Climate Change and...

Annotated Bibliography

Carbon Dynamics

Boreal Forest Soils

Aerts, R., Logtestijn, R., Karlsson, P. (2005). Nitrogen supply differentially affects litter decomposition rates and nitrogen dynamics of sub-arctic bog species.. Oecologia 146 (4): 652-658

ABSTRACT: High-latitude peatlands are important soil carbon sinks. In these ecosystems, the mineralization of carbon and nitrogen are constrained by low temperatures and low nutrient concentrations in plant litter and soil organic matter. Global warming is predicted to increase soil N availability for plants at high-latitude sites. We applied N fertilizer as an experimental analogue for this increase. In a three-year field experiment we studied N fertilization effects on leaf litter decomposition and N dynamics of the four dominant plant species (comprising >75% of total aboveground biomass) in a sub-arctic bog in northern Sweden. The species wereEmpetrum nigrum (evergreen shrub),Eriophorum vaginatum (graminoid),Betula nana (deciduous shrub) andRubus chamaemorus (perennial forb). In the controls, litter mass loss rates increased in the order:Empetrum <Eriophorum <Betula <Rubus . Increased N availability had variable, species-specific effects: litter mass loss rates (expressed per unit litter mass) increased inEmpetrum , did not change inEriophorum andBetula and decreased inRubus . In the leaf litter from the controls, we measured no or only slight net N mineralization even after three years. In the N-fertilized treatments we found strong net N immobilization, especially inEriophorum andBetula . This suggests that, probably owing to substantial chemical and/or microbial immobilization, additional N supply does not increase the rate of N cycling for at least the first three years.

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.

Akselsson, C., Berg, B., Meentemeyer, V., Westling, O. (2005). Carbon sequestration rates in organic layers of boreal and temperate forest soils — Sweden as a case study. Global Ecology and Biogeography 14 (1): 77-84

ABSTRACT:The aim of this work was to estimate C sequestration rates in the organic matter layer in Swedish forests.The region encompassed the forested area (23 × 106 ha) of Sweden ranging from about 55° N to 69° N.We used the concept of limit values to estimate recalcitrant litter remains, and combined it with amount of litter fall. Four groups of tree species were identified (pine, spruce, birch and 'other deciduous species'). Annual actual evapotranspiration (AET) was estimated for 5 × 5 km grids covering Sweden. For each grid, data of forested area and main species composition were available. The annual input of foliar litter into each grid was calculated using empirical relationships between AET and foliar litter fall in the four groups. Litter input was combined with average limit values for decomposition for the four groups of litter, based on empirical data. Finally, C sequestration rate was calculated using a constant factor of the C concentration in the litter decomposed to the limit value, thus forming soil organic matter (SOM).We obtained a value of 4.8 × 106 metric tons of C annually sequestered in SOM in soils of mature forests in Sweden, with an average of 180 kg ha−1 and a range from 40 to 410 kg ha−1 . Norway spruce forests accumulated annually an average of 200 kg C ha−1 . The pine and birch groups had an average of 150 kg ha−1 and for the group of other deciduous trees, which is limited to south Sweden, the C sequestration was around 400 kg ha−1 .There is a clear C sequestration gradient over Sweden with the highest C sequestration in the south-west, mainly corresponding to the gradient in litter fall. The limit-value method appears useful for scaling up to a regional level to describe the C sequestration in SOM. A development of the limit value approach in combination with process-orientated dynamic models may have a predictive value.

Akselsson, C., Westling, O., Sverdrup, H., Gundersen, P. (2007). Nutrient and carbon budgets in forest soils as decision support in sustainable forest management. Forest Ecology and Management 238 (1-3): 167-174

ABSTRACT: Knowledge about the nutrient and carbon budgets in forest soils is essential to maintain sustainable production, but also in several environmental issues, such as acidification, eutrophication and climate change. The budgets are strongly influenced by atmospheric deposition as well as forestry. This study demonstrates how budget calculations for nitrogen (N), carbon (C) and base cations (BC) can be used as a basis for policy decisions on a regional level in Sweden.

The study was based on existing nutrient and C budget calculations on a regional scale in Sweden. The nutrient budgets have been calculated for each square in a national 5 km × 5 km net by means of mass balances including deposition, harvest losses, leaching, weathering (BC) and fixation (N). Scenarios with different deposition and forestry intensity have been run and illustrated on maps. A simplified C budget has been estimated by multiplying the N accumulation with the C/N ratio in the organic layer, based on the assumption that the C/N ratio in the accumulating organic matter is equal to the ratio in the soil organic matter pool. The budget approaches differ from earlier budget studies since they involve regional high resolution data, combine deposition and forestry scenarios and integrate different environmental aspects.

The results indicate that whole-tree harvesting will cause net losses of N and base cations in large parts of Sweden, which means that forestry will not be sustainable unless nutrients are added through compensatory fertilization. To prevent net losses following whole-tree harvesting, compensatory fertilization of base cations would be required in almost the whole country, whereas N fertilization would be needed mainly in the northern half of Sweden. The results further suggest that today's recommendations for N fertilization should be revised in southern Sweden by applying the southwest–northeast gradient of the N budget calculations. The C and N accumulation calculations show that C sequestration in Swedish forest soils is not an effective or sustainable way to decrease the net carbon dioxide emissions. A better way is to apply whole-tree harvesting and use the branches, tops and needles as biofuel replacing fossil fuels. This could reduce the present carbon dioxide emissions from fossil fuels substantially.

The study shows that high resolution budget calculations that illuminate different aspects of sustainability in forest ecosystems are important tools for identifying problem areas, investigating different alternatives through scenario analyses and developing new policies. Cooperation with stakeholders increases the probability that the research will be useful

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.

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.

Bonan, G.B., Van Cleve, K. (1991). Soil temperature, nitrogen mineralization, and carbon source-sink relationships in boreal forests. Canadian Journal of Forest Research 22 (5): 629-639

ABSTRACT: Boreal forests contain large quantities of soil carbon, prompting concern that climatic warming may stimulate decomposition and accentuate increasing atmospheric CO2 concentrations. While soil warming increases decomposition rates, the accompanying increase in nutrient mineralization may promote tree growth in these nutrient-poor soils and thereby compensate for the increased carbon loss during decomposition. We used a model of production and decomposition to test this hypothesis. In black spruce (Piceamariana (Mill.) B.S.P.), white spruce (Piceaglauca (Moench) Voss), and paper birch (Betulapapyrifera Marsh.) forests, decomposition increased with the soil warming caused by a 5 °C increase in air temperature. However, increased nitrogen mineralization promoted tree growth, offsetting the increased carbon loss during decomposition. In the black spruce forest, increased tree production was maintained for the 25 years of simulation. Whether this can be maintained indefinitely is unknown. In the birch forest, tree production decreased to prewarming levels after about 10 years. Our analyses examined only the consequences of belowground feedbacks that affect ecosystem carbon uptake with climatic warming. These analyses highlight the importance of interactions among net primary production, decomposition, and nitrogen mineralization in determining the response of forest ecosystems to climatic change.

Bronson, Dustin R., Gower, S. T., Tanner, M., Linder, S., Van Herk, I. (2008). Response of soil surface CO2 flux in a boreal forest to ecosystem warming. Global Change Biology 14 (4): 856-867

ABSTRACT: Soil surface carbon dioxide (CO2 ) flux (RS) was measured for 2 years at the Boreal Soil and Air Warming Experiment site near Thompson, MB, Canada. The experimental design was a complete random block design that consisted of four replicate blocks, with each block containing a 15 m × 15 m control and heated plot. Black spruce [Picea mariana (Mill.) BSP] was the overstory species andEpilobium angustifolium was the dominant understory. Soil temperature was maintained (~5 °C) above the control soil temperature using electric cables inside water filled polyethylene tubing for each heated plot. Air inside a 7.3-m-diameter chamber, centered in the soil warming plot, contained approximately nine black spruce trees was heated ~5 °C above control ambient air temperature allowing for the testing of soil-only warming and soil+air warming. Soil surface CO2 flux (RS) was positively correlated (P < 0.0001) to soil temperature at 10 cm depth. Soil surface CO2 flux (RS) was 24% greater in the soil-only warming than the control in 2004, but was only 11% greater in 2005, while RS in the soil+air warming treatments was 31% less than the control in 2004 and 23% less in 2005. Live fine root mass (< 2 mm diameter) was less in the heated than control treatments in 2004 and statistically less (P < 0.01) in 2005. Similar root mass between the two heated treatments suggests that different heating methods (soil-only vs. soil+air warming) can affect the rate of decomposition.

Burke, R., Zepp, R.G. (1997). Effect of fire on soil-atmosphere exchange of methane and carbon dioxide in Canadian boreal forest sites. Journal of Geophysical Research 102: 29,289- 29,300

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.

Carrasco, J., Neff, J.C., Harden, J.W. (2006). Modeling the long-term accumulation of carbon in boreal forest soils: influence of physical and chemical factors. Journal of Geophysical Research - Biogeosciences

ABSTRACT: Boreal soils are important to the global C cycle owing to large C stocks, repeated disturbance from fire, and the potential for permafrost thaw to expose previously stable, buried C. To evaluate the primary mechanisms responsible for both short- and long-term C accumulation in boreal soils, we developed a multi-isotope (12, 14 C) soil C model with dynamic soil layers that develop through time as soil organic matter burns and reaccumulates. We then evaluated the mechanisms that control organic matter turnover in boreal regions including carbon input rates, substrate recalcitrance, soil moisture and temperature, and the presence of historical permafrost to assess the importance of these factors in boreal C accumulation. Results indicate that total C accumulation is controlled by the rate of carbon input, decomposition rates, and the presence of historical permafrost. However, unlike more temperate ecosystems, one of the key mechanisms involved in C preservation in boreal soils examined here is the cooling of subsurface soil layers as soil depth increases rather than increasing recalcitrance in subsurface soils. The propagation of the14 C bomb spike into soils also illustrates the importance of historical permafrost and twentieth century warming in contemporary boreal soil respiration fluxes. Both14 C and total C simulation data also strongly suggest that boreal SOM need not be recalcitrant to accumulate; the strong role of soil temperature controls on boreal C accumulation at our modeling test site in Manitoba, Canada, indicates that carbon in the deep organic soil horizons is probably relatively labile and thus subject to perturbations that result from changing climatic conditions in the future.

Czimczik, C.I., Preston, C.M., Schmidt, M.W.I., Schulze, E. (2003). How surface fire in Siberian Scots pine forests affects soil organic carbon in the forest floor: Stocks, molecular structure, and conversion to black carbon (charcoal). Global Biogeochemical Cycles 17 (1): doi:10.1029/2002GB001956

ABSTRACT: In boreal forests, fire is a frequent disturbance and converts soil organic carbon (OC) to more degradation-resistant aromatic carbon, i.e., black carbon (BC) which might act as a long-term atmospheric-carbon sink. Little is known on the effects of fires on boreal soil OC stocks and molecular composition. We studied how a surface fire affected the composition of the forest floor of Siberian Scots pine forests by comparing the bulk elemental composition, molecular structure (13 C-MAS NMR), and the aromatic carbon fraction (BC and potentially interfering constituents like tannins) of unburned and burned forest floor. Fire reduced the mass of the forest floor by 60%, stocks of inorganic elements (Si, Al, Fe, K, Ca, Na, Mg, Mn) by 30–50%, and of OC, nitrogen, and sulfur by 40–50%. In contrast to typical findings from temperate forests, unburned OC consisted mainly of (di-)O-alkyl (polysaccharides) and few aromatic structures, probably due to dominant input of lichen biomass. Fire converted OC into alkyl and aromatic structures, the latter consisting of heterocyclic macromolecules and small clusters of condensed carbon. The small cluster size explained the small BC concentrations determined using a degradative molecular marker method. Fire increased BC stocks (16 g kg−1 OC) by 40% which translates into a net-conversion rate of 0.7% (0.35% of net primary production) unburned OC to BC. Here, however, BC was not a major fraction of soil OC pool in unburned or burned forest floor, either due to rapid in situ degradation or relocation.

Czimczik, C.I., Trumbore, S.E., Carbone, M.S., Winston, G.C. (2006). Changing sources of soil respiration with time since fire in a boreal forest. Global Change Biology 12 (6): 957-971

ABSTRACT: Radiocarbon signatures (Δ14 C) of carbon dioxide (CO2 ) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2-year period, we measured Δ14 C of soil respiration and soil CO2 in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand-replacing fire. Comparing bulk respiration Δ14 C with Δ14 C of CO2 evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ14 C of respired CO2 indicated marked variation in respiration sources in space and time.The14 C signature of respired CO2 respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ14 C greater (averaging~120‰) than autotrophic respiration. TheΔ14 C of autotrophically respired CO2 in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004).CO2 respired by black spruce roots in stands >40 years old hadΔ14 C up to 30‰ higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants.

Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ~50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soilCO2 hadΔ14 C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ14 C of soil respiration in younger successional stands dropped below those of the atmospheric CO2 .

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.

Gulledge, J., Schimel, J. P. (1998). Moisture control over atmospheric CH4 consumption and CO2 production in diverse Alaskan soils. Soil Biology and Biochemistry 30 (8-9): 1127-1132

ABSTRACT: Moisture is an important control on atmospheric CH4 consumption and CO2 production in soil. Wet conditions limit these microbial activities by restricting CH4 and O2 diffusion and dry conditions limit microbial activity due to physiological water stress. We examined the relationship between soil moisture and these biogeochemical activities in five Alaskan soils with varying physical properties. Three expressions of soil moisture, absolute water content (g H2O g−1 dry soil), water potential and percent of water-holding capacity (%WHC), were compared for their abilities to predict microbial activity in the different soils. We also examined the physiological responses of CH4 oxidizers and the general microbial community to changes in water potential. The quantitative relationship between absolute water content and microbial activity varied widely among soils with different textures. The relationship between microbial activity and water potential was asymmetrical and differed between upland and wetland soils. In contrast, the parabolic relationship between %WHC and CH4 consumption was symmetrical and similar among the five soils. CO2 production also related to %WHC similarly across soils. Maximum atmospheric CH4 consumption occurred between 20-40% WHC in all soils with a mean optimum of 34% WHC, whereas CO2 production was maximal above 50% WHC. For CH4 oxidation, optimum water potential was −0.3 to −0.2 MPa in upland soils, and about −0.02 MPa in a wetland soil. Our results demonstrate that %WHC is a powerful expression for quantitatively relating microbial activity responses to moisture across physically diverse soils and may be useful for modeling the response of biogeochemical processes, especially atmospheric CH4 consumption, to climate change. Our data also suggest that CH4 oxidizers in upland soils are adapted to growth on atmospheric CH4 and that CH4 consumption in upland taiga soils may be decreased by altered soil moisture, regardless of whether conditions become wetter or drier.

Gulledge, J., Schimel, J. P. (2000). Controls on soil carbon dioxide and methane fluxes in a variety of taiga forest stands in interior Alaska. Ecosystems 3 (3): 269-282

ABSTRACT: CO2 and CH4 fluxes were monitored over 4 years in a range of taiga forests along the Tanana River in interior Alaska. Floodplain alder and white spruce sites and upland birch/aspen and white spruce sites were examined. Each site had control, fertilized, and sawdust amended plots; flux measurements began during the second treatment year. CO2 emissions decreased with successional age across the sites (alder, birch/aspen, and white spruce, in order of succession) regardless of landscape position. Although CO2 fluxes showed an exponential relationship with soil temperature, the response of CO2 production to moisture fit an asymptotic model. Of the manipulations, only N fertilization had an effect on CO2 flux, decreasing flux in the floodplain sites but increasing it in the birch/aspen site. Landscape position was the best predictor of CH4 flux. The two upland sites consumed CH4 at similar rates (approximately 0.5 mg C m−2 d−1 ), whereas the floodplain sites had lower consumption rates (0–0.3 mg C m−2 d−1 ). N fertilization and sawdust both inhibited CH4 consumption in the upland birch/aspen and floodplain spruce sites but not in the upland spruce site. The biological processes driving CO2 fluxes were sensitive to temperature, moisture, and vegetation, whereas CH4 fluxes were sensitive primarily to landscape position and biogeochemical disturbances. Hence, climate change effects on C-gas flux in taiga forest soils will depend on the relationship between soil temperature and moisture and the concomitant changes in soil nutrient pools and cycles.

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.

Kane,E., Valentine,D., Schuur,E., Dutta,K. (2005). Soil carbon stabilization along climate and stand productivity gradients in black spruce forests of interior Alaska. Canadian Journal of Forest Research 35 (9): 2118-2129

ABSTRACT: The amount of soil organic carbon (SOC) in stable, slow-turnover pools is likely to change in response to climate warming because processes mediating soil C balance (net primary production and decomposition) vary with environmental conditions. This is important to consider in boreal forests, which constitute one of the world's largest stocks of SOC. We investigated changes in soil C stabilization along four replicate gradients of black spruce productivity and soil temperature in interior Alaska to develop empirical relationships between SOC and stand and physiographic features. Total SOC harbored in mineral soil horizons decreased by 4.4 g C·m–2 for every degree-day increase in heat sum within the organic soil across all sites. Furthermore, the proportion of relatively labile light-fraction (density <1.6 g·cm–3 ) soil organic matter decreased significantly with increased stand productivity and soil temperature. Mean residence times of SOC (as determined byΔ14 C) in dense-fraction (>1.6 g·cm–3 ) mineral soil ranged from 282 to 672 years. The oldest SOC occurred in the coolest sites, which also harbored the most C and had the lowest rates of stand production. These results suggest that temperature sensitivities of organic matter within discrete soil pools, and not just total soil C stocks, need to be examined to project the effects of changing climate and primary production on soil C balance.

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 .

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.

Liski, J., Ivelsniemi, H., Makela, A., Starr, M. (1998). Model analysis of the efects of soil age, fires and harvesting on the carbon storage of boreal forest soils. European Journal of the Soil Science 49: 407-416

ABSTRACT: Potential causes for changes in the amounts of carbon (C) stored in the soils of boreal forests were studied by measuring the C in the soil along a 5000-year chronosequence in coastal western Finland and using a simple dynamic model of decomposition. The amount of soil C stabilized at an age of about 2000 years. This suggests that the youth of many boreal soils does not make them sinks for atmospheric C. Simulated repeated fires kept the amount of soil C reduced by about 25%, but if fires were prevented then the C in the soil increased. Stored C may thus be less than the potential storage where fires are frequent, and it could be increased by preventing fires. Simulated clear-cutting caused a temporary 5–10% decrease in the amount of soil C over a 20-year period after the harvesting. It also caused a long-term decrease in the amount of soil C such that, after two 100-year rotations, the amount had been decreased by 14%. Stored C is almost certainly less than the potential storage and decreasing where forests are harvested.

Liski, J., Westman, C. J. (1997). Carbon storage in forest soil of Finland: 2. Size and regional patterns. Biogeochemistry 36 (3): 261-274

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.

Liski, J., Westman, C. J. (1997). Carbon storage in forest soil of Finland: 1. Effect of thermoclimate. Biogeochemistry 36 (3): 239-260

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.

Maljanen, M., Nykanen, H., Moilanen, M., Martikainen, P. J. (2006). Greenhouse gas fluxes of coniferous forest floors as affected by wood ash addition. Forest Ecology and Management 237 (1-3): 143-149

ABSTRACT: Wood ash has been used to alleviate nutrient deficiencies of peat forests and to combat acidification of forest soils. Ash may change the activities of soil microbes, including those producing or consuming greenhouse gases, such as methane (CH4 ), nitrous oxide (N2 O) and carbon dioxide (CO2 ). We studied the effects of wood ash (loose wood ash originating from pulp mill or power plants) on the fluxes of CH4 , N2 O and CO2 in forests with mineral or peat soils in northern Finland. The ash doses were from 3 to 8 t ha−1 . Gas fluxes were measured with a closed chamber method from five recently fertilized experiments for 1 year after application of ash and from five long-term trials 14–50 years after application. Wood ash did not affect N2 O gas fluxes. In the long-term experiments, wood ash increased the soil CO2 production and the CH4 uptake and lowered the CH4 emissions.

Manies, K.L., Harden, J.W., Yoshikawa, K., Randerson, J.T., Galloway, J. (2003). The effect of soil drainage on fire and carbon cycling in central Alaska. U.S. Geological Survey: 145-152

ABSTRACT: Relatively high rates of plant production coupled with low rates of decomposition allow boreal forests to store large amounts of carbon. Fire, the main disturbance of this ecosystem, also plays a key role in regulating this biome’s C storage. All three of these factors are sensitive to climate change. For this reason, it is important to understand the interactions between fire, productivity, and decomposition, as well as how these interactions vary with soil drainage.

We are currently investigating the effects of fire on soil temperature and vegetative regrowth for different soil-drainage classes. Various soil, thermal, and vegetative properties are being measured within different-age black spruce (Picea mariana (Mill.) BSP) stands in well-drained, moderately well drained, and poorly drained areas. While the absolute amount of organic matter lost to fire is greater at moderately well drained sites, the relative amount of organic-matter loss is greatest at well-drained sites. Loss of any organic matter profoundly affects soil temperature—differences between burned and unburned plots ranged as high as 13°C. Soil drainage also affected which species were dominant postburn. Quantifying the effect of soil drainage on such factors as depth of organic matter, soil temperature, and vegetative regrowth will aid in understanding the impact of fire on boreal-forest C storage.

Mansson, K. F., Falkengren-Grerup, U. (2003). The effect of nitrogen deposition on nitrification, carbon and nitrogen mineralisation and litter C : N ratios in oak (Quercus robur L.) forests. Forest Ecology and Management 179 (1-3): 455-467

ABSTRACT: The present study addresses the question why there is a positive relationship between nitrogen deposition and potential net nitrogen mineralisation and nitrification in oak (Quercus robur L.) forest soils in south Sweden (Falkengren-Grerup et al., 1998), and how this is related to the carbon mineralisation. We tested three hypotheses based on European studies ( Persson et al., 2000a and Persson et al., 2000b) that postulate lower availability of carbon due to chemical binding of nitrogen to lignin remains and phenolic compounds or a more decomposed, recalcitrant organic matter due to faster initial decomposition rates of fresh litter. This in turn leads to increased net nitrogen mineralisation, and nitrifiers that may adapt to acid soils when ammonium availability increases. We used soils from two regions exposed to a total deposition of 17 and 10 kg N ha−1 per year and incubated the soils in the laboratory separately as well as in mixtures between the regions. To be able to evaluate how the microbial communities and organic matter interacted in the soil mixtures, we divided the observed values of the net carbon and nitrogen mineralisation and nitrification for the soil mixtures by the calculated expected values. C:N ratios of litter, fresh leafs ofDeschampsia flexuosa and microbial biomass were also measured. Contrary to the assumptions in the hypothesis, the soil respiration was somewhat higher in soils subjected to high nitrogen deposition. Furthermore, the observed rate of nitrogen mineralisation was higher than expected in the majority of soil mixtures, while observed rates of carbon mineralisation only showed a weak tendency to be higher than expected. All the results taken together indicate that there has been a positive change in litter quality that leads to increased carbon and nitrogen mineralisation. This conclusion is supported by the C:N ratio of oak litter and fresh leaves ofDeschampsia flexuosa that was lower in the most nitrogen-exposed sites and which might indicate an increase in decomposability. The observed values of nitrification were significantly higher than the calculated expected values. Thus, the increased net nitrogen mineralisation in the region with high nitrogen deposition seems to allow nitrifiers to adapt to these acid soils when they are no longer limited by ammonium.

Meyer, H., Kaiser, C., Biasi, C., Haemmerle, R., Rusalimova, O., Lashchinsky, N., Baranyi, C., Daims, H., Barsukov, P., Richter, A. (2006). Soil carbon and nitrogen dynamics along a latitudinal transect in western Siberia, Russia. Biogeochemistry 81 (2): 239-252

ABSTRACT: An 1800-km South to North transect (N 53°43′ to 69°43′) through Western Siberia was established to study the interaction of nitrogen and carbon cycles. The transect comprised all major vegetation zones from steppe, through taiga to tundra and corresponded to a natural temperature gradient of 9.5°C mean annual temperature (MAT). In order to elucidate changes in the control of C and N cycling along this transect, we analyzed physical and chemical properties of soils and microbial structure and activity in the organic and in the mineral horizons, respectively. The impact of vegetation and climate exerted major controls on soil C and N pools (e.g., soil organic matter, total C and dissolved inorganic nitrogen) and process rates (gross N mineralization and heterotrophic respiration) in the organic horizons. In the mineral horizons, however, the impact of climate and vegetation was less pronounced. Gross N mineralization rates decreased in the organic horizons from south to north, while remaining nearly constant in the mineral horizons. Especially, in the northern taiga and southern tundra gross nitrogen mineralization rates were higher in the mineral compared to organic horizons, pointing to strong N limitation in these biomes. Heterotrophic respiration rates did not exhibit a clear trend along the transect, but were generally higher in the organic horizon compared to mineral horizons. Therefore, C and N mineralization were spatially decoupled at the northern taiga and tundra. The climate change implications of these findings (specifically for the Arctic) are discussed.

Olsson, P., Linder, S., Giesler, R., Hogberg, P. (2005). Fertilization of boreal forest reduces both autotrophic and heterotrophic soil respiration. Global Change Biology 11 (10): 1745-1753

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.

ONeill,Katherine, Richter,Daniel, Kasischke,Eric (2006). Succession-driven changes in soil respiration following fire in black spruce stands of interior Alaska. Biogeochemistry 80 (1): 1-20

ABSTRACT: This study examined the nitrogen (N) dynamics of a black spruce (Picea mariana (Mill.) BSP)-dominated chronosequence in Manitoba, Canada. The seven sites studied each contained separate well- and poorly drained stands, originated from stand-killing wildfires, and were between 3 and 151 years old. Our goals were to (i) measure total N concentration ([N]) of all biomass components and major soil horizons; (ii) compare N content and select vegetation N cycle processes among the stands; and (iii) examine relationships between ecosystem C and N cycling for these stands. Vegetation [N] varied significantly by tissue type, species, soil drainage, and stand age; woody debris [N] increased with decay state and decreased with debris size. Soil [N] declined with horizon depth but did not vary with stand age. Total (live + dead) biomass N content ranged from 18.4 to 99.7 g N m−2 in the well-drained stands and 37.8–154.6 g N m−2 in the poorly drained stands. Mean soil N content (380.6 g N m−2 ) was unaffected by stand age. Annual vegetation N requirement (5.9 and 8.4 g N m−2 yr−1 in the middle-aged well- and poorly drained stands, respectively) was dominated by trees and fine roots in the well-drained stands, and bryophytes in the poorly drained stands. Fraction N retranslocated was significantly higher in deciduous than evergreen tree species, and in older than younger stands. Nitrogen use efficiency (NUE) was significantly lower in bryophytes than in trees, and in deciduous than in evergreen trees. Tree NUE increased with stand age, but overall stand NUE was roughly constant (~150 g g−1 N) across the entire chronosequence.

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.

Peltoniemi, M., Palosuo, T., Monni, S., Makipaa, R. (2006). Factors affecting the uncertainty of sinks and stocks of carbon in Finnish forests soils and vegetation. Forest Ecology and Management 232 (1-3): 75-85

ABSTRACT: Monitoring and transparent reporting of forest carbon sinks are currently needed under the Climate Convention. From 2005 onwards, national GHG inventories should also provide uncertainty estimates of the reported emissions and removals. Comprehensive uncertainty analysis and key category analysis of the carbon inventory can provide guidance for prioritizing efforts in further development of the inventory. In this study, the estimates of the forest carbon stock and carbon sink were obtained by combining forest inventory data, models of biomass and turnover, and a dynamic decomposition model for SOM and litter, Yasso. To study the decisive factors affecting uncertainties of forest carbon sink and stock estimates, we conducted a Monte Carlo analysis for the calculation of the forest carbon budget of Finnish forests for the period 1989–2004.

Uncertainty of the vegetation carbon sink was affected mostly by input data on growth variation and drain. Uncertainty of the soil carbon sink was dominated by the soil model initialization, but the effect decreased with time. After few years, the effect of initialization leveled with the effect of temperature and drain, both of which were given as input data to the system and which varied inter-annually. The contribution of these variables was less important to uncertainty of stocks in vegetation and soil than the contribution of model parameters. The most influential parameters for vegetation C stock were carbon density and conversion factors for tree and ground vegetation biomass, and for soil C stock, they were soil model parameters, and biomass conversion factors and turnover rates of fine roots and ground vegetation.

After short initialization period for soil C, uncertainty of soil sink can be reduced by improving the precision of input data (harvests on upland soils, annual temperature). Precision of vegetation sink can be improved mainly by improving the quality of input data on growth variation and harvests. There is an unknown error source related to inter-annual variability of the forest ecosystems, which cannot be represented with the system. Vegetation sink was compiled with biomass models that are based on long-term averages and they do not support year-to-year variations which may occur in forest ecosystems. Averaged biomass models with averaged turnover models, produce highly auto-correlated series of litter input, which result in relatively precise annual soil sink estimates. Due to these reasons, the current inventory-based approach is more justified for the estimation of average sinks for longer periods than 1 year.

Potter, C. (2004). Predicting climate change effects on vegetation, soil thermal dynamics, and carbon cycling in ecosystems of interior Alaska. Ecological Modelling 175 (1): 1-24

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.

Schimel, J. P., Clein, J. S. (1996). Microbial response to freeze-thaw cycles in tundra and taiga soils. Soil Biology and Biochemistry 28 (8): 1061-1066

ABSTRACT: Four tundra and taiga soils were experimentally subjected to three freeze-thaw cycles (5 days each at −5°C and +5°C). After each thaw, there was an initial pulse (<24 h) in microbial respiration. The total amount of C respired in each thaw period was largest during the first cycle and decreased in successive cycles. Three cycles caused a net increase in total respired C relative to the +5°C control in wet meadow tundra soil, a reduction in birch soil, and had no net effect in either alder-poplar or tussock tundra soil. These different patterns apparently resulted from differences in the quality of the soil organic matter and the relative activity of the microbial biomass. Net N mineralization was generally enhanced relative to the +5°C controls in the first cycle, but was inhibited in the third cycle, similar to what was observed with C. Over multiple freeze-thaw cycles, the initial response of C and N mineralization appear to be driven by release from the microbial biomass, while over the longer-term, the response is driven by the reduction in attack on soil organic matter resulting from a reduced microbial population.

Schimel, J. P., Gulledge, J. M., Clein-Curley, J. S., Lindstrom, J. E., Braddock, J. F. (1999). Moisture effects on microbial activity and community structure in decomposing birch litter in the Alaskan taiga. Soil Biology and Biochemistry 31 (6): 831-838

ABSTRACT: We carried out a field experiment to evaluate the effect of moisture regime on microbial biomass and activity in birch litter in the Alaskan taiga. Litter bags were placed in one of three treatments: continuously moist (0.5 cm water d−1 ), cycling (0.5 cm water weekly), and ‘natural', which experienced two natural dry–wet cycles of 2 weeks dry followed by rain. The experiment lasted for 1 month. Each week we collected litter bags and analyzed microbial respiration and biomass C and N. In the last two cycles we analyzed bacterial substrate use on Biolog GN plates. There were strong overall correlations between biomass, respiration and litter moisture content. However, the different treatments had significantly different rates of respiration, biomass and respiratory quotient (qCO2 ) that could not be explained by moisture content directly. The natural treatment had lower respiration rates and biomass than the wet or cycling samples, indicating that the 2-week droughts in the natural treatment reduced microbial populations and activity to a greater degree than did shorter droughts. Episodic drying and rewetting considerably decreased the number of Biolog substrates used. This experiment showed that the size and functioning of the litter microbial community was strongly affected by its stress history.

Schuur, E. A. G., Trumbore, S. E. (2006). Partitioning sources of soil respiration in boreal black spruce forest using radiocarbon. Global Change Biology 12 (2): 165-176

ABSTRACT: Separating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ14 C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently-fixed C that fuels plant or microbial metabolism has Δ14 C values close to that of current atmospheric CO2 , while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ14 C of C respired by recently excised black spruce roots averaged 14‰ greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. TheΔ14 C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60‰ higher than the contemporary atmosphereΔ14 CO2 , indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ14C end members with measurements of the Δ14 C of total soil respiration, we calculated that 47–63% of soil CO2 emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for highΔ14 C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration.

Ullah, S., Frasier, R., King, L., Picotte-Anderson, N., Moore, T.R. (2008). Potential fluxes of N2 O and CH4 from soils of three forest types in Eastern Canada. Soil Biology and Biochemistry 40 (4): 986-994

ABSTRACT: We conducted laboratory incubation experiments to elucidate the influence of forest type and topographic position on emission and/or consumption potentials of nitrous oxide (N2 O) and methane (CH4 ) from soils of three forest types in Eastern Canada. Soil samples collected from deciduous, black spruce and white pine forests were incubated under a control, an NH4 NO3 amendment and an elevated headspace CH4 concentration at 70% water-filled pore space (WFPS), except the poorly drained wetland soils which were incubated at 100% WFPS. Deciduous and boreal forest soils exhibited greater potential of N2 O and CH4 fluxes than did white pine forest soils. Mineral N addition resulted in significant increases in N2 O emissions from wetland forest soils compared to the unamended soils, whereas well-drained soils exhibited no significant increase in N2 O emissions in-response to mineral N additions. Soils in deciduous, boreal and white pine forests consumed CH4 when incubated under an elevated headspace CH4 concentration, except the poorly drained soils in the deciduous forest, which emitted CH4 . CH4 consumption rates in deciduous and boreal forest soils were twice the amount consumed by the white pine forest soils. The results suggest that an episodic increase in reactive N input in these forests is not likely to increase N2 O emissions, except from the poorly drained wetland soils; however, long-term in situ N fertilization studies are required to validate the observed results. Moreover, wetland soils in the deciduous forest are net sources of CH4 unlike the well-drained soils, which are net sinks of atmospheric CH4 . Because wetland soils can produce a substantial amount of CH4 and N2 O, the contribution of these wetlands to the total trace gas fluxes need to be accounted for when modeling fluxes from forest soils in Eastern Canada.

Vedrova, E.F., Shugalei, L.S., Stakanov, V.D. (2002). The carbon balance in natural and disturbed forests of the southern taiga in central Siberia. Journal of Vegetation Science 13 (3): 341-350

ABSTRACT: We evaluated the balance of production and decomposition in natural ecosystems ofPinus sylvestris ,Larix sibirica andBetula pendulain the southern boreal forests of central Siberia, using the Yenisei transect. We also investigated whether anthropogenic disturbances (logging, fire and recreation pressure) influence the carbon budget.Pinus andLarix stands up to age class VI act as a net sink for atmospheric carbon. Mineralization rates in youngBetula forests exceed rates of uptake via photosynthesis assimilation. Old-growth stands of all three forest types are CO2 sources to the atmosphere. The prevalence of old-growthLarix in the southern taiga suggests thatLarix stands are a net source of CO2 . The CO2 flux to the atmosphere exceeds the uptake of atmospheric carbon via photosynthesis by 0.23 t C.ha−1 .yr−1 (47%).Betula andPinus forests are net sinks, as photosynthesis exceeds respiration by 13% and 16% respectively. The total carbon flux fromPinus ,Larix andBetula ecosystems to the atmosphere is 10 387 thousand tons C.yr−1 . Net Primary Production (0.935 t-C.ha−1 ) exceeds carbon release from decomposition of labile and mobile soil organic matter (Rh) by 767 thousand tons C (0.064 t-C.ha−1 ), so that these forests are net C-sinks. The emissions due to decomposition of slash (101 thousand tons C; 1.0%) and from fires (0.21%) are very small. The carbon balance of human-disturbed forests is significantly different. A sharp decrease in biomass stored inPinus andBetula ecosystems leads to decreased production. As a result, the labile organic matter pool decreased by 6–8 times; course plant residues with a low decomposition rate thus dominate this pool. Annual carbon emissions to the atmosphere from these ecosystems are determined primarily by decomposing fresh litterfall. This source comprises 40–79% of the emissions from disturbed forests compared to only 13–28% in undisturbed forests. The ratio of emissions to production (NPP) is 20–30% in disturbed and 52–76% in undisturbed forests.

Lovett, G.M., K.C. Weathers, M.A. Arthur (2002). Control of nitrogen loss from forested watersheds by soil carbon:nitrogen ratio and tree species composition. Ecosystems 5 (7): 712-718

ABSTRACT: Leaching losses of nitrate from forests can have potentially serious consequences for soils and receiving waters. In this study, based on extensive sampling of forested watersheds in the Catskill Mountains of New York State, we examine the relationships among stream chemistry, the properties of the forest floor, and the tree species composition of watersheds. We report the first evidence from North America that nitrate export from forested watersheds is strongly influenced by the carbon:nitrogen (C:N) ratio of the watershed soils. We also show that variation in soil C:N ratio is associated with variation in tree species composition. This implies that N retention and release in forested watersheds is regulated at least in part by tree species composition and that changes in species composition caused by introduced pests, climate change, or forest management could affect the capacity of a forest ecosystem to retain atmospherically deposited N.

G. D. Thackray (2001). Extensive early and middle Wisconsin glaciation on the western Olympic Peninsula, Washington, and the variability of Pacific moisture delivery to the Northwestern United States. Quaternary Research 55 (3): 257-270

ABSTRACT: Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,00014 C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,00014 C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,30014 C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,30014 C yr B.P.; the Twin Creeks 1 advance, 19,100–18,30014 C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.

Waldrop, M. P., Harden, J. W. (2008). Interactive effects of wildfire and permafrost on microbial communities and soil processes in an Alaskan black spruce forest.. Global Change Biology 14 (11): 2591-2602

ABSTRACT: Boreal forests contain significant quantities of soil carbon that may be oxidized to CO2 given future increases in climate warming and wildfire behavior. At the ecosystem scale, decomposition and heterotrophic respiration are strongly controlled by temperature and moisture, but we questioned whether changes in microbial biomass, activity, or community structure induced by fire might also affect these processes. We particularly wanted to understand whether postfire reductions in microbial biomass could affect rates of decomposition. Additionally, we compared the short-term effects of wildfire to the long-term effects of climate warming and permafrost decline. We compared soil microbial communities between control and recently burned soils that were located in areas with and without permafrost near Delta Junction, AK. In addition to soil physical variables, we quantified changes in microbial biomass, fungal biomass, fungal community composition, and C cycling processes (phenol oxidase enzyme activity, lignin decomposition, and microbial respiration). Five years following fire, organic surface horizons had lower microbial biomass, fungal biomass, and dissolved organic carbon (DOC) concentrations compared with control soils. Reductions in soil fungi were associated with reductions in phenol oxidase activity and lignin decomposition. Effects of wildfire on microbial biomass and activity in the mineral soil were minor. Microbial community composition was affected by wildfire, but the effect was greater in nonpermafrost soils. Although the presence of permafrost increased soil moisture contents, effects on microbial biomass and activity were limited to mineral soils that showed lower fungal biomass but higher activity compared with soils without permafrost. Fungal abundance and moisture were strong predictors of phenol oxidase enzyme activity in soil. Phenol oxidase enzyme activity, in turn, was linearly related to both13 C lignin decomposition and microbial respiration in incubation studies. Taken together, these results indicate that reductions in fungal biomass in postfire soils and lower soil moisture in nonpermafrost soils reduced the potential of soil heterotrophs to decompose soil carbon. Although in the field increased rates of microbial respiration can be observed in postfire soils due to warmer soil conditions, reductions in fungal biomass and activity may limit rates of decomposition.

bottom right