Global Environmental Change Research Pathways for the Next Decade (1999) / Chapter Skim
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2 Changes to the Biology and Biochemistry of Ecosystems
2 Changes to the Biology and Biochemistry of Ecosystems
Pages 19-86
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From page 19... ...
Improved fundamental understanding of marine and terrestrial ecosystems and hydrology has already led to practical applications in weather and climate modeling, air quality, and better management and natural hazards responses for water, forest, fisheries, and rangeland resources. The development of spatially resolved global-scale ecosystem models has occurred only during the past five years.
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The Research Imperatives for the future are as follows: • Land surface and climate. Understand the relationships between land surface processes and weather prediction and changing land cover and climate change.
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national assessment of the potential consequences of climate variability and change to provide a detailed understanding of the consequences of climate change for the nation, including the interactive effects of environmental changes to climate, atmospheric chemistry, sea level, water quality, and land use. This chapter describes the research that is under way to provide appropriate links to that activity by emphasizing the scientific aspects of managed ecosystems, especially at the regional scale.
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From page 22... ...
. is to obtain additional experimental data, so that new models can be developed to extrapolate ecological responses to environmental changes that have not been experienced in the past." That report called for "laboratory and field experiments at the organism level and compilation of existing data on population and community patterns." It also observed that "experiments are needed on intact ecosystems, using large-scale manipulations" and that "in the long-term, ecosystem models must be assembled that couple populationcommunity models with process-functional models." This agenda had a major influence on the approaches taken by ecologists for both marine and terrestrial ecosystems, and all three of the NRC's recommended agenda areas were pursued in parallel by the community.
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From page 23... ...
In addition, the International Global Atmospheric Chemistry (IGAC) program became involved in studying biological sources of trace gases,3 the Biological Aspects of the Hydrological Cycle Core Project began activities in biophysical research, and the Scientific Committee on Problems of the Environment (SCOPE)
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From page 24... ...
The relationship of terrestrial carbon storage to climate is fundamental to understanding the interactions between climate and ecosystems that may occur during future climate changes. This subject has been addressed for terrestrial systems by a combination of experimental lab and field studies and by observational programs and data synthesis.7 In addition, there has been vigorous model a Inverse modeling is defined as modeling where the chain of inference runs opposite the chain of causation: in the case of the carbon cycle, sources and sinks are modeled from atmospheric concentrations and transport.
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From page 25... ...
ing of climate effects on both terrestrial and marine ecosystems.8 Although the response of ecosystem processes to climate has long been of interest, recent work has led to a much more general understanding of temperature and moisture effects on biota and on the interactions of climate effects with internal ecosystem processes such as succession and the nitrogen cycle.9 This work includes manipulative experimentsb in the lab and field that have led to an improved understanding of microclimatic effects on biological processes and the specific behavior of particular ecosystems, whereas comparative studies and data syntheses have led to better understanding of ecosystem to global patterns. Long-term flux observations have illuminated the effects of climate on carbon storage: measurements over the past 25 years in the Arctic have shown tundra systems shifting from being a sink to a source of CO2 as conditions became warmer and drier.10 Recent advances in measurement techniques, especially the advent of eddy covariance techniques and their application in long-term studies, have produced unique data on climate effects on net ecosystem exchange (NEE)
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From page 26... ...
It has led to dramatic developments in science and in observing systems. Beginning with a few provocative papers on the potential effects of land cover and inhomogeneities in land cover on climate, research on land surface processes has expanded to encompass a large and diverse theoretical and modeling effort validated by a series of highly successful international field campaigns.19 The field campaigns have in turn led to a number of payoffs.
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NSF Arctic System Science program's Land-Atmosphere-Ice Interaction Study, and the importance of disturbance processes (such as large-scale fires) in land surface water and carbon exchange.
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Research on land surface processes exemplifies a constructive partnership among many groups: global climate modelers, organism- to ecosystem-oriented bio- and microclimatologists, ecologists, and remote sensing scientists, as well as geographers, plant physiologists, soil scientists, and hydrologists. Having defined the need for improved satellite algorithms early on, the community carried out the necessary theoretical, modeling, and empirical demonstrations of such a capability.
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The proposed remote sensing strategy is shown for the physical climate system and carbon cycle studies.
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inception, as well as later in the International Human Dimensions Programme for Global Environmental Change. Whereas early attention was given to the carbon cycle, interest increased about the role of land use in altering trace gas budgets through soil processes and biomass burning28 (see Figure 2.4)
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From page 31... ...
Global retrieval remains a major problem, which is being addressed, albeit in a piecemeal and somewhat halting fashion at the present. However, understanding contemporary vegetation patterns, patterns of human land use, and the interactions of human and natural ecosystem processes is increasingly emerging as an important research area, and a vigorous community is developing.33 This area of science has a strong natural link to biodiversity research since climate impacts on biodiversity will occur synergistically with other environmental changes.
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From page 32... ...
Yet a 4°C temperature increase resulted in persistent carbon storage under elevated CO2.43 CO2 enrichment under the more fertile conditions of the Chesapeake Bay wetlands leads to continued stimulation of photosynthesis and plant growth, especially below-ground growth, for at least nine years.44 The contrast between the Arctic and saltmarsh studies shown in these examples suggests a key regulatory role for nutrient limitation; however, shorter-term experiments indicate no consistent modulation of CO2 responses by nutrient availability.45 Soil warming could drive increased decomposition and mineralization, but it could also lead to increased drought and consequent decreases in primary production, decomposition, and mineralization. With nine years of summertime warming, Arctic tundra primary production changed only slightly, though the increased abundance of deciduous shrubs presages decreased decomposition and future declines in nutrient availability.46 Warming in montane meadows also stimulated dramatic changes in plant species composition, with increased dominance by sagebrush, indicating likely future changes in carbon storage, nutrient cycling, and biodiversity.47 Long-term nutrient additions can also stimulate dramatic
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From page 33... ...
Multiple Stresses on Ecosystems The dominant concern originally motivating global change research was global climate change. As ecologists and their colleagues from other disciplines began to address ecology at regional and larger scales, the importance of multiple large-scale environmental changes became apparent.
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Progress in this area is critical because carbon cycle research forms the basis for setting targets in international negotiations to mitigate climate change.59 Understanding contemporary and possible future fluxes of carbon is the essential underpinning of sound policy to manage radiative forcing of the atmosphere. The development of accurate and reliable measurement techniques for carbon fluxes is a prerequisite for evaluating the success of measures undertaken to comply with the Framework Convention on Climate Change and to monitor international compliance with other treaty measures.
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From page 35... ...
, or the inability of models to account for the fate of the excess carbon, is referred to as the "missing sink," and the excess carbon was assumed to be in terrestrial ecosystems because models of ocean uptake of CO2 indicated actual uptake of 2 to 2.5 Gt, based on calibration against isotopic 14CO (from thermonuclear bomb testing) ,61 and fossil fuel emissions and atmo 2 spheric accumulation were known.
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with fossil fuel emissions, seasonal vegetation (no net annual source or sink) , tropical deforestation of 0.3 Gt of C per year, and three different cases of ocean uptake: (b)
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The ways that ecosystem models deal with biota have changed substantially in the past decade. It is now widely recognized that functional differences between vegetation types influence ecosystem processes.
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to population-community processes is in its infancy at global scales, it is widely viewed as the next major challenge. One influential, if highly simplified paper, suggested that future changes to global ecosystem carbon storage will be dominated by the population processes of mortality, recruitment, and rates of migration or recovery.69 The importance of considering processes across biological levels of organization was highlighted by a recent large-scale international collaboration, the Vegetation and Ecosystem Modeling and Analysis Project.70 VEMAP compared models of biogeochemistry and biogeography for the conterminous United States under current and general circulation model (GCM)
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Despite this not unexpected result, all of the biogeochemical models showed that the nitrogen cycle influenced the sensitivity of the carbon cycle to CO2 and climate.73 The paradox of VEMAP -- that the models agreed on NPP and carbon storage under current conditions but simulated divergent behavior under altered conditions -- led to a detailed analysis of model mechanisms.74 The analysis showed that, while the models agreed on continental average NPP and carbon stocks (see Table 2.2) , they disagreed on spatial patterns within vegetation types and, more fundamentally, predicted different relationships of NPP and decomposition to nutrient and biophysical controls.75 The differing spatial sensitivity of NPP to water versus nutrient regulation was a clear predictor of response to changes in the modeled climate conditions.
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(Tg C) Carbon storage (Gt C)
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From page 41... ...
Other efforts are in progress within the IGCP-GCTE program and in numerous investigators' programs. By illustrating quantitatively the sensitivity of ecosystem function to biogeography -- functional diversity among plant types, disturbance, and demography -- the study emphasizes the importance of linking population/communitylevel ecology to biogeochemistry, a critical development for the eventual synthesis of global change and biodiversity research.
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Research Imperatives and Key Scientific Questions The preceding discussion indicates there are four Research Imperatives that should guide ecosystem studies in the USGCRP for the coming decade: • Land surface and climate. Understand the relationships between land surface processes and weather prediction, and changing land cover and climate change.
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• How is terrestrial carbon storage regulated by land use, changes to ma rine ecosystems, internal ecosystem processes, and climate, and how might this storage change in response to future environmental changes? • What are the consequences of the anthropogenically accelerated nitrogen cycle?
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Early work on biogenic trace gases focused on radiatively active trace species. Measurements made during the USGCRP and by IGBP projects have demonstrated that biogenic gases from microbes, plants, and biomass burning influence atmospheric photochemistry and aerosol formation, thereby impacting the atmospheric cycles of ozone, the hydroxyl radical, and reactive nitrogen.
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Biodiversity Imperative As an ecosystem modeling framework has been developed for understanding biophysical and biogeochemical processes at regional to global scales, the interaction of ecosystem function with species ecology has emerged as a topic of increasing urgency. Although not all research on biological diversity lies within the global change research agenda, there is an important interface between the two areas.
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• How does the functional diversity of organisms in ecosystems affect carbon uptake and sequestration, nutrient cycling, biophysical interac tions with climate, and trace gas emissions? • What information on plant, microbial, and animal function is needed to model the role of organisms in large-scale changes in community compo sition and ecosystem function?
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From page 47... ...
, many of these time series must span years to decades. As discussed later under "Lessons Learned", observational time series and long-term experi TABLE 2.3 Key Measurements and Research Imperatives: Need for a Crosscutting Research Agenda Multiple Biodiversity Sources And Stresses And And Changing Ecosystems And Sinks Of Ecosystem Ecosystem Physical Climate Biogenic Gases Change Function Key Measurement X X X X Network of sites.
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From page 48... ...
Many human impacts are cumulative, and analyses of land-use effects on terrestrial carbon storage show the need for time series data on human impacts.85 In any program evaluating global changes to ecosystems, direct human impacts must play a central role.86 Human use of ecosystems also alters their susceptibility to modification by climate and other "natural" processes.87 Understanding ecosystem interactions with the physical environment requires knowing the type and state of ecosystems present, making knowledge of changing community composition and
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From page 49... ...
. Observations of atmospheric change Time series observations of atmospheric CO2 have long been a mainstay of global change research, and, as that time series has lengthened, the degree of insight it provides into the dynamics of the global carbon cycle has continued to increase.
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In addition to using the vertical sampling scheme, continental sites must be located to sample the range of ecosystems and human systems present. The experimental design of the full sampling approach is critical and must be carefully analyzed before an investment in data gathering is made.
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From page 51... ...
On a larger scale, satellite measurements of vegetation activity have increasingly proven their utility for observing seasonal and interannual variations in vegetation activity.91 Global observations of vegetation state are important to understanding climate-land surface interactions, biogeochemical cycles, and hydrology.92 Although direct observations of canopy gas exchange provide information on ecosystem interactions with climate at local scales, remote observations of vegetation provide a spatial perspective. As the next generation of space-borne sensors and algorithms become available, with improved calibration and atmospheric correction, the detection of variability and secular trends will become more quantitative.
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Coupled measurements of controls over productivity, including climate variables, soil moisture, and nutrient availability, also will be of great value. In addition, as ecosystems change, their emissions of other trace gases may change.
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The biophysical and biogeochemical processes are naturally linked because biophysical processes are critical regulators of trace gas exchange and biogeochemical processes influence biophysical exchanges. Although site measurements of trace gas flux are useful for detecting change and testing models, regional measurements using atmospheric techniques are crucial for constructing budgets.
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It should be researched through a partnership between the global change and biological diversity communities. Research on this topic is internationally coordinated by Focus 4 of the IGBP Global Change in Terrestrial Ecosystems
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program for marine ecosystems. Aspects of ecosystem function that must be addressed include, but are not limited to, carbon exchange, nitrogen cycling, productivity, albedo, and hydrological processes.
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This may be historically consistent with ecology's past as a largely experimental field and the structure developed for global change research, but much has been gained from minimally intrusive long-term observations. Ecologists, who have always used such data opportunistically, have derived from statisticians and earth scientists rigorous analytical methods of observational records (using time series analysis and inverse modeling)
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From page 57... ...
Many phenomena in marine and terrestrial ecosystems occur over long periods of time, including soil carbon accumulation and turnover, nutrient accumulation, forest succession and plant community responses to climate, ocean circulation changes and consequent physical and nutrient influences on marine ecosystems, and population and evolutionary responses to environmental change.98 Long time series are also critical for model validation. Global change encompasses changes to many environmental controls over ecosystem processes.
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FIGURE 2.8 (a) Monthly average atmospheric carbon dioxide concentration at Mauna Loa Observatory, Hawaii.
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change. Experimental plots established and sustained for over a decade by the NSF's LTER program in temperate grasslands have led to critical understanding of the role of species diversity in sustaining ecosystem processes through drought cycles and of the impact of cumulative nitrogen stress on ecosystem carbon storage and nitrogen cycling.
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Several key lessons can be distilled from the scientific community's experience with time series measurements. FIGURE 2.10 Daily net CO2 exchange (NEE)
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• Second, the time series should include both the response of interest (e.g., atmospheric CO2 or net primary productivity) and the hypothesized con trols over the response variable's dynamics.
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Thus, variations in atmospheric deposition at mesoscales or larger, and in hydrology at the watershed or larger scale, are key controls on N cycling over very long timescales.110 Nitrogen cycling in marine ecosystems is similarly dominated by a fast cycle on short timescales, governed by local trophic interactions and microbial recycling. But on seasonal to millennia timescales, transport of nitrogen to the euphotic (sunlit)
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From page 63... ...
Need for Technical Infrastructure in Ecology Major advances in understanding terrestrial ecosystems have resulted from the use of new measurement and data analytical techniques. Such measurement techniques have included flux measurements by eddy covariance, isotopic measurements, remote sensing of vegetation canopies, and measurements of atmospheric O2.
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Encouraging the application of advanced techniques for interpreting observational data and for model-data fusion will build on a strong foundation. There are clear needs for the infusion of quantitative inferential methods such as time series analysis, Kalman filters, data assimilation and adjoint techniques, inverse modeling, and many other approaches in ecology, as well as novel adaptation of these methods to important ecological problems.
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From page 65... ...
Similar lessons could be drawn from studies of vegetation and hydrometeorology and of biological sources of trace gases. Study of the carbon cycle required an interdisciplinary approach for several reasons: • Both terrestrial and marine biological processes are central to its regula tion.
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From page 66... ...
Even within the IGBP the carbon cycle is just emerging as a coherent program-level activity, spanning the terrestrial Global Change and Terrestrial Ecosystems Core Project, oceanic (JGOFS, Land-Ocean Interactions in the Coastal Zone, IGAC) , and integrative (Global Analysis, Interpretation, and Modeling)
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From page 67... ...
When research areas, like the carbon cycle, require the partner ship of disciplines whose funding mechanisms are not traditionally coor dinated, mechanisms must be developed to allow the establishment of problem-oriented rather than disciplinary priorities within and across agencies. RESEARCH IMPERATIVES Again, critical Research Imperatives for ecosystem studies in the USGCRP for the next decade include the following: • Land surface and climate: Understand the relationships between land surface processes and weather prediction and changing land cover and climate change.
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From page 68... ...
Time Series Observations of Ecosystem State Global time series of vegetation and phytoplankton state, derived from NOAA's AVHRR and Coastal Zone Color Scanner sensors, for land and ocean, respectively, have proven their value in understanding the seasonal and spatial characteristics, interannual variability, and trends of large-scale biogeochemistry and biophysical processes.118 Space-based measurements of ecosystem state are fundamental in determining the link of terrestrial ecosystems to climate, the biogeochemistry of the land and oceans, and the impacts of climate and other disturbances. While the measurements of "greenness" and ocean color are not direct ecological properties, they have proven to be highly correlated with the spatiotemporal dynamics of ecosystems.
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These requirements apply generally for both terrestrial and marine ecosystems: marine ecosystems add additional instrument requirements to avoid saturation by sun glint or high reflection. Note that data for land cover change require higher spatial resolution but lower temporal resolution.
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From page 70... ...
Measurements of Diversity, Functional Diversity, and Ecosystem Function The issue of diversity and species composition changes has emerged as a critical topic of global change in recent years. It is clear that the functional diversity of the Earth's biota is a first-order control over global ecosystem function, but how changes to the biota will affect global ecosystem function still is a young research topic.122 Designing a global observing system and network of experimental studies, analogous to those described above for biogeochemical fluxes, is premature; the necessary monitoring and manipulations at global scales are currently far from obvious.
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From page 71... ...
. The next generation of models, the so-called dynamic global vegetation models, couple the above three types of models because the time-dependent change of vegetation results from interactive biophysical, biogeochemical, and population-community processes.125 Marine ecosystem models often simulate the coupled dynamics of carbon and nutrients (usually nitrogen)
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From page 72... ...
These models must simulate biophysical processes, biotic uptake and storage of carbon, and ecosystem effects on atmospheric CO2 and other trace gases. • Changes to the distribution of species and/or functional types will have profound effects on ecosystem functioning.
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It is essential to assess proposed mitigation measures for greenhouse gas emissions and experimental manipulations with ecosystem models prior to implementing such measures. These efforts may depend on the response of long-lived species such that experimental evaluation would require decades.
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; land surface processes, Henderson-Sellers (1993) , Sellers et al.
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(1997) interactions with nitrogen cycle, VEMAP Participants (1995)
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82. Carbon storage, Goulden et al.
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1989. Exchange of Trace Gases Between Terrestrial Eco systems and the Atmosphere.
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Global Biogeochemical Cycles 11:111-124. Carleton, J.T.
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Global Biogeochemical Cycles 10:603-628. Friedlingstein, P., I
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Global Biogeochemical Cycles 10:431 456. Hurst, D.F., P.S.
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Global Biogeochemical Cycles 3:281-285. Martin, J.H.
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Long-term impact of land-use change. Global Biogeochemical Cycles 11:29-42.
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Global Biogeochemical Cycles 10:677-692. Schimel, D.S., VEMAP Participants, and B.H.
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Global Biogeochemical Cycles 10:711-726. Tilman, D., and J.A.
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: Com paring biogeography and biogeochemistry models in a continental-scale study of terrestrial ecosystem responses to climate change and CO2 doubling. Global Biogeochemical Cycles 9:407-438.
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