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Role of the Terrestrial Biosphere in the Earth System RECENT ADVANCES The terrestrial biosphere plays a central role in global change as the cause of anthropogenic changes in the atmosphere and as the point of many critical feed- backs that govern the behavior of the earth system. In this section recent advances in our understanding of the role of the terrestrial biosphere in the earth system are briefly reviewed. These advances give us confidence that continued research in this area can rapidly reduce uncertainty about future global change. Rising atmospheric CO2 concentrations, as documented in the 35-year mea- surement record on Mauna Loa (Keeling et al., 1989) and the 200-year ice core record (Neftel et al., 1985), demonstrate that human-induced changes in the ter- restrial biosphere strongly influence atmospheric chemistry. Data from the geo- graphic network of monitoring stations, when incorporated into tracer models based on general circulation models, and supporting data from oceans and forests suggest that the terrestrial biosphere of the northern hemisphere may exert a strong negative feedback on the rate of atmospheric CO2 accumulation through increases in terrestrial carbon storage (Tans et al., 1990; Innes, 1991; Quay et al., 1992; Kauppietal., 1992). Human impact on the terrestrial biosphere is also largely responsible for the rapid increase in atmospheric concentrations of greenhouse gases other than CO2 (e.g., methane, nitrous oxide; see Cicerone and Oremland, 1988; Schimel et al., 1989; Matson and Vitousek, 1990). Although methane production can be partially predicted from patterns of soil moisture, its transport to the atmosphere is strongly influenced by microbial oxidation at the soil surface and by transport through
10 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE plants (Sharkey et al., 1991). In unflooded soils, nitrogen deposition and fertiliza- tion are reducing the strength of soils as a sink for atmospheric methane, thus contributing to rising atmospheric concentrations (Steudler et al., 1989; Mosier et al., 1991). Changing land use, particularly in the tropics, is a major contributor to increased nitrous oxide flux to the atmosphere (Matson and Vitousek, 1990). To- gether, increases in terrestrial sources of trace-gas flux (including consumption of fossil fuels) to the atmosphere (CO2, CH4, N2O, chlorofluorocarbons) are largely responsible for the increased radiative forces that causes global warming. The seasonal and geographic variations in terrestrial productivity can be qualitatively monitored with satellite-based remote sensing (Tucker et al., 1984), documenting the location and timing of terrestrial CO2 uptake and demonstrating changes in the land use of the globe. Simulation models that predict productivity from climate and nutrient cycling are now much more sophisticated and testable than even 5 years ago (e.g., Parton et al., 1987; Pastor and Post, 1988; Running et al., 1989; Rastetteret al., 1991). When linked to geographic information systems, these models provide reasonable estimates of productivity and CO2 flux on re- gional to continental scales (Vorosmarty et al., 1989; Burke et al., 1991). Global data bases of model inputs (e.g., biomass, soil carbon stores, physical properties) are being assembled and extrapolated globally with remote sensing (Matthews, 1983; Post et al., 1985; Botkin and Simpson, in press; Wessman et al., 1988; Aber et al., 1990). These models and data bases provide a means of predicting the current and future roles of the terrestrial biosphere as a source and/or sink of CO2, trace gases, water, and energy that are, some of the major parameters responsible for global changes in the earth system. Integration of confusing information from remote sensing, paleoecological studies and field research by means of models indicates that humans are substan- tially changing the productive capacity of the earth as a result of deforestation, pollution, overgrazing and other disturbances (Tucker et al., 1984; Schlesinger et al., 1990). For this reason the future behavior of the earth system cannot be pre- dicted from simple extrapolation of its present response to climate. Research into factors governing sustainability of managed systems (NRC, 1991; Lubchenco et al., 1991) has provided information on the long-term consequences of different patterns of land use. The global extent of changes in land use and their socioeco- nomic causes (Stern et al., 1992) must be documented to predict how humans respond to and effect further changes in the earth system. Major advances in our understanding of the role of the terrestrial biosphere in the earth system have come from whole-ecosystem experiments that document responses to CO2 (Curtis et al., 1989; Hendrey and Kimball, 1990; Grulke et al., 1990; Mooney et al., 1991), acid rain (Schindler et al., 1990), and climate (Lauenroth et al., 1978; Chapin and Shaver, 1985). These experiments demon- strate that ecosystems are remarkably resistant to many environmental changes, but that critical thresholds exist, beyond which alterations in the environment cause dramatic, and often unexpected, changes in ecosystem function (Carpenter
ROLE OF THE TERRESTRIAL BIOSPHERE IN THE EARTH SYSTEM 11 et al., in press). Thus, the future role of the terrestrial biosphere in the earth system cannot be predicted without understanding the internal controls over eco- system response to the environment (Schimel et al., 1990). In the past decade there have been major breakthroughs in understanding the roles of individual species in community and ecosystem processes. Certain spe- cies are strong interactors or "keystone species" that dramatically alter feedbacks with the atmosphere through changes in canopy structure, water use, trophic dy- namics, nitrogen cycling, productivity, and disturbance regime (Vitousek and Walker, 1989; Bryant et al., 1991; Carpenter et al., in press). Paleoecological and experimental research demonstrates that each species has a unique distribution or growth response to climate (Davis, 1981; Chapin and Shaver, 1985; COHMAP, 1988) and that dispersal can limit the rate at which species come into equilibrium with climate (Davis, 1981). Consequently, predictions of the future distribution and productivity of ecosystems must incorporate understanding of environmental responses and controls over migration of individual species or functional groups of species (Pastor and Post, 1988). Recent research has demonstrated the large impact of terrestrial vegetation on local and regional climate. The canopy structure and the physiology of individual plants govern water loss and energy budgets of vegetated surfaces (Jarvis and McNaughton, 1986; Rosenberg et al., 1989) in a way that can have dramatic and long-lasting effects on temperature and precipitation at regional and continental scales (Running et al., 1989; Shukla et al., 1990). Albedo of vegetation can also strongly influence regional climate (Schlesinger et al., 1990). REMAINING UNCERTAINTIES The brief summary presented above demonstrates that detailed knowledge of the terrestrial biosphere is essential to understanding the causes and consequences of global changes in the earth system. The rapid progress in understanding the global role of the terrestrial biosphere permits a focus on six key research ques- tions that will substantially reduce uncertainty in predicting the response of the earth system to global change: 1. What are the interactive effects of changes in COy climate, and bio- geochemistry on the terrestrial carbon cycle and on food and fiber production? Previous research has largely ignored interactions of CO2 with other factors and provides only limited evidence of CO2 effects on ecosystem processes. 2. What factors control trace-gas flux between terrestrial ecosystems and the atmosphere? Current understanding is based on environmental correlations rather than on whole-ecosystem experiments. 3. What are reasonable scenarios of the future distribution, structure and productivity of both managed and unmanaged ecosystems based on changes in land use, disturbance regime and climate? Predictions of patterns of land use,
12 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE disturbance regimes and species movements are critical to predicting the future rate of global change. 4. How will global change alter biotic diversity and what are the ecosystem consequences? Little is known about consequences of losses in biological diver- sity to ecosystems or to global processes. 5. How will global change affect biotic interactions with the hydrologic cycle and surface energy balance? The general understanding of physiological and canopy controls of plant-water relations must be integrated into landscape- level analyses. 6. How will global change affect biotic controls over transport of water, nutrients, and materials from land to freshwater ecosystems and to coastal zones of the ocean? The impact of global change on the integration of terrestrial, fresh- water and coastal ocean ecosystems has received little attention. For each of these six research topics the remaining uncertainties, the neces- sary research, and the timing and nature of results that can be expected from the research are described in Chapter 3. The new research programs needed are em- phasized, as well as ongoing ones whose continuation is critical.