The effects of fire on methane and carbon dioxide exchange in black spruce stands developed on clay soils were evaluated by measuring fluxes with dark chambers in unburned stands and stands burned in 1994, 1992, and 1987. Similar measurements were made in jack pine stands developed on sandy soils, one unburned and the other burned in 1989. All of the sites were net sinks of atmospheric methane with median fluxes ranging from −0.3 to −1.4 mg CH4 -C m−2 d−1 . Median fluxes of carbon dioxide from the forest floor to the atmosphere ranged between 1 and 2 g C m−2 d−1 . Both ecosystem characteristics (e.g., soil and vegetation type) and burning history (time since burn and fire intensity) appear to have some effect on atmospheric methane consumption and carbon dioxide emission by these forest soils. In general, the jack pine sites were stronger methane sinks and had lower carbon dioxide emissions than the black spruce sites. After a few years of recovery, the burned sites tended to be slightly stronger methane sinks than unburned controls. Our results suggest that soil CO2 effluxes from upland black spruce stands may not be immediately impacted by fire, possibly maintained at preburn levels by microbial decomposition of labile compounds released as a result of the fire. By 2 years postfire there appears to be a significant reduction in soil CO2 flux, due to the loss of tree root and moss respiration and possibly to the depletion of fire-related labile compounds. The observed recovery of soil respiration rates to preburn levels by 7 years postburn is probably due to the respiration of regrowing vegetation and the combined effects of elevated soil temperatures (about 4° to 5°C warmer than unburned sites) and improved litter quality on soil microbial activities. We estimate that soil CO2 emissions from recently burned boreal forest soils in the northern hemisphere could be of the order of 0.35 Pg C yr−1 , which is in good agreement with a previous estimate that was derived in a different manner.
Callaghan, T. V., Bjorn, L. O., Chernov, Y., Chapin, T., Christensen, T. R., Huntley, B., Ims, R. A., Johansson, M., Jolly, D., Jonasson, S., Matveyeva, N., Panikov, N., Oechel, W., Shaver, G. R. (2004). Effects on the function of Arctic ecosystems in the short- and long-term perspectives. Ambio 33 (7): 448-458
ABSTRACT: Historically, the function of Arctic ecosystems in terms of cycles of nutrients and carbon has led to low levels of primary production and exchanges of energy, water and greenhouse gases have led to low local and regional cooling. Sequestration of carbon from atmospheric CO2 , in extensive, cold organic soils and the high albedo from low, snow-covered vegetation have had impacts on regional climate. However, many aspects of the functioning of Arctic ecosystems are sensitive to changes in climate and its impacts on biodiversity. The current Arctic climate results in slow rates of organic matter decomposition. Arctic ecosystems therefore tend to accumulate organic matter and elements despite low inputs. As a result, soil-available elements like nitrogen and phosphorus are key limitations to increases in carbon fixation and further biomass and organic matter accumulation. Climate warming is expected to increase carbon and element turnover, particularly in soils, which may lead to initial losses of elements but eventual, slow recovery. Individual species and species diversity have clear impacts on element inputs and retention in Arctic ecosystems. Effects of increased CO2 and UV-B on whole ecosystems, on the other hand, are likely to be small although effects on plant tissue chemisty, decomposition and nitrogen fixation may become important in the long-term. Cycling of carbon in trace gas form is mainly as CO2 and CH4 . Most carbon loss is in the form of CO2 , produced by both plants and soil biota. Carbon emissions as methane from wet and moist tundra ecosystems are about 5% of emissions as CO2 and are responsive to warming in the absence of any other changes. Winter processes and vegetation type also affect CH4 emissions as well as exchanges of energy between biosphere and atmosphere. Arctic ecosystems exhibit the largest seasonal changes in energy exchange of any terrestrial ecosystem because of the large changes in albedo from late winter, when snow reflects most incoming radiation, to summer when the ecosystem absorbs most incoming radiation. Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. Albedo during the period of snow cover declines from tundra to forest tundra to deciduous forest to evergreen forest. Shrubs and trees increase snow depth which in turn increases winter soil temperatures. Future changes in vegetation driven by climate change are therefore, very likely to profoundly alter regional climate.
Clein, J. S., McGuire, A. D., Zhang, X., Kicklighter, D. W., Melillo, J. M., Wofsy, S. C., Jarvis, P. G., Massheder, J. M. (2002). Historical and projected carbon balance of mature black spruce ecosystems across North America: the role of carbon-nitrogen interactions. Plant And SoilPlant Soil 242 (1): 15-32
ABSTRACT: The role of carbon (C) and nitrogen (N) interactions on sequestration of atmospheric CO2 in black spruce ecosystems across North America was evaluated with the Terrestrial Ecosystem Model (TEM) by applying parameterizations of the model in which C–N dynamics were either coupled or uncoupled. First, the performance of the parameterizations, which were developed for the dynamics of black spruce ecosystems at the Bonanza Creek Long-Term Ecological Research site in Alaska, were evaluated by simulating C dynamics at eddy correlation tower sites in the Boreal Ecosystem Atmosphere Study (BOREAS) for black spruce ecosystems in the northern study area (northern site) and the southern study area (southern site) with local climate data. We compared simulated monthly growing season (May to September) estimates of gross primary production (GPP), total ecosystem respiration (RESP), and net ecosystem production (NEP) from 1994 to 1997 to available field-based estimates at both sites. At the northern site, monthly growing season estimates of GPP and RESP for the coupled and uncoupled simulations were highly correlated with the field-based estimates (coupled: R2 = 0.77, 0.88 for GPP and RESP; uncoupled: R2 = 0.67, 0.92 for GPP and RESP). Although the simulated seasonal pattern of NEP generally matched the field-based data, the correlations between field-based and simulated monthly growing season NEP were lower (R2 = 0.40, 0.00 for coupled and uncoupled simulations, respectively) in comparison to the correlations between field-based and simulated GPP and RESP. The annual NEP simulated by the coupled parameterization fell within the uncertainty of field-based estimates in two of three years. On the other hand, annual NEP simulated by the uncoupled parameterization only fell within the field-based uncertainty in one of three years. At the southern site, simulated NEP generally matched field-based NEP estimates, and the correlation between monthly growing season field-based and simulated NEP (R2 = 0.36, 0.20 for coupled and uncoupled simulations, respectively) was similar to the correlations at the northern site. To evaluate the role of N dynamics in C balance of black spruce ecosystems across North America, we simulated historical and projected C dynamics from 1900 to 2100 with a global-based climatology at 0.5° resolution (latitude × longitude) with both the coupled and uncoupled parameterizations of TEM. From analyses at the northern site, several consistent patterns emerge. There was greater inter-annual variability in net primary production (NPP) simulated by the uncoupled parameterization as compared to the coupled parameterization, which led to substantial differences in inter-annual variability in NEP between the parameterizations. The divergence between NPP and heterotrophic respiration was greater in the uncoupled simulation, resulting in more C sequestration during the projected period. These responses were the result of fundamentally different responses of the coupled and uncoupled parameterizations to changes in CO2 and climate.
Dunn, A. L., Barford, C. C., Wofsy, S. C., Goulden, M. L., Daube, B. C. (2007). A long-term record of carbon exchange in a boreal black spruce forest: means, responses to interannual variability, and decadal trends. Global Change Biology 13 (3): 577-590
ABSTRACT: We present a decadal (1994–2004) record of carbon dioxide flux in a 160-year-old black spruce forest/veneer bog complex in central Manitoba, Canada. The ecosystem shifted from a source (+41 g C m−2 , 1995) to a sink (−21 g C m−2 , 2004) of CO2 over the decade, with an average net carbon balance near zero. Annual mean temperatures increased 1–2° during the period, consistent with the decadal trend across the North American boreal biome. We found that ecosystem carbon exchange responded strongly to air temperature, moisture status, potential evapotranspiration, and summertime solar radiation. The seasonal cycle of ecosystem respiration significantly lagged that of photosynthesis, limited by the rate of soil thaw and the slow drainage of the soil column. Factors acting over long time scales, especially water table depth, strongly influenced the carbon budget on annual time scales. Net uptake was enhanced and respiration inhibited by multiple years of rainfall in excess of evaporative demand. Contrary to expectations, we observed no correlation between longer growing seasons and net uptake, possibly because of offsetting increases in ecosystem respiration. The results indicate that the interactions between soil thaw and water table depth provide critical controls on carbon exchange in boreal forests underlain by peat, on seasonal to decadal time scales, and these factors must be simulated in terrestrial biosphere models to predict response of these regions to future climate.
ABSTRACT: The area burned in the North American boreal forest is controlled by the frequency of midtropospheric blocking highs that cause rapid fuel drying. Climate controls the area burned through changing the dynamics of large-scale teleconnection patterns (Pacific Decadal Oscillation/El Niño Southern Oscillation and Arctic Oscillation, PDO/ENSO and AO) that control the frequency of blocking highs over the continent at different time scales. Changes in these teleconnections may be caused by the current global warming. Thus, an increase in temperature alone need not be associated with an increase in area burned in the North American boreal forest. Since the end of the Little Ice Age, the climate has been unusually moist and variable: large fire years have occurred in unusual years, fire frequency has decreased and fire–climate relationships have occurred at interannual to decadal time scales. Prolonged and severe droughts were common in the past and were partly associated with changes in the PDO/ENSO system. Under these conditions, large fire years become common, fire frequency increases and fire–climate relationships occur at decadal to centennial time scales. A suggested return to the drier climate regimes of the past would imply major changes in the temporal dynamics of fire–climate relationships and in area burned, a reduction in the mean age of the forest, and changes in species composition of the North American boreal forest.
SUMMARY: On Independence Day, 1999, a storm system that originated over the Gulf of Mexico and passed through North Dakota dealt a severe blow to nearly half a million acres of the Superior National Forest in northern Minnesota. The blowdown, or derecho, packed winds exceeding 90 miles per hour and left in its wake downed and damaged trees and a dangerously high fuel load. Nearly half a million acres of forest were affected, primarily in the Boundary Waters Canoe Area Wilderness (BWCAW) and the Gunflint Trail Corridor, a strip of land in public and private ownership that supports a thriving tourist trade in the world’s premier canoe wilderness. Immediately after the blowdown, the Forest Service began to implement a strategy to reduce risks to life and property in the corridor. Within the BWCAW, some exceptions to the Wilderness Act regulating human activity in primitive areas were allowed in accordance with the Forest Service mandate to ensure wildfire does not exit the wilderness. Prescribed fire applied on strategic sites at the boundary of the wilderness area later proved successful at containing a wildfire in 2006, but a second, human-caused fire in 2007 caused significant damage to buildings in the corridor. A number of research projects comparing treatments allowed in the corridor, including salvage logging and prescribed fire, helped guide the long-term management plan for the area. Results are not always clear, however, and managers have to consider a number of tradeoffs, balancing the risks to life and property versus the overall health of an ecosystem and the flora and fauna that have evolved along with moderate to severe fire with a return interval of approximately 70 years. Moreover, as the climate warms, managers may confront more-frequent severe weather events that will challenge their ability to respond.
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%.
ABSTRACT: Forests are an important source for fiber and fuel for humans and contain the majority of the total terrestrial carbon (C). The amount of C stored in the vegetation and soil are strongly influenced by environmental constraints on annual C uptake and decomposition and time since disturbance. Increasing concentrations of atmospheric carbon dioxide (CO2 ), nitrogen deposition, and climate warming induced by greater greenhouse gas (GHG) concentrations in the atmosphere influence C accumulation rates of forests, but their effects will likely differ in direction and magnitude among forest ecosystems. The net interactive effect of global change on the forest C cycle is poorly understood. The growing demand for wood fiber and fuel by humans and the ongoing anthropogenic perturbations of the climate have changed the natural disturbance regimes (i.e., frequency and intensity); these changes influence the net exchange of CO2 between forests and the atmosphere. To date, the role of forest products in the global C cycle have largely been ignored, and important emissions associated with the production, transport, and utilization of the forest products have been excluded, leading to erroneous conclusions about net C storage in forest products.
Grant, R.F., Black, T.A., Gaumont-Guay, D., Klujn, N., Barr, A.G., Morgenstern, K., Nesic, Z. (2006). Net ecosystem productivity of boreal aspen forests under drought and climate change: Mathematical modelling with Ecosys. Agricultural and Forest Meteorology 140 (1-4): 152-170
ABSTRACT: The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration (Ea ) and precipitation (Pa ). Changes in Ea versus Pa during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of Ea versus Pa on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO2 uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO2 and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO2 effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO2 effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO2 exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in g C m−2 : 275 versus 367 ± 110 in 2001, 82 versus 144 ± 43 in 2002 and 23 versus 104 ± 31 in 2003). Lower modelled NEP was attributed to the larger modelled CO2 effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP = NEP − C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4 g C m−2 year−1 . NBP rose by 10, 108 and 126 g C m−2 year−1 during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP.
Harden, J. W., Mack, M., Veldhuis, H., Gower, S. T. (2002). Fire dynamics and implications for nitrogen cycling in boreal forests. Journal of Geophysical Research - Atmospheres 107 (8223): doi:10.1029/2001JD000494
ABSTRACT: We used a dynamic, long-term mass balance approach to track cumulative carbon (C) and nitrogen (N) losses to fire in boreal Manitoba over the 6500 years since deglaciation. Estimated C losses to decomposition and fire, combined with measurements of N pools in mature and burned forest floors, suggest that loss of N by combustion has likely resulted in a long-term loss that exceeds the amount of N stored in soil today by 2 to 3 times. These estimates imply that biological N fixation rates could be as high as 5 to 10 times atmospheric deposition rates in boreal regions. At the site scale, the amount of N lost is due to N content of fuels, which varies by stand type and fire severity, which in turn vary with climate and fire dynamics. The interplay of fire frequency, fire severity, and N partitioning during regrowth are important for understanding rates and sustainability of nutrient and carbon cycling over millenia and over broad regions.
ABSTRACT: To reconcile observations of decomposition rates, carbon inventories, and net primary production (NPP), we estimated long-term averages for C exchange in boreal forests near Thompson, Manitoba. Soil drainage as defined by water table, moss cover, and permafrost dynamics, is the dominant control on direct fire emissions. In upland forests, an average of about 10–30% of annual NPP was likely consumed by fire over the past 6500 years since these landforms and ecosystems were established. This long-term, average fire emission is much larger than has been accounted for in global C cycle models and may forecast an increase in fire activity for this region. While over decadal to century times these boreal forests may be acting as slight net sinks for C from the atmosphere to land, periods of drought and severe fire activity may result in net sources of C from these systems.
Hazlett, P. W., Gordon, A. M., Sibley, P. K., Buttle, J. M. (2005). Stand carbon stocks and soil carbon and nitrogen storage for riparian and upland forests of boreal lakes in northeastern Ontario. Forest Ecology and Management 219 (1): 56-68
ABSTRACT: The establishment of shoreline reserves (buffer strips) has guided riparian forest management in Ontario for many years. A riparian area is defined as the transitional zone between the aquatic and terrestrial environments and therefore is also known as the aquatic/terrestrial ecotone. While many functions of riparian forests have been recognized and well studied, less is known about their potential to sequester C and whether this potential differs from other areas in the boreal forest landscape. Increased harvesting pressure due to decreased wood supply in Ontario and debate about the effectiveness of the current reserve guidelines has resulted in a renewed interest in harvesting riparian forests. In this study riparian and upslope forest C and soil C and N storage were quantified for 21 lakes shorelines at the Esker Lakes Research Area, a boreal forest ecosystem in northeastern Ontario, Canada. Objectives were to compare the C and N storage potential of riparian forests with those of adjacent upland forests, and to examine the potential impacts of harvesting on C stocks in riparian zones of the boreal forest.
Riparian forests did not differ from upslope stands in terms of total aboveground overstory C storage although there were significant differences in stocking density and species composition. However, a greater proportion of total site C in riparian areas was stored in the overstory tree layer (>5 cm dbh) compared to upslope areas. Forest floor layers were deeper and stored more C and N in riparian forest stands in comparison to upslope stands. In contrast, mineral soil in upslope stands had greater C and N storage than mineral soil horizons within the riparian forest. As a result, the riparian organic horizons comprise a larger percentage of the overall soil storage of C and N than upslope layers. Currently practiced full-tree harvesting would result in a removal of approximately 76% of total aboveground C (17% of the ecosystem C) in upslope stands compared to 98% of total aboveground C (35% of the ecosystem C) in riparian forests. Selective or modified harvesting in riparian zones could decrease C removal to levels equal to that obtained by full-tree harvesting in upslope areas.
ABSTRACT: We calculated carbon budgets for a chronosequence of harvested jack pine (Pinus banksiana Lamb.) stands (0-, 5-, 10-, and ~29-year-old) and a ~79-year-old stand that originated after wildfire. We measured total ecosystem C content (TEC), above-, and belowground net primary productivity (NPP) for each stand. All values are reported in order for the 0-, 5-, 10-, 29-, and 79-year-old stands, respectively, for May 1999 through April 2000. Total annual NPP (NPPT) for the stands (Mg C ha−1 yr−1 ±1 SD) was 0.9±0.3, 1.3±0.1, 2.7±0.6, 3.5±0.3, and 1.7±0.4. We correlated periodic soil surface CO2 fluxes (RS) with soil temperature to model annual RS for the stands (Mg C ha−1 yr−1 ±1 SD) as 4.4±0.1, 2.4±0.0, 3.3±0.1, 5.7±0.3, and 3.2±0.2. We estimated net ecosystem productivity (NEP) as NPPT minus RH (where RH was calculated using a Monte Carlo approach as coarse woody debris respiration plus 30–70% of total annual RS). Excluding C losses during wood processing, NEP (Mg C ha−1 yr−1 ±1 SD) for the stands was estimated to be −1.9±0.7, −0.4±0.6, 0.4±0.9, 0.4±1.0, and −0.2±0.7 (negative values indicate net sources to the atmosphere.) We also calculated NEP values from the changes in TEC among stands. Only the 0-year-old stand showed significantly different NEP between the two methods, suggesting a possible mismatch for the chronosequence. The spatial and methodological uncertainties allow us to say little for certain except that the stand becomes a source of C to the atmosphere following logging.
Ilvesniemi, H., Kahkonen, M. A., Pumpanen, J., Rannik, U., Wittmann, C., Peramaki, M., Keronen, P., Hari, P., Vesala, T., Salkinoja-Salonen, M. (2005). Wintertime CO2 evolution from a boreal forest ecosystem. Boreal Environment Research 10 (5): 401-408
ABSTRACT: We investigated wintertime ecosystem activity and CO2 efflux over three winters (1 November–28 February 1997–2000) in a boreal Scots pine stand in Finland. During the three winters the cumulative wintertime CO2 efflux measured with continuously operating soil chambers directly from the soil surface was between 103 and 144 g m–2 , and between 240 and 330 g m–2 when measured by an eddy covariance method or estimated from the soil sample endogenous CO2 production. The flux measured directly from the soil surface is probably an underestimation due to the ice formation within the chamber. Photosynthesis was found to be active also during winter and metabolic activity was found to show extrapolated zero at –5 °C to –10 °C.
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.
A. R. Keyser, J. S. Kimball, R. R. Nemani, S. W. Running (2000). Simulating the effects of climate change on the carbon balance of North American high-latitude forests. Global Change Biology 6 (S1): 185-195
ABSTRACT: The large magnitude of predicted warming at high latitudes and the potential feedback of ecosystems to atmospheric CO2 concentrations make it important to quantify both warming and its effects on high-latitude carbon balance. We analysed long-term, daily surface meteorological records for 13 sites in Alaska and north-western Canada and an 82-y record of river ice breakup date for the Tanana River in interior Alaska. We found increases in winter and spring temperature extrema for all sites, with the greatest increases in spring minimum temperature, average 0.47 °C per 10 y, and a 0.7-day per 10 y advance in ice breakup on the Tanana River. We used the climate records to drive an ecosystem process model, BIOME_BGC, to simulate the effects of climate change on the carbon and water balances of boreal forest ecosystems. The growing season has lengthened by an average of 2.6 days per 10 y with an advance in average leaf onset date of 1.10 days per 10 y. This advance in the start of the active growing season correlates positively with progressively earlier ice breakup on the Tanana River in interior Alaska. The advance in the start of the growing season resulted in a 20% increase in net primary production for both aspen (Populus tremuloides ) and white spruce (Picea glauca ) stands. Aspen had a greater mean increase in maintenance respiration than spruce, whereas spruce had a greater mean increase in evapotranspiration. Average decomposition rates also increased for both species. Both net primary production and decomposition are enhanced in our simulations, suggesting that productive forest types may not experience a significant shift in net carbon flux as a result of climate warming.
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.
ABSTRACT: Despite growing evidence for an effect of species composition on carbon (C) storage and sequestration, few projects have examined the implications of such a relationship for forestry and agriculture-based climate change mitigation activities. We worked with a community in Eastern Panama to determine the average above- and below-ground C stocks of three land-use types in their territory: managed forest, agroforests and pasture. We examined evidence for a functional relationship between tree-species diversity and C storage in each land-use type, and also explored how the use of particular tree species by community members could affect C storage. We found that managed forests in this landscape stored an average of 335 Mg C ha−1 , traditional agroforests an average of 145 Mg C ha−1 , and pastures an average of 46 Mg C ha−1 including all vegetation-based C stocks and soil C to 40 cm depth. We did not detect a relationship between diversity and C storage; however, the relative contributions of species to C storage per hectare in forests and agroforests were highly skewed and often were not proportional to species’ relative abundances. We conclude that protecting forests from conversion to pasture would have the greatest positive impact on C stocks, even though the forests are managed by community members for timber and non-timber forest products. However, because several of the tree species that contribute the most to C storage in forests were identified by community members as preferred timber species, we suggest that species-level management will be important to avoiding C-impoverishment through selective logging in these forests. Our data also indicate that expanding agroforests into areas currently under pasture could sequester significant amounts of carbon while providing biodiversity and livelihood benefits that the most common reforestation systems in the region – monoculture teak plantations – do not provide.
Dan Berggren Kleja, Magnus Svensson, Hooshang Majdi, Per-Erik Jansson, Ola Langvall, Bo Bergkvist, Maj-Britt Johansson, Per Weslien, Laimi Truusb, Anders Lindroth, Göran I. Ågren (2007). Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden. Biogeochemistry 89 (1): 7-25
ABSTRACT: This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m−2 year−1 ). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m−2 year−1 in the north to +9 g C m−2 year−1 in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m−2 year−1 .
Krishnan, P., Black, T. A., Grant, N. J., Barr, A. G., Hogg, E. T. H., Jassal, R. S., Morgenstern, K. (2006). Impact of changing soil moisture distribution on net ecosystem productivity of a boreal aspen forest during and following drought. Agricultural and Forest Meteorology 139 (3-4): 208-223
ABSTRACT: The interannual and seasonal variability of gross ecosystem photosynthesis (P), ecosystem respiration (R) and evapotranspiration (E), and their relationships to environmental factors were used to explain changes in net ecosystem productivity (FNEP) at the onset of, during, and following a 3-year-long (2001–2003) drought in a mature boreal aspen stand in central Saskatchewan, Canada. The forest was a moderate carbon (C) sink over its entire 11-year data record (FNEP = 153 ± 99 g C m−2 year−1 ), including the peak drought years of 2002 and 2003. In 2001, the depletion of water near the soil surface likely reduced heterotrophic soil respiration while water remaining deep in the root zone maintained P above the pre-drought mean, resulting in above-average FNEP. In 2002 and 2003, the forest remained a C sink even though P was below average because R was also below average—a likely consequence of the influence of low soil water content in deep and shallow soil layers on both autotrophic and heterotrophic respiration. In 2004, the recharge of soil water in shallow soil layers allowed R to recover to its pre-drought values, whereas low spring temperatures, the slow recharge of soil water in deep soil layers in spring, late leaf emergence and diminished leaf area index combined to suppress P and produce the lowest annual FNEP of the 11-year record (4 g C m−2 year−1 ). The low FNEP and P were mirrored in the lowest stem growth and LAI values of the 11-year record. In 2005, a warm wet year, both the annual values and seasonal variations of FNEP, P and R returned to those of pre-drought years; the partial recovery of LAI to pre-drought values suggests that aspen P was able to adjust to this restriction on C assimilation. Growing season average dry surface conductance (gsv), the Priestley–Taylor coefficient (α) and light use efficiency (LUE) reached their lowest values in 2003 and became similar to pre-drought years in 2004–2005. Water use efficiency (WUE) was highest in 2003 and remained above average in 2004 and 2005. At the ecosystem scale, the above-average gains made in C sequestration in the first year of the drought were significantly offset by below-average stand FNEP in the final 2 years of the drought, and in the year following the drought.
W. A. Kurz, G. Stinson, G. J. Rampley, C. C. Dymond, E. T. Neilson (2008). Risk of natural disturbances makes future contribution of Canada's forests to the global carbon cycle highly uncertain. Proceedings of the National Academy of Sciences 105 (5): 1551-1555
ABSTRACT: A large carbon sink in northern land surfaces inferred from global carbon cycle inversion models led to concerns during Kyoto Protocol negotiations that countries might be able to avoid efforts to reduce fossil fuel emissions by claiming large sinks in their managed forests. The greenhouse gas balance of Canada's managed forest is strongly affected by naturally occurring fire with high interannual variability in the area burned and by cyclical insect outbreaks. Taking these stochastic future disturbances into account, we used the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to project that the managed forests of Canada could be a source of between 30 and 245 Mt CO2 e yr−1 during the first Kyoto Protocol commitment period (2008–2012). The recent transition from sink to source is the result of large insect outbreaks. The wide range in the predicted greenhouse gas balance (215 Mt CO2 e yr−1 ) is equivalent to nearly 30% of Canada's emissions in 2005. The increasing impact of natural disturbances, the two major insect outbreaks, and the Kyoto Protocol accounting rules all contributed to Canada's decision not to elect forest management. In Canada, future efforts to influence the carbon balance through forest management could be overwhelmed by natural disturbances. Similar circumstances may arise elsewhere if global change increases natural disturbance rates. Future climate mitigation agreements that do not account for and protect against the impacts of natural disturbances, for example, by accounting for forest management benefits relative to baselines, will fail to encourage changes in forest management aimed at mitigating climate change.
ABSTRACT: Assessments made over the past few decades have suggested that boreal forests may act as a sink for atmospheric carbon dioxide. However, the fate of the newly accumulated carbon in the living forest biomass is not well understood, and the estimates of carbon sinks vary greatly from one assessment to another. Analysis of remote sensing data has indicated that the carbon sinks in the Russian forests are larger than what has been estimated from forest inventories. In this study, we show that over the past four decades, the allometric relationships among various plant parts have changed in the Russian forests. To this end, we employ two approaches: (1) analysis of the database, which contains 3196 sample plots; and (2) application of developed models to forest inventory data. Within the forests as a whole, when assessed at the continental scale, we detect a pronounced increase in the share of green parts (leaves and needles). However, there is a large geographical variation. The shift has been largest within the European Russia, where summer temperatures and precipitation have increased. In the Northern Taiga of Siberia, where the climate has become warmer but drier, the fraction of the green parts has decreased while the fractions of aboveground wood and roots have increased. These changes are consistent with experiments and mathematical models that predict a shift of carbon allocation to transpiring foliage with increasing temperature and lower allocation with increasing soil drought. In light of this, our results are a possible demonstration of the acclimation of trees to ongoing warming and changes in the surface water balance. Independent of the nature of the observed changes in allometric ratios, the increase in the share of green parts may have caused a misinterpretation of the satellite data and a systematic overestimation by remote sensing methods of the carbon sink for living biomass of the Russian forest.
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.
S. Lavorel, M. D. Flannigan, E. F. Lambin, M. C. Scholes (2007). Vulnerability of land systems to fire: Interactions among humans, climate, the atmosphere, and ecosystems. Mitigation and Adaptation Strategies for Global Change 12 (1): 33-53
ABSTRACT: Fires are critical elements in the Earth System, linking climate, humans, and vegetation. With 200–500 Mha burnt annually, fire disturbs a greater area over a wider variety of biomes than any other natural disturbance. Fire ignition, propagation, and impacts depend on the interactions among climate, vegetation structure, and land use on local to regional scales. Therefore, fires and their effects on terrestrial ecosystems are highly sensitive to global change. Fires can cause dramatic changes in the structure and functioning of ecosystems. They have significant impacts on the atmosphere and biogeochemical cycles. By contributing significantly to greenhouse gas (e.g., with the release of 1.7–4.1 Pg of carbon per year) and aerosol emissions, and modifying surface properties, they affect not only vegetation but also climate. Fires also modify the provision of a variety of ecosystem services such as carbon sequestration, soil fertility, grazing value, biodiversity, and tourism, and can hence trigger land use change. Fires must therefore be included in global and regional assessments of vulnerability to global change. Fundamental understanding of vulnerability of land systems to fire is required to advise management and policy. Assessing regional vulnerabilities resulting from biophysical and human consequences of changed fire regimes under global change scenarios requires an integrated approach. Here we present a generic conceptual framework for such integrated, multidisciplinary studies. The framework is structured around three interacting (partially nested) subsystems whose contribute to vulnerability. The first subsystem describes the controls on fire regimes (exposure). A first feedback subsystem links fire regimes to atmospheric and climate dynamics within the Earth System (sensitivity), while the second feedback subsystem links changes in fire regimes to changes in the provision of ecological services and to their consequences for human systems (adaptability). We then briefly illustrate how the framework can be applied to two regional cases with contrasting ecological and human context: boreal forests of northern America and African savannahs.
ABSTRACT: The potential equilibrium response of Canadian vegetation under two doubled-CO2 climatic scenarios was investigated at three levels in the vegetation mosaic using the rule-based, Canadian Climate-Vegetation Model (CCVM) and climatic response surfaces. The climatic parameters employed as model drivers (i.e., degree-days, minimum temperature, snowpack, actual evapotranspiration, and soil moisture deficit) have a more direct influence on the distribution of vegetation than those commonly used in equilibrium models. Under both scenarios, CCVM predicted reductions in the extent of the tundra and subarctic woodland formations, a northward shift and some expansion in the distributions of boreal and the temperate forest, and an expansion of the dry woodland and prairie formations that was especially pronounced under one of the scenarios. Results of the response surface analysis suggest the potential for significant changes in the probability of dominance for eight boreal tree species. A dissimilarity coefficient was used to identify forest-types under the future climatic scenarios that were analogous to boreal forest-types derived from cluster analysis of the current probabilities of species dominance. All of the current forest-types persisted under the doubled-CO2 scenarios, but no-analog areas were also identified within which an empirically derived threshold of the distance coefficient was exceeded. Maps showing the highest level in the vegetation hierarchy where change was predicted suggest the relative impact of the response under the two climatic scenarios.
Liski, J., Lehtonen, A., Palosuo, T., Peltoniemi, M., Eggers, T., Muukkonen, P., Makipaa, R. (2006). Carbon accumulation in Finland's forests 1922-2004 - an estimate obtained by combination of forest inventory data with modelling of biomass, litter and soil. Annals of Forest Science 63 (7): 687-697
ABSTRACT: Comparable regional scale estimates for the carbon balance of forests are needed for scientific and political purposes. We developed a method for deriving these estimates from readily available forest inventory data by using statistical biomass models and dynamic modelling of litter and soil. Here, we demonstrate this method and apply it to Finland's forests between 1922 and 2004. The method was reliable, since the results obtained were comparable to independent data. The amount of carbon stored in the forests increased by 29%, 79% of which was found in the biomass and 21% in the litter and soil. The carbon balance varied annually, depending on the climate and level of harvesting, with each of these factors having effects on the biomass differing from those on the litter and soil. Our results demonstrate the importance of accounting for all forest carbon pools to avoid misleading pictures of short- and long-term forest carbon balance.
ABSTRACT: For confidently estimating the amount of carbon stored in boreal forest soil, better knowledge of smaller regions is needed. In order to estimate the amount of soil C in forests on mineral soil in Finland, i.e. excluding peatland forests, and illustrate the regional patterns of the storage, statistical models were first made for the C densities of the organic and 0–1 m mineral soil layers. A forest type, which indicated site productivity, and the effective temperature sum were used as explanatory variables of the models. In addition, a constant C density was applied for the soil layer below the depth of 1 m on sorted sediments. Using these models the C densities were calculated for a total of 46673 sites of the National Forest Inventory (NFI). The amount of the soil C was then calculated in two ways: 1) weighting the C densities of the NFI sites by the land area represented by these sites and 2) interpolating the C densities of the NFI sites for 4 ha blocks to cover the whole land area of Finland and summing up the blocks on forested mineral soil. The soil C storage totalled 1109 Tg and 1315 Tg, when calculated by the areal weighting and the interpolated blocks, respectively. Of that storage, 28% was in the organic layer, 68% in the 0–1 m mineral soil layer and 4% in the layer below 1 m. The total soil C equals more than two times the amount of C in tree biomass and 20% of the amount of C in peat in Finland. Soil C maps made using the interpolated blocks indicated that the largest soil C reserves are located in central parts of southern Finland. The C storage of the organic layer was assessed to be overestimated at largest by 13% and that of the 0–1 m mineral soil layer by 29%. The largest error in the organic layer estimate is associated with the effects of forest harvesting and in the mineral soil estimate with the stone content of the soil.
ABSTRACT: A total of 30 coniferous forest sites representing two productivity classes, forest types, were investigated on a temperature gradient (effective temperature sum using +5°C threshold 800–1300 degree-days and annual mean temperature –0.6–+3.9°C) in Finland for studying the effect of thermoclimate on the soil C storage. Other soil forming factors were standardized within the forest types so that the variation in the soil C density could be related to temperature. According to the applied regression model, the C density of the 0–1 m mineral soil layer increased 0.266 kg m–2 for every 100 degree-day increase in the temperature sum, and the layer contained 57% and 28% more C under the warmest conditions of the gradient compared to the coolest in the less and more productive forest type, respectively. Accordingly, this soil layer was estimated to contain 23 more C in a new equilibrium with a 4°C higher annual mean temperature in Finland. The C density of the organic layer was not associated with temperature. Both soil layers contained more C at the sites of the more productive forest type, and the forest type explained 36% and 70% of the variation in the C density of the organic and 0–1 m layers, respectively. Within the forest types, the temperature sum accounted for 33–41% of the variation in the 0–1 m layer. These results suggest that site productivity is a cause for the large variation inthe soil C density within the boreal zone, and relating the soil C density to site productivity and temperature would help to estimate the soil C reserves more accurately in the boreal zone.
ABSTRACT: Mixedwood forests are an ecologically and economically important forest type in central Canada, but the ecology of these forests is not as well studied as that of single-species dominated stands in the boreal forest. Northern boreal mixedwood forests have only recently been harvested and the effects of harvesting on carbon content in these stands are unknown. We quantified the carbon content and aboveground net primary production (NPP) for four different-aged mixedwood boreal forest stands in northern Manitoba, Canada. The stands included 11-, 18-, and 30-year-old stands that originated from harvesting and a 65-year-old fire-originated stand that typifies the origin of all northern boreal mixed-wood forests that are coming under management. Trees included black spruce (Picea mariana (Mill.) B.S.P.), jack pine (Pinus banksiana Lamb.), balsam poplar (Populus balsamifera L.), and quaking aspen (Populus tremuloides Michx.). Overstory biomass was estimated using species-specific allometric models that generally explained greater than 95% of the observed variation in biomass. Carbon content of the overstory vegetation was greatest in the 65-year-old stand and was 74% larger than the 11-year-old stand and showed a positive relationship with stand age (F1, 2=122.62, P=0.0081 R2 =0.99). The slope of mineral soil carbon did not differ significantly among stands (F1, 2=0.39, P=0.5956, R2=0.16). Coarse woody debris carbon content followed a U-shaped pattern among stands. Aboveground NPP differed by 24% between the youngest and oldest stand. Mean annual carbon accumulation and aboveground NPP rates of the mixedwood forests were on average two times greater than nearby relatively pure stands studied during the BOREAS (BOReal Ecosystem Atmospheric Study) project. The trends in the results, along with other field studies, suggest that harvesting does not significantly affect the total soil carbon content. The results of this study suggest that scientists should be cautious about extrapolating results from BOREAS stands to a broader region until more data on other forest types and regions are available.
Mkhabela, M. S., Amiro, B. D., Barr, A. G., Black, T. A., Hawthorne, I., Kidston, J., McCaughey, J. H., Orchansky, A. L., Nesic, Z.; Sass, A., Shashkov, A., Zha, T. (2009). Comparison of carbon dynamics and water use efficiency following fire and harvesting in Canadian boreal forests. Agricultural and Forest Meteorology 149 (5): 783-794
ABSTRACT; Fire and harvesting are major forest renewal processes in the Canadian boreal forest. The eddy covariance method was used to compare ecosystem fluxes of carbon dioxide between harvested and burned boreal forest sites in Saskatchewan, Canada. The harvest chronosequence had sites harvested in 2002 (HJP02), 1994 (HJP94) and 1975 (HJP75), whereas the fire chronosequence sites were burned in 1998 (F98), 1989 (F89), 1977 (F77) and 1929 (OJP). All sites were dominated by jack pine prior to the disturbance. During 2004 and 2005, net ecosystem production showed an average carbon gain (g C m−2 year−1 ) at F89 = 84, HJP75 = 80, HJP94 = 14 and OJP = 20. The other sites lost carbon (g C m−2 year−1 ) at HJP02 = −139, F98 = −20, and F77 = −58. Gross ecosystem production (GEP), ecosystem respiration (Re ) and evapotranspiration tended to be greater at the burned sites than the harvested sites. The F89 and F77 sites had the strongest response of GEP to photosynthetically active radiation, and the strongest response of Re to soil temperature at the 2-cm depth. HJP02 had the weakest responses, followed by HJP94. This apparent greater ecosystem activity at the burned sites is likely caused by local differences in soil moisture and nutrients, differences in vegetation development, and differences in the decomposition of coarse woody debris.
Mohan, J. E., Cox, R. M., Iverson, L. R. (2009). Composition and carbon dynamics of forests in northeastern North America in a future, warmer world.. Canadian Journal of Forest Research 39 (2): 213-230
ABSTRACT: Increasing temperatures, precipitation extremes, and other anthropogenic influences (pollutant deposition, increasing carbon dioxide) will influence future forest composition and productivity in the northeastern United States and eastern Canada. This synthesis of empirical and modeling studies includes tree DNA evidence suggesting tree migrations since the last glaciation were much slower, at least under postglacial conditions, than is needed to keep up with current and future climate warming. Exceedances of US and Canadian ozone air quality standards are apparent and offset CO2 -induced gains in biomass and predispose trees to other stresses. The deposition of nitrogen and sulfate in the northeastern United States changes forest nutrient availability and retention, reduces reproductive success and frost hardiness, causes physical damage to leaf surfaces, and alters performance of forest pests and diseases. These interacting stresses may increase future tree declines and ecosystem disturbances during transition to a warmer climate. Recent modeling work predicts warmer climates will increase suitable habitat (not necessarily actual distribution) for most tree species in the northeastern United States. Species whose habitat is declining in the northeastern United States currently occur in Canadian forests and may expand northward with warming. Paleoecological studies suggest local factors may interact with, even overwhelm, climatic effects, causing lags and thresholds leading to sudden large shifts in vegetation.
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 .
ABSTRACT: The boreal forest is expected to experience the greatest warming of all forest biomes, raising concerns that some of the large quantities of soil carbon in these systems may be added to the atmosphere as CO2 . However, nitrogen deposition or fertilization has the potential to increase boreal forest production and retard the decomposition of soil organic matter, hence increasing both tree stand and soil C storage.
The major contributors to soil-surface CO2 effluxes are autotrophic and heterotrophic respiration. To evaluate the effect of nutrient additions on the relative contributions from autotrophic and heterotrophic respiration, a large-scale girdling experiment was performed in a long-term nutrient optimization experiment in a 40-year-old stand of Norway spruce in northern Sweden. Trees on three nonfertilized plots and three fertilized plots were girdled in early summer 2002, and three nonfertilized and three fertilized plots were used as control plots. Each plot was 0.1 ha and contained around 230 trees. Soil-surface CO2 fluxes, soil moisture, and soil temperature were monitored in both girdled and nongirdled plots.
In late July, the time of the seasonal maximum in soil-surface CO2 efflux, the total soil-CO2 efflux in nongirdled plots was 40% lower in the fertilized than in the nonfertilized plots, while the efflux in girdled fertilized and nonfertilized plots was 50% and 60% lower, respectively, than in the corresponding nongirdled controls. We attribute these reductions to losses of the autotrophic component of the total soil-surface CO2 efflux. The estimates of autotrophic respiration are conservative as root starch reserves were depleted more rapidly in roots of girdled than in nongirdled trees. Thus, heterotrophic activity was overestimated.
Calculated on a unit area basis, both the heterotrophic and autotrophic soil respiration was significantly lower in fertilized plots, which is especially noteworthy given that aboveground production was around three times higher in fertilized than in nonfertilized plots.
O'Neill, K.P., Kasischke, E.S., Richter, D.D. (2003). Seasonal and decadal patterns of soil carbon uptake and emission along an age-sequence of burned black spruce stands in interior Alaska. Journal of Geophysical Research 108 (D1): doi:1029/2001JD000443
ABSTRACT: Postfire changes in the local energy balance and soil chemistry may significantly alter rates of carbon turnover in organic-rich soils of boreal forests. This study combines field measurements of soil carbon uptake and emission along a 140-year chronosequence of burned black spruce stands to evaluate the timescales over which these disturbance effects operate. Soil CO2 efflux increased as a function of stand age at a mean rate of 0.12 Mg C ha− 1 yr−1 up to a maximum of 2.2 Mg C ha− 1 yr−1 in the 140-year-old stand. During this same time period, organic soil horizons sequestered carbon and nitrogen at rates of 0.28–0.54 and 0.0076 Mg N ha− 1 yr−1 , respectively. A mass balance model based on field measurements suggests that postfire changes in root and microbial respiration caused these soils to function as a net source of carbon for 7–15 years after fire, releasing between 1.8 and 11.0 Mg C ha−1 to the atmosphere (12.4–12.6% of total soil organic matter). These estimates are on the same order of magnitude as carbon losses during combustion and suggest that current models may underestimate the effect of fire on carbon emissions by a factor of 2.
ABSTRACT: A dynamic vegetation model has been used to predict patterns of recent past and potential future change in taiga forest ecosystems of interior Alaska. The model, called CASA (Carnegie Ames Stanford Approach), is a process-based ecosystem depiction of plant and soil processes, including all major cycles of water and carbon. CASA’s dynamic vegetation component is intended to facilitate coupling to general circulation models of the atmosphere, which require mechanistic fluxes and feedbacks from terrestrial vegetation. Simulation results for selected Alaska sites of Denali National Park suggest that the past 50-year climate trends of warming temperatures may shift the taiga ecosystem from dominance by evergreen needleleaf trees to a more mixed broadleaf–needleleaf tree composition. For other (higher elevation) areas of Denali, our model predicts that a difference of only about 3 °C in mean annual air temperatures appears to differentiate the permanent presence of tundra vegetation forms over taiga forest. The model predicts that over the 1950–1999 climate record at Denali station, the changing taiga ecosystems were net sinks for atmospheric CO2 of about 1.3 kg C m−2 . During the warm 1990s, these forests were predicted to be net carbon sinks of more than 15 g C m−2 per year in 8 out of 10 years. Predicted NPP for the forest continues to increase with a projected warming trend for the next 25 years at a mean rate of about +1.2 g C m−2 per year. On the basis of these model results, a series of crucial field site measurements can be identified for inclusion in subsequent long-term ecological studies of the changing taiga forest.