2

Changes to the Biology and Biochemistry of Ecosystems

SUMMARY

The study of large-scale ecosystems has become a rapidly maturing field of science. With the impetus of global change research, such studies have shown major successes over the past decade. 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. Computing capability and remote sensing technology have further driven change in the nature of the field. The capability has emerged not only to model at global scales but also to exploit data at these scales. Models have been developed and rejected based on the use of such data. Historically, large-scale ecosystem studies have also been integrative and multi-disciplinary, with problems often worked from beginning to end with significant interactions with the human dimension components. Some of this experience stems from applied roots in the field and traditional links with agricultural, forestry, and fisheries issues, as well as environmental policy and assessment. In fact, as in atmospheric chemistry (see Chapter 5), there is a rich history of assessment at all spatial scales.

Areas of success in large-scale ecosystem studies include the following:

  • Field and theoretical studies that have laid the foundation to understand the roles of vegetation and soils in weather and climate and that have advanced our methods for interpreting satellite data. Field experiments



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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade 2 Changes to the Biology and Biochemistry of Ecosystems SUMMARY The study of large-scale ecosystems has become a rapidly maturing field of science. With the impetus of global change research, such studies have shown major successes over the past decade. 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. Computing capability and remote sensing technology have further driven change in the nature of the field. The capability has emerged not only to model at global scales but also to exploit data at these scales. Models have been developed and rejected based on the use of such data. Historically, large-scale ecosystem studies have also been integrative and multi-disciplinary, with problems often worked from beginning to end with significant interactions with the human dimension components. Some of this experience stems from applied roots in the field and traditional links with agricultural, forestry, and fisheries issues, as well as environmental policy and assessment. In fact, as in atmospheric chemistry (see Chapter 5), there is a rich history of assessment at all spatial scales. Areas of success in large-scale ecosystem studies include the following: Field and theoretical studies that have laid the foundation to understand the roles of vegetation and soils in weather and climate and that have advanced our methods for interpreting satellite data. Field experiments

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade planned for the Mississippi and Amazon River basins will complete this series of studies. Development of satellite observation techniques, ground-based observations, and models to determine changes in land cover type and spatial and seasonal changes of vegetation. Clarifying the role of nutrients in large-scale interactions of ecosystems with the atmosphere. The effects of nutrients such as nitrogen and phosphorus must now be systematically incorporated into global models of land-atmosphere interactions. Implementation of an ambitious program to measure and model the sources and sinks of CO2 and trace gases from biological and biomass burning sources. This new information will facilitate the development of an observing system to determine trends and patterns of emissions and uptake at continental scales. Oceanic time series observations that have revealed previously unknown year-to-year variations in coupled ocean biology, chemistry, and physics, linked to climate variability. Regional ocean carbon studies that have quantified seasonal marine ecosystem effects on atmosphere-ocean CO2 exchange, and El Niño-related variations in equatorial Pacific sources and sinks of CO 2. Modeling the impacts of climate change and variability on agricultural and forest ecosystems. Overall, the U.S. Global Change Research Program (USGCRP) has been successful in advancing the science and tools required for space-based assessment of ecosystem change. The synergistic instrument complement consisting of the Earth Observing System (EOS) AM-1 and PM-1 platforms, combined with data from other ocean-sensing satellites, will largely satisfy the satellite data needs of the ecosystems community and will result in a massive improvement in the quality of remote observations. The ground- and ocean-based components of the program have had varying degrees of success. Atmospheric science components (biophysics and trace gases) have had the strongest programs. The more ecological components (vegetation and land cover) and integrative components (ecosystem manipulation experiments) have been supported on a rather ad hoc basis. 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. Biogeochemistry. Understand the changing global biogeochemical cycles of carbon and nitrogen. Multiple stresses. Understand the responses of ecosystems to multiple stresses.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade Biodiversity. Understand the relationship between changing biological diversity and ecosystem function. INTRODUCTION The ecosystems of the world are critical foundations of human society. People depend on ecosystems extensively for goods and services. Ecosystems provide such commodities as food, construction materials, and pharmaceuticals. In the context of global change, humanity's dependence on the biosphere for climate regulation, air quality, and clean water has also become starkly apparent. Thus, research on ecosystems in global change plays a dual role. First, decisions relating to climate variability, climate change, and other environmental problems require that we understand the impacts of climate, air pollution, and changing ultraviolet radiation on forests, agriculture, livestock, water resources, fisheries, biological diversity, and other critical life support systems. “Impacts research ” builds on the foundation of basic and applied research in agronomy and soil science, forest science, fisheries, ecology, and other well-established disciplines, in a context increasingly influenced by new concerns (climate change, tropospheric pollution, ultraviolet-B). An ambitious effort is under way to conduct a U.S. 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. Second, managing global change must also recognize the role that ecosystems play in modifying the atmosphere and hence the ocean-atmosphere-land climate system. We now know that vegetation and soils influence climate by controlling the amount of radiation reflected or absorbed, the evaporation of water, and other direct feedbacks to temperature, precipitation, and weather systems. Terrestrial ecosystems store a great deal of carbon and influence atmospheric CO2 both by releasing carbon as a result of land use (such as deforestation and agriculture) and by taking up carbon (the so-called missing sink). Marine ecosystems also influence oceanic carbon storage, interacting with physical and chemical processes. Ecosystems are also potent sources and sinks of other trace gases such as methane and nitrous oxide. Climate cannot be viewed as a force external to ecosystems: ecosystems participate in the shaping of weather, climate, atmospheric composition, and climate change. This view of ecosystems, as both responding to and controlling environmental change, is one of the great intellectual and practical contributions of global change research. It has implications for a wide range of issues, from the improvement of weather forecasts (by taking into account the state of vegetation) to

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade decisions about whether fossil fuel emissions should be limited and by how much. Ecology contributes a unique perspective to global change research. While the geophysical sciences begin conceptually with a unified physical-chemical view of systems (based in fluid dynamics, thermodynamics, and photochemistry), the underlying paradigm of ecology emphasizes the diversity of ecosystems, resulting from the evolutionary history of organisms, and the soils and landforms or water bodies they inhabit. This perspective, of seeking understanding from the similarities and differences of processes across a range of environments, has become important in interdisciplinary Earth system research. Planning the Program At the outset of global change research, ecology as a discipline emphasized organism- to local-scale investigations, and the field 's ability to address problems at even the landscape scale was limited. In addition, collaborations between ecologists, climatologists, and atmospheric chemists only began in the early 1980s. As a result, when the USGCRP and International Geosphere-Biosphere Program (IGBP) began, there was a substantial effort to develop the intellectual infrastructure within ecology to tackle problems at the global scale. A series of National Research Council (NRC) reports on global change and the IGBP planning process paid substantial attention to large-scale ecological issues, and the community also organized many important workshops and meetings. As a result, ecology has become much better prepared to take on science issues at scales from landscape to global on a breadth of issues and using tools not imagined a decade and a half ago. Some issues have remained over that period. The focus on both ecosystem feedbacks and ecosystem impacts has been consistent in NRC and IGBP guidance throughout this period. However, as the science has evolved, the specific Research Imperatives have evolved substantially. It is worthwhile to review NRC guidance on global change and ecological research. For example, in a 1986 report1 the summary recommendation (given with reference to the IGBP) was that “the initial priority . . . 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 population-community 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. In addition, long-term

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade and large-scale observational (as opposed to manipulative) studies became a major component of research, both building on the foundation of the National Science Foundation's (NSF) Long-Term Ecological Research (LTER) program and arising from the increasingly fruitful collaboration with the Earth sciences community (in which observational campaigns play a larger role than in the largely experimental discipline of ecology). By the 1990s the research agenda had come into sharper focus. In 1994 an NRC report, the “Chapin report”2 on terrestrial ecosystem research listed six major research areas: the interactive effects of CO2, climate, and biogeochemistry; factors that control trace gas fluxes; scenarios for managed and unmanaged ecosystems; how global change will influence biodiversity; how global change will affect biotic interactions with the hydrological cycle; and how global change will affect the transport of water, nutrients, and materials from land to freshwater and coastal zones. The Chapin report (NRC, 1994) also specifies the following needs: experiments that determine ecosystem responses to interactions among elevated CO2, temperature, water, and nutrients; research to predict the role of landscape-scale processes, including land use; and research to determine how changes in species composition affect the functions of ecosystems, which is urgently needed and unlikely to proceed without focused attention. Finally, the report lists as a major theme “the development and use of comprehensive models of ecological and physical systems” as a means of linking small-scale understanding to large-scale processes. The NRC was not the only forum in which the role of ecosystems in global change was discussed. The late 1980s to early 1990s saw the rapid development of the IGBP Global Change and Terrestrial Ecosystems Core Project (GCTE) and the Joint Global Ocean Flux Study Core Project (JGOFS), addressing terrestrial and marine ecosystems, respectively. 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) organized several important collaborations on ecosystems and global change.4 The IGBP elaborated considerably on the science agenda for global change research, and its deliberations have been documented in extensive reports. The IGBP GCTE research plan is in many

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade ways consistent with priorities enunciated in the Chapin report and earlier NRC documents, though it presents a substantially more detailed vision for studying managed (especially agricultural) ecosystems. Critical Results and the Development of Large-Scale Ecology In view of this scientific vision, what were the critical results and advances in ecosystem research during the first decade of the USGCRP? The areas of progress have been diverse, and many critical advances have been highly interdisciplinary. Discussed below are some of the most important areas of progress. Climate and Ecosystem Change: Evolution of Models and Observations Techniques and datasets for inferring the behavior of ecosystems from large-scale observations evolved rapidly during the 1990s. The use of inverse modeling a to deduce spatial and, later, temporal patterns of terrestrial and oceanic CO2 exchange produced a qualitative change in perceptions of the likely nature of terrestrial sinks. Although the use of inverse modeling began in the geophysical community, where both inverse modeling techniques and CO2 global observations were developed,5 collaborations to expand use of the technique rapidly grew to include ecologists.6 Inverse modeling showed a sink of CO2 in northern latitudes, through discrepancies between the observed interhemispheric gradient of CO2 and the values predicted based on fossil emissions and characteristics of interhemispheric transport. Whereas initial analyses had reached different conclusions about the distribution of this sink between marine and terrestrial systems, later analyses using 13C in CO2 and measured O 2 indicated a substantial terrestrial sink (see Figure 2.1). This sink has been and remains difficult to quantify or even detect in forest and soil inventory measurements; atmospheric measurements remain the most conclusive evidence for the location of the so-called missing sink. Applications of the inverse methodology over time have also suggested correlations between climate and terrestrial CO2 exchange at hemispheric to global scales. These observations remain preliminary but provide a foundation for future monitoring of global source and sink patterns. 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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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 2.1 Latitudinal ocean/land partitioning of the sources and sinks of CO2 versus latitude. The continuous line is the net flux of CO2 after removal of fossil fuels. The dashed line is the net flux of CO2 exchanged with the oceans. The dotted line is the net flux exchanged with land ecosystems. The sum of ocean and land fluxes equals the total net flux of CO2. SOURCE: Ciais et al. (1995b). Courtesy of the American Geophysical Union. 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).11 Eddy covariance time series in forests have provided direct observations of the effects of unusually warm, cold, and dry conditions on carbon exchange. 12 Because the temperature (T) responses of respiration (R) tend to be larger than those of photosynthesis (A) (T versus R is exponential, whereas T versus A b Manipulative experiments are ones where some variable or variables are deliberately altered and the response of the system is observed (e.g., experiments with artificially elevated atmospheric CO2).

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade is saturating), it has been hypothesized that global warming should lead to global net CO2 emissions from ecosystems. 13 Recent studies using USGCRP global datasets have tentatively confirmed this hypothesis for short timescales (<1 year) while suggesting more complex interactions on interannual timescales. 14 Modeling studies have probed this type of relationship quantitatively, suggesting a dependence on rates of mortality, major effects in soils, interactions with the nitrogen cycle, and interactions between physiology and biogeography.15 As observing techniques improve and time series lengthen, it has become increasingly possible to distinguish among alternative hypotheses about how future warming might affect ecosystems. An exciting development is the use of models to understand the emerging time series of climate and carbon exchange at the global scale. 16 Early efforts to model climate effects on ecosystems were extremely hypothetical.17 The models used then had been tested at a limited array of sites, against “typical” or average conditions. The ability of the models to simulate the dynamic response of ecosystems to varying climate had in general not been examined. Currently, models used to project future ecosystem responses are being tested against observed dynamic changes resulting from inter-annual climate variations using data collected at local to global scales.18 Land Surface Processes and Climate Research on the role of the land surface in climate has been a prominent area of research at the interface of ecology and atmospheric science over the past decade. 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. The use of eddy covariance flux measurements in ecological applications (in which measurement of the vertical winds and concentrations of a gas are measured together) was pioneered and verified in the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE),20 and the datasets collected during FIFE remain a touchstone for validating land surface models. A key scaling principle for canopies was first combined with land surface models as a result of FIFE collaboration, greatly increasing the simplicity and success of canopy modeling (see Figure 2.2). 21 The validity of “vegetation index”-based estimates of surface conductance was tested against observations during FIFE, and this validation remains a cornerstone of the communities ' confidence in satellite-driven land surface models. Results are just appearing from the Boreal Ecosystem-Atmosphere Study (BOREAS) campaign, which extended the FIFE paradigm to forested ecosystems. These results suggest a strong role for vegetation-atmosphere interactions in northern climates,22 as do results from the

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 2.2 Range of spatial scales addressed by the First ISLSCP Field Experiment (FIFE). SOURCE: Sellers et al. (1992). Courtesy of the American Geophysical Union. 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. They are also likely to make major contributions to understanding isotopic exchanges between ecosystems and the atmosphere. 23 Completion of the final planned experiment, the Brazilian-led Large-Scale Biosphere Experiment in the Amazon, will lead to a broad understanding of ecosystem-climate interactions in boreal, temperate, and tropical ecosystems. This work has had, and will continue to have, a major impact on our understanding of paleoclimate, contemporary weather and climate, weather forecasting, and climate projections.24 The need for comprehensive information on the land surface has also spawned a large effort to develop remote sensing algorithms for land surface variables. Major progress has been made in developing satellite algorithms to infer surface resistance to evaporation, temperature, soil moisture, and land cover.25 Experience

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade in using satellite data was gained using extant satellite systems such as the Thematic Mapper (TM), the Systeme Probatoire pour l' Observatión de la Terre (SPOT), and the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) systems. These data were successfully combined with algorithm development and evaluation in field campaigns such as FIFE, BOREAS, and the Hydrological and Atmospheric Pilot Experiment.26 The intensive, decade-long effort in this area resulted in the experimental design of two major EOS satellites (AM-1 and PM-1) being optimized for synergistic measurements of land surface variables. The primary platform, AM-1, orbits with a morning overpass time, chosen to minimize cloud contamination for land surface imaging, and includes a synergistic combination of three instruments: the Moderate-Resolution Imaging Spectroradiometer (MODIS), the Multi-Angle Imaging Spectroradiometer (MISR), and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). MODIS has capability in the visible and near infrared for remote sensing of vegetation characteristics with high time resolution. MISR takes multiangle measurements that allow determination of albedo and better constrain so-called vegetation indices related to conductance to water and photosynthesis. 27 ASTER and LANDSAT provide high spatial resolution information on land cover. The proposed remote sensing strategy for the physical climate system and carbon cycle studies is shown in Figure 2.3. 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. These science requirements are now largely executed on the EOS AM-1 spacecraft. AM-1 is a large and expensive mission, but the community has confidence in the quality of its land surface mission. This forms one component of AM-1's full scientific agenda, which also includes cloud, aerosol, atmospheric chemistry, and oceanographic experiments. Human Use and Modification of Ecosystems At the beginning of carbon cycle research, the carbon cycle appeared to be roughly in balance, with fossil emissions balanced approximately by ocean uptake and atmospheric accumulation. Beginning in the 1970s, ecologists led by George Woodwell began to make the case that emissions from land use, largely deforestation, had to contribute significant inputs to the atmosphere. As this hypothesis became better and better documented, it became clear that to balance land-use emissions an additional sink process, dubbed the “missing sink,” was required. The significance of land use in the carbon cycle was recognized prior to the USGCRP and the IGBP and has been a major focus of both programs since their

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 2.3 Satellite remote land-atmosphere interactions. Schematic of the diurnal variation of atmospheric boundary layer height and cloudiness for a humid continental region during the growing season; surface fluxes of net radiation sensible heat flux, water vapor, and CO2 are depicted. The proposed remote sensing strategy is shown for the physical climate system and carbon cycle studies. I. Geosynchronous observations of cloud fields, reflectances, temperatures. (GOES, METEOSAT). II. Two polar platforms with sounding instruments and imagers capable of resolving cloud fields. Platforms are spaced in time to characterize diurnal variations of atmospheric variables (POEM-1, EOS-pm). III. Polar platform with surface imaging payload. A morning crossing time is preferred to minimize cloud contamination (EOS-am). SOURCE: Sellers and Schimel (1993). Courtesy of Elsevier Science-NL.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade 80. Chameides et al. (1994). 81. Howarth et al. (1996). 82. Carbon storage, Goulden et al. (1996); trace species, Bakwin et al. (1995). 83. Mosier et al. (1997). 84. Responses to trends, Oechel et al. (1993), climatic events, Schimel et al. (1996), Wedin and Tilman (1996). 85. Cumulative impacts, e.g., Walker et al. (1987); Holland et al. (1997); analyses of land-use effects, e.g., Houghton et al. (1983). 86. Ambio vol. 23 (1994), Turner et al. (1995). 87. Asner et al. (1997), Holland et al. (1997). 88. Schimel et al. (1997a). 89. E.g., Dlugokencky et al. (1994), Hein et al. (1997). 90. Bakwin et al. (1995), Hurst et al. (1997). 91. Myneni et al. (1996, 1997); Braswell et al. (1997); Randerson et al. (1997). 92. Sellers et al. (1997), Braswell et al. (1997). 93. Harriss et al. (1994). 94. Groffman and Likens (1994). 95. Bolker et al. (1995), Schimel et al. (1996), Wedin and Tilman (1996). 96. Stability, Lawton (1995); vulnerability, Wedin and Tilman (1996); in their experiment the presence of low C-to-N ratio C3 plants reduced system-level nitrogen retention. 97. http:\\www.unesco.org/mab/collab/diver1.htm. 98. Pastor and Post (1986), Harden et al. (1992). 99. Keeling et al. (1996). 100. Schimel (1995). 101. Randerson et al. (1997). 102. Keeling and Shertz (1992). 103. Breymeyer et al. (1996). 104. Goulden et al. (1996). 105. Global Change Biology, vol. 2 (1996). 106. McNaughton and Jarvis (1991). 107. Schimel et al. (1991). 108. McNaughton and Jarvis (1991). 109. Aber and Melillo (1982). 110. Asner et al. (1997). 111. Najjar (1992). 112. Carpenter and Capone (1983). 113. Rosswall et al. (1988). 114. E.g., Shugart et al. (1988). 115. Hibbard and Shaulis (1997). 116. Levin (1992). 117. NRC (1997). 118. Sellers et al. (1997), Fung et al. (1987), Myneni et al. (1997), Potter et al. (1993), Randerson et al. (1997), Braswell et al. (1997). 119. Braswell et al. (1997) and Myneni et al. (1997). 120. Skole and Tucker (1993). 121. Wessman (1988). 122. Vitousek et al. (1996), Braswell et al. (1997), Schimel et al. (submitted). 123. Models, Sarmiento and Le Quéré (1996), Sellers et al. (1996a); data analyses, Denning et al. (1995), Sarmiento et al. (1995), Schimel et al. (1996); assessments, Melillo et al. (1993), VEMAP Participants (1995), Sarmiento and Le Quéré (1996). 124. (1) Sellers and Schimel (1993), (2) VEMAP Participants (1995); (3) Ibid.

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