McGuire A. David

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McGuire

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A. David

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Long-term ecological research in a human-dominated world

(American Institute of Biological Sciences, 2012-04) Robertson, G. Philip ; Collins, Scott L. ; Foster, David R. ; Brokaw, Nicholas ; Ducklow, Hugh W. ; Gragson, Ted L. ; Gries, Corinna ; Hamilton, Stephen K. ; McGuire, A. David ; Moore, John C. ; Stanley, Emily H. ; Waide, Robert B. ; Williams, Mark W.

The US Long Term Ecological Research (LTER) Network enters its fourth decade with a distinguished record of achievement in ecological science. The value of long-term observations and experiments has never been more important for testing ecological theory and for addressing today's most difficult environmental challenges. The network's potential for tackling emergent continent-scale questions such as cryosphere loss and landscape change is becoming increasingly apparent on the basis of a capacity to combine long-term observations and experimental results with new observatory-based measurements, to study socioecological systems, to advance the use of environmental cyberinfrastructure, to promote environmental science literacy, and to engage with decisionmakers in framing major directions for research. The long-term context of network science, from understanding the past to forecasting the future, provides a valuable perspective for helping to solve many of the crucial environmental problems facing society today.
Methane fluxes between terrestrial ecosystems and the atmosphere at northern high latitudes during the past century : a retrospective analysis with a process-based biogeochemistry model

(American Geophysical Union, 2008-08-18) Zhuang, Qianlai ; Melillo, Jerry M. ; Kicklighter, David W. ; Prinn, Ronald G. ; McGuire, A. David ; Steudler, Paul A. ; Felzer, Benjamin S. ; Hu, Shaomin

We develop and use a new version of the Terrestrial Ecosystem Model (TEM) to study how rates of methane (CH4) emissions and consumption in high-latitude soils of the Northern Hemisphere have changed over the past century in response to observed changes in the region's climate. We estimate that the net emissions of CH4 (emissions minus consumption) from these soils have increased by an average 0.08 Tg CH4 yr−1 during the twentieth century. Our estimate of the annual net emission rate at the end of the century for the region is 51 Tg CH4 yr−1. Russia, Canada, and Alaska are the major CH4 regional sources to the atmosphere, responsible for 64%, 11%, and 7% of these net emissions, respectively. Our simulations indicate that large interannual variability in net CH4 emissions occurred over the last century. Our analyses of the responses of net CH4 emissions to the past climate change suggest that future global warming will increase net CH4 emissions from the Pan-Arctic region. The higher net CH4 emissions may increase atmospheric CH4 concentrations to provide a major positive feedback to the climate system.
The impacts of recent permafrost thaw on land–atmosphere greenhouse gas exchange

(IOP Publishing, 2014-04-06) Hayes, Daniel J. ; Kicklighter, David W. ; McGuire, A. David ; Chen, Min ; Zhuang, Qianlai ; Yuan, Fengming ; Melillo, Jerry M. ; Wullschleger, Stan D.

Permafrost thaw and the subsequent mobilization of carbon (C) stored in previously frozen soil organic matter (SOM) have the potential to be a strong positive feedback to climate. As the northern permafrost region experiences as much as a doubling of the rate of warming as the rest of the Earth, the vast amount of C in permafrost soils is vulnerable to thaw, decomposition and release as atmospheric greenhouse gases. Diagnostic and predictive estimates of high-latitude terrestrial C fluxes vary widely among different models depending on how dynamics in permafrost, and the seasonally thawed 'active layer' above it, are represented. Here, we employ a process-based model simulation experiment to assess the net effect of active layer dynamics on this 'permafrost carbon feedback' in recent decades, from 1970 to 2006, over the circumpolar domain of continuous and discontinuous permafrost. Over this time period, the model estimates a mean increase of 6.8 cm in active layer thickness across the domain, which exposes a total of 11.6 Pg C of thawed SOM to decomposition. According to our simulation experiment, mobilization of this previously frozen C results in an estimated cumulative net source of 3.7 Pg C to the atmosphere since 1970 directly tied to active layer dynamics. Enhanced decomposition from the newly exposed SOM accounts for the release of both CO2 (4.0 Pg C) and CH4 (0.03 Pg C), but is partially compensated by CO2 uptake (0.3 Pg C) associated with enhanced net primary production of vegetation. This estimated net C transfer to the atmosphere from permafrost thaw represents a significant factor in the overall ecosystem carbon budget of the Pan-Arctic, and a non-trivial additional contribution on top of the combined fossil fuel emissions from the eight Arctic nations over this time period.
Importance of recent shifts in soil thermal dynamics on growing season length, productivity, and carbon sequestration in terrestrial high-latitude ecosystems

( 2005-10-07) Euskirchen, Eugenie ; McGuire, A. David ; Kicklighter, David W. ; Zhuang, Qianlai ; Clein, Joy S. ; Dargaville, R. J. ; Dye, D. G. ; Kimball, John S. ; McDonald, Kyle C. ; Melillo, Jerry M. ; Romanovsky, Vladimir ; Smith, N. V.

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, 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 NOAA satellite observations collected between the years 1972-2000, with Pearson rank correlation coefficients between 0.58-0.65. 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 1988-2000. Between 1988 and 2000, satellite records yield a slightly stronger trend in thaw and the onset of the growing season, averaging between 5-8 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 carbon and increases in vegetation carbon, with greatest losses of soil carbon 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 carbon uptake, indicating that prediction of terrestrial carbon dynamics from one decade to the next will require that large-scale models adequately take into account the corresponding changes in soil thermal regimes.
An analysis of the carbon balance of the Arctic Basin from 1997 to 2006

( 2010-06-18) McGuire, A. David ; Hayes, Daniel J. ; Kicklighter, David W. ; Manizza, Manfredi ; Zhuang, Qianlai ; Chen, Min ; Follows, Michael J. ; Gurney, Kevin R. ; McClelland, James W. ; Melillo, Jerry M. ; Peterson, Bruce J. ; Prinn, Ronald G.

This study used several model-based tools to analyze the dynamics of the Arctic Basin between 1997 and 2006 as a linked system of land-ocean-atmosphere C exchange. The analysis estimates that terrestrial areas of the Arctic Basin lost 62.9 Tg C yr-1 and that the Arctic Ocean gained 94.1 Tg C yr-1. Arctic lands and oceans were a net CO2 sink of 108.9 Tg C yr-1, which is within the range of uncertainty in estimates from atmospheric inversions. Although both lands and oceans of the Arctic were estimated to be CO2 sinks, the land sink diminished in strength because of increased fire disturbance compared to previous decades, while the ocean sink increased in strength because of increased biological pump activity associated with reduced sea ice cover. Terrestrial areas of the Arctic were a net source of 41.5 Tg CH4 yr-1 that increased by 0.6 Tg CH4 yr-1 during the decade of analysis, a magnitude that is comparable with an atmospheric inversion of CH4. Because the radiative forcing of the estimated CH4 emissions is much greater than the CO2 sink, the analysis suggests that the Arctic Basin is a substantial net source of green house gas forcing to the climate system.
Influence of changes in wetland inundation extent on net fluxes of carbon dioxide and methane in northern high latitudes from 1993 to 2004

(IOP Science, 2015-09-10) Zhuang, Qianlai ; Zhu, Xudong ; He, Yujie ; Prigent, Catherine ; Melillo, Jerry M. ; McGuire, A. David ; Prinn, Ronald G. ; Kicklighter, David W.

Estimates of the seasonal and interannual exchanges of carbon dioxide (CO2) and methane (CH4) between land ecosystems north of 45°N and the atmosphere are poorly constrained, in part, because of uncertainty in the temporal variability of water-inundated land area. Here we apply a process-based biogeochemistry model to evaluate how interannual changes in wetland inundation extent might have influenced the overall carbon dynamics of the region during the time period 1993–2004. We find that consideration by our model of these interannual variations between 1993 and 2004, on average, results in regional estimates of net methane sources of 67.8 ± 6.2 Tg CH4 yr−1, which is intermediate to model estimates that use two static inundation extent datasets (51.3 ± 2.6 and 73.0 ± 3.6 Tg CH4 yr−1). In contrast, consideration of interannual changes of wetland inundation extent result in regional estimates of the net CO2 sink of −1.28 ± 0.03 Pg C yr−1 with a persistent wetland carbon sink from −0.38 to −0.41 Pg C yr−1 and a upland sink from −0.82 to −0.98 Pg C yr−1. Taken together, despite the large methane emissions from wetlands, the region is a consistent greenhouse gas sink per global warming potential (GWP) calculations irrespective of the type of wetland datasets being used. However, the use of satellite-detected wetland inundation extent estimates a smaller regional GWP sink than that estimated using static wetland datasets. Our sensitivity analysis indicates that if wetland inundation extent increases or decreases by 10% in each wetland grid cell, the regional source of methane increases 13% or decreases 12%, respectively. In contrast, the regional CO2 sink responds with only 7–9% changes to the changes in wetland inundation extent. Seasonally, the inundated area changes result in higher summer CH4 emissions, but lower summer CO2 sinks, leading to lower summer negative greenhouse gas forcing. Our analysis further indicates that wetlands play a disproportionally important role in affecting regional greenhouse gas budgets given that they only occupy approximately 10% of the total land area in the region.
Is the northern high-latitude land-based CO2 sink weakening?

(American Geophysical Union, 2011-08-30) Hayes, Daniel J. ; McGuire, A. David ; Kicklighter, David W. ; Gurney, Kevin R. ; Burnside, T. J. ; Melillo, Jerry M.

Studies indicate that, historically, terrestrial ecosystems of the northern high-latitude region may have been responsible for up to 60% of the global net land-based sink for atmospheric CO2. However, these regions have recently experienced remarkable modification of the major driving forces of the carbon cycle, including surface air temperature warming that is significantly greater than the global average and associated increases in the frequency and severity of disturbances. Whether Arctic tundra and boreal forest ecosystems will continue to sequester atmospheric CO2 in the face of these dramatic changes is unknown. Here we show the results of model simulations that estimate a 41 Tg C yr−1 sink in the boreal land regions from 1997 to 2006, which represents a 73% reduction in the strength of the sink estimated for previous decades in the late 20th century. Our results suggest that CO2 uptake by the region in previous decades may not be as strong as previously estimated. The recent decline in sink strength is the combined result of (1) weakening sinks due to warming-induced increases in soil organic matter decomposition and (2) strengthening sources from pyrogenic CO2 emissions as a result of the substantial area of boreal forest burned in wildfires across the region in recent years. Such changes create positive feedbacks to the climate system that accelerate global warming, putting further pressure on emission reductions to achieve atmospheric stabilization targets.
Net emissions of CH4 and CO2 in Alaska : implications for the region's greenhouse gas budget

(Ecological Society of America, 2007-01) Zhuang, Qianlai ; Melillo, Jerry M. ; McGuire, A. David ; Kicklighter, David W. ; Prinn, Ronald G. ; Steudler, Paul A. ; Felzer, Benjamin S. ; Hu, Shaomin

We used a biogeochemistry model, the Terrestrial Ecosystem Model (TEM), to study the net methane (CH4) fluxes between Alaskan ecosystems and the atmosphere. We estimated that the current net emissions of CH4 (emissions minus consumption) from Alaskan soils are 3 Tg CH4/yr. Wet tundra ecosystems are responsible for 75% of the region's net emissions, while dry tundra and upland boreal forests are responsible for 50% and 45% of total consumption over the region, respectively. In response to climate change over the 21st century, our simulations indicated that CH4 emissions from wet soils would be enhanced more than consumption by dry soils of tundra and boreal forests. As a consequence, we projected that net CH4 emissions will almost double by the end of the century in response to high-latitude warming and associated climate changes. When we placed these CH4 emissions in the context of the projected carbon budget (carbon dioxide [CO2] and CH4) for Alaska at the end of the 21st century, we estimated that Alaska will be a net source of greenhouse gases to the atmosphere of 69 Tg CO2 equivalents/yr, that is, a balance between net methane emissions of 131 Tg CO2 equivalents/yr and carbon sequestration of 17 Tg C/yr (62 Tg CO2 equivalents/yr).
Assessing the carbon balance of circumpolar Arctic tundra using remote sensing and process modeling

(Ecological Society of America, 2007-01) Sitch, Stephen ; McGuire, A. David ; Kimball, John S. ; Gedney, Nicola ; Gamon, John ; Engstrom, Ryan ; Wolf, Annett ; Zhuang, Qianlai ; Clein, Joy S. ; McDonald, Kyle C.

This paper reviews the current status of using remote sensing and process-based modeling approaches to assess the contemporary and future circumpolar carbon balance of Arctic tundra, including the exchange of both carbon dioxide and methane with the atmosphere. Analyses based on remote sensing approaches that use a 20-year data record of satellite data indicate that tundra is greening in the Arctic, suggesting an increase in photosynthetic activity and net primary production. Modeling studies generally simulate a small net carbon sink for the distribution of Arctic tundra, a result that is within the uncertainty range of field-based estimates of net carbon exchange. Applications of process-based approaches for scenarios of future climate change generally indicate net carbon sequestration in Arctic tundra as enhanced vegetation production exceeds simulated increases in decomposition. However, methane emissions are likely to increase dramatically, in response to rising soil temperatures, over the next century. Key uncertainties in the response of Arctic ecosystems to climate change include uncertainties in future fire regimes and uncertainties relating to changes in the soil environment. These include the response of soil decomposition and respiration to warming and deepening of the soil active layer, uncertainties in precipitation and potential soil drying, and distribution of wetlands. While there are numerous uncertainties in the projections of process-based models, they generally indicate that Arctic tundra will be a small sink for carbon over the next century and that methane emissions will increase considerably, which implies that exchange of greenhouse gases between the atmosphere and Arctic tundra ecosystems is likely to contribute to climate warming.
Simulating the impacts of disturbances on forest carbon cycling in North America : processes, data, models, and challenges

(American Geophysical Union, 2011-11-08) Liu, Shuguang ; Bond-Lamberty, Benjamin ; Hicke, Jeffrey A. ; Vargas, Rodrigo ; Zhao, Shuqing ; Chen, Jing ; Edburg, Steven L. ; Hu, Yueming ; Liu, Jinxun ; McGuire, A. David ; Xiao, Jingfeng ; Keane, Robert ; Yuan, Wenping ; Tang, Jianwu ; Luo, Yiqi ; Potter, Christopher ; Oeding, Jennifer

Forest disturbances greatly alter the carbon cycle at various spatial and temporal scales. It is critical to understand disturbance regimes and their impacts to better quantify regional and global carbon dynamics. This review of the status and major challenges in representing the impacts of disturbances in modeling the carbon dynamics across North America revealed some major advances and challenges. First, significant advances have been made in representation, scaling, and characterization of disturbances that should be included in regional modeling efforts. Second, there is a need to develop effective and comprehensive process-based procedures and algorithms to quantify the immediate and long-term impacts of disturbances on ecosystem succession, soils, microclimate, and cycles of carbon, water, and nutrients. Third, our capability to simulate the occurrences and severity of disturbances is very limited. Fourth, scaling issues have rarely been addressed in continental scale model applications. It is not fully understood which finer scale processes and properties need to be scaled to coarser spatial and temporal scales. Fifth, there are inadequate databases on disturbances at the continental scale to support the quantification of their effects on the carbon balance in North America. Finally, procedures are needed to quantify the uncertainty of model inputs, model parameters, and model structures, and thus to estimate their impacts on overall model uncertainty. Working together, the scientific community interested in disturbance and its impacts can identify the most uncertain issues surrounding the role of disturbance in the North American carbon budget and develop working hypotheses to reduce the uncertainty.
CO2 and CH4 exchanges between land ecosystems and the atmosphere in northern high latitudes over the 21st century

(American Geophysical Union, 2006-09-15) Zhuang, Qianlai ; Melillo, Jerry M. ; Sarofim, Marcus C. ; Kicklighter, David W. ; McGuire, A. David ; Felzer, Benjamin S. ; Sokolov, Andrei P. ; Prinn, Ronald G. ; Steudler, Paul A. ; Hu, Shaomin

Terrestrial ecosystems of the northern high latitudes (above 50oN) exchange large amounts of CO2 and CH4 with the atmosphere each year. Here we use a process-based model to estimate the budget of CO2 and CH4 of the region for current climate conditions and for future scenarios by considering effects of permafrost dynamics, CO2 fertilization of photosynthesis and fire. We find that currently the region is a net source of carbon to the atmosphere at 276 Tg C yr-1. We project that throughout the 21st century, the region will most likely continue as a net source of carbon and the source will increase by up to 473 Tg C yr-1 by the end of the century compared to the current emissions. However our coupled carbon and climate model simulations show that these emissions will exert relatively small radiative forcing on global climate system compared to large amounts of anthropogenic emissions.
Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire : an expert assessment

(IOPScience, 2016-03-07) Abbott, Benjamin W. ; Jones, Jeremy B. ; Schuur, Edward A. G. ; Chapin, F. Stuart ; Bowden, William B. ; Bret-Harte, M. Syndonia ; Epstein, Howard E. ; Flannigan, Michael ; Harms, Tamara K. ; Hollingsworth, Teresa N. ; Mack, Michelle C. ; McGuire, A. David ; Natali, Susan M. ; Rocha, Adrian V. ; Tank, Suzanne E. ; Turetsky, Merritt R. ; Vonk, Jorien E. ; Wickland, Kimberly ; Aiken, George R. ; Alexander, Heather D. ; Amon, Rainer M. W. ; Benscoter, Brian ; Bergeron, Yves ; Bishop, Kevin ; Blarquez, Olivier ; Bond-Lamberty, Benjamin ; Breen, Amy L. ; Buffam, Ishi ; Cai, Yihua ; Carcaillet, Christopher ; Carey, Sean K. ; Chen, Jing M. ; Chen, Han Y. H. ; Christensen, Torben R. ; Cooper, Lee W. ; Cornelissen, Johannes H. C. ; de Groot, William J. ; DeLuca, Thomas Henry ; Dorrepaal, Ellen ; Fetcher, Ned ; Finlay, Jacques C. ; Forbes, Bruce C. ; French, Nancy H. F. ; Gauthier, Sylvie ; Girardin, Martin ; Goetz, Scott J. ; Goldammer, Johann G. ; Gough, Laura ; Grogan, Paul ; Guo, Laodong ; Higuera, Philip E. ; Hinzman, Larry ; Hu, Feng Sheng ; Hugelius, Gustaf ; JAFAROV, ELCHIN ; Jandt, Randi ; Johnstone, Jill F. ; Karlsson, Jan ; Kasischke, Eric S. ; Kattner, Gerhard ; Kelly, Ryan ; Keuper, Frida ; Kling, George W. ; Kortelainen, Pirkko ; Kouki, Jari ; Kuhry, Peter ; Laudon, Hjalmar ; Laurion, Isabelle ; Macdonald, Robie W. ; Mann, Paul J. ; Martikainen, Pertti ; McClelland, James W. ; Molau, Ulf ; Oberbauer, Steven F. ; Olefeldt, David ; Paré, David ; Parisien, Marc-André ; Payette, Serge ; Peng, Changhui ; Pokrovsky, Oleg ; Rastetter, Edward B. ; Raymond, Peter A. ; Raynolds, Martha K. ; Rein, Guillermo ; Reynolds, James F. ; Robards, Martin ; Rogers, Brendan ; Schädel, Christina ; Schaefer, Kevin ; Schmidt, Inger K. ; Shvidenko, Anatoly ; Sky, Jasper ; Spencer, Robert G. M. ; Starr, Gregory ; Striegl, Robert ; Teisserenc, Roman ; Tranvik, Lars J. ; Virtanen, Tarmo ; Welker, Jeffrey M. ; Zimov, Sergey A.

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced.
Nitrogen attenuation of terrestrial carbon cycle response to global environmental factors

(American Geophysical Union, 2009-12-30) Jain, Atul K. ; Yang, Xiaojuan ; Kheshgi, Haroon ; McGuire, A. David ; Post, Wilfred ; Kicklighter, David W.

Nitrogen cycle dynamics have the capacity to attenuate the magnitude of global terrestrial carbon sinks and sources driven by CO2 fertilization and changes in climate. In this study, two versions of the terrestrial carbon and nitrogen cycle components of the Integrated Science Assessment Model (ISAM) are used to evaluate how variation in nitrogen availability influences terrestrial carbon sinks and sources in response to changes over the 20th century in global environmental factors including atmospheric CO2 concentration, nitrogen inputs, temperature, precipitation and land use. The two versions of ISAM vary in their treatment of nitrogen availability: ISAM-NC has a terrestrial carbon cycle model coupled to a fully dynamic nitrogen cycle while ISAM-C has an identical carbon cycle model but nitrogen availability is always in sufficient supply. Overall, the two versions of the model estimate approximately the same amount of global mean carbon uptake over the 20th century. However, comparisons of results of ISAM-NC relative to ISAM-C reveal that nitrogen dynamics: (1) reduced the 1990s carbon sink associated with increasing atmospheric CO2 by 0.53 PgC yr−1 (1 Pg = 1015g), (2) reduced the 1990s carbon source associated with changes in temperature and precipitation of 0.34 PgC yr−1 in the 1990s, (3) an enhanced sink associated with nitrogen inputs by 0.26 PgC yr−1, and (4) enhanced the 1990s carbon source associated with changes in land use by 0.08 PgC yr−1 in the 1990s. These effects of nitrogen limitation influenced the spatial distribution of the estimated exchange of CO2 with greater sink activity in high latitudes associated with climate effects and a smaller sink of CO2 in the southeastern United States caused by N limitation associated with both CO2 fertilization and forest regrowth. These results indicate that the dynamics of nitrogen availability are important to consider in assessing the spatial distribution and temporal dynamics of terrestrial carbon sources and sinks.
Insights and issues with simulating terrestrial DOC loading of Arctic river networks

(Ecological Society of America, 2013-12) Kicklighter, David W. ; Hayes, Daniel J. ; McClelland, James W. ; Peterson, Bruce J. ; McGuire, A. David ; Melillo, Jerry M.

Terrestrial carbon dynamics influence the contribution of dissolved organic carbon (DOC) to river networks in addition to hydrology. In this study, we use a biogeochemical process model to simulate the lateral transfer of DOC from land to the Arctic Ocean via riverine transport. We estimate that, over the 20th century, the pan-Arctic watershed has contributed, on average, 32 Tg C/yr of DOC to river networks emptying into the Arctic Ocean with most of the DOC coming from the extensive area of boreal deciduous needle-leaved forests and forested wetlands in Eurasian watersheds. We also estimate that the rate of terrestrial DOC loading has been increasing by 0.037 Tg C/yr2 over the 20th century primarily as a result of climate-induced increases in water yield. These increases have been offset by decreases in terrestrial DOC loading caused by wildfires. Other environmental factors (CO2 fertilization, ozone pollution, atmospheric nitrogen deposition, timber harvest, agriculture) are estimated to have relatively small effects on terrestrial DOC loading to Arctic rivers. The effects of the various environmental factors on terrestrial carbon dynamics have both offset and enhanced concurrent effects on hydrology to influence terrestrial DOC loading and may be changing the relative importance of terrestrial carbon dynamics on this carbon flux. Improvements in simulating terrestrial DOC loading to pan-Arctic rivers in the future will require better information on the production and consumption of DOC within the soil profile, the transfer of DOC from land to headwater streams, the spatial distribution of precipitation and its temporal trends, carbon dynamics of larch-dominated ecosystems in eastern Siberia, and the role of industrial organic effluents on carbon budgets of rivers in western Russia.
Reconciling carbon-cycle concepts, terminology, and methods

( 2006-01-06) Chapin, F. Stuart ; Woodwell, G. M. ; Randerson, James T. ; Rastetter, Edward B. ; Lovett, G. M. ; Baldocchi, Dennis D. ; Clark, D. A. ; Harmon, Mark E. ; Schimel, David S. ; Valentini, R. ; Wirth, C. ; Aber, J. D. ; Cole, Jonathan J. ; Goulden, Michael L. ; Harden, J. W. ; Heimann, M. ; Howarth, Robert W. ; Matson, P. A. ; McGuire, A. David ; Melillo, Jerry M. ; Mooney, H. A. ; Neff, Jason C. ; Houghton, Richard A. ; Pace, Michael L. ; Ryan, M. G. ; Running, Steven W. ; Sala, Osvaldo E. ; Schlesinger, William H. ; Schulze, E.-D.

Recent patterns and projections of climatic change have focused increased scientific and public attention on patterns of carbon (C) cycling and its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric CO2. Net ecosystem production (NEP), a central concept in C-cycling research, has been used to represent two different concepts by C-cycling scientists. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary production (GPP) and ecosystem respiration (ER), and that a new term—net ecosystem carbon balance (NECB)—be applied to the net rate of C accumulation in (or loss from; negative sign) ecosystems. NECB differs from NEP when C fluxes other than C fixation and respiration occur or when inorganic C enters or leaves in dissolved form. These fluxes include leaching loss or lateral transfer of C from the ecosystem; emission of volatile organic C, methane, and carbon monoxide; and soot and CO2 from fire. C fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to measuring C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle. Key words: Net ecosystem production, net ecosystem carbon balance, gross primary production, ecosystem respiration, autotrophic respiration, heterotrophic respiration, net ecosystem exchange, net biome production, net primary production.
The role of historical fire disturbance in the carbon dynamics of the pan-boreal region : a process-based analysis

(American Geophysical Union, 2007-06-20) Balshi, M. S. ; McGuire, A. David ; Zhuang, Qianlai ; Melillo, Jerry M. ; Kicklighter, David W. ; Kasischke, E. ; Wirth, C. ; Flannigan, M. ; Harden, J. W. ; Clein, Joy S. ; Burnside, T. J. ; McAllister, J. ; Kurz, Werner A. ; Apps, M. ; Shvidenko, Anatoly

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.

McGuire A. David

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