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Research Program INTERACTIVE EFFECTS OF CO2, CLIMATE, AND BIOGEOCHEMISTRY The Problem Since the beginning of the Industrial Revolution, the atmospheric CO2 con- centration has risen 21 percent (Houghton et al., 1990). This rise has led to pre- dictions of increased global temperature and changes in the global hydrologic cycle that may already be occurring (Hansen and Lebedeff, 1987). However, atmospheric CO2 concentration is increasing less rapidly than would be predicted from known rates of fossil fuel use, deforestation, and CO2 uptake by the land and oceans (Houghton et al., 1990). Several lines of evidence based on interhemi- spheric gradients in atmospheric CO2 concentration, intra-annual variations in atmospheric CO2, models of ocean-atmosphere CO2 exchange, and patterns of forest growth together suggest that the terrestrial biosphere of the northern hemi- sphere may be the sink for this "missing carbon" (Tans et al., 1990; Innes, 1991; Quay et al., 1992; Kauppi et al., 1992). However, current estimates of CO2 ex- change, based on carbon inventories of ecosystems and patterns of land-use change, suggest that the terrestrial biosphere should be a net carbon source (Houghton et al., 1990). Until the current global carbon budget can be balanced, there is little hope of predicting future changes in atmospheric CO2 concentration and, therefore, the radiative properties of the atmosphere that will determine fu- ture climatic changes. 13
14 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE One mechanism by which the terrestrial biosphere could sequester additional carbon is "CO2 fertilization" of plant production. Many greenhouse studies and a few field studies of agricultural and natural ecosystems suggest that vegetation and soils could sequester much of the CO2 added to the atmosphere by human activities (Curtis et al., 1989; Bazzaz, 1990; Idso and Kimball, 1991). Other eco- system studies suggest that much of the terrestrial vegetation is so strongly limited by other resources that, in the short term, it shows little response to increases in atmospheric CO2 (Mooney et al., 1991; Oechel and Billings, 1992). Until this issue is resolved, the nature or strength of the negative feedback exerted by the terrestrial biosphere on rising atmospheric CO2 concentrations cannot be pre- dicted, and global atmospheric models will be unable to project future climate. These CO2 responses and their interactions with other environmental factors must also be known in order to predict future food and fiber production and, therefore, the human carrying capacity of the earth. Current Efforts The Global Change and Terrestrial Ecosystems (GCTE) project of the Inter- national Geosphere-Biosphere Program (IGBP, 1992) has developed a research agenda that includes study of the consequences of simultaneous changes of mul- tiple resources for ecosystems (GCTE, focus 1). The project's research plan calls for a series of large-scale field manipulations of CO2, temperature, nutrients and water coupled with measurements of changes at various levels of organization; including those at the physiological, population, community and ecosystem lev- els. The field manipulations are to be supplemented by controlled-environment experiments, as needed, to provide the mechanistic basis for process models. More thorough documentation of current terrestrial carbon pools and fluxes also is a component of the IGBP plans. This compilation is essential to establish baseline conditions in regional and global models of biogeochemical cycles. Sev- eral research groups are now engaged in compiling such data. Research Questions A major question motivating this research is whether terrestrial ecosystems are a net source or sink for atmospheric CO2. Is the net primary productivity of terrestrial ecosystems changing? To predict future changes in terrestrial carbon storage and its sensitivity to changes in climate, it must be known under what conditions (particularly conditions of temperature, nutrients and water), terrestrial ecosystems will respond to rising atmospheric CO2 concentrations. How will experimental manipulations of ecosystems affect patterns of productivity and nu- trient cycling, the competitive interactions among plants, and the interactions among plants, herbivores, pathogens, and other trophic levels?
RESEARCH PROGRAM 15 General Strategy The international planning efforts for research that will increase our under- standing of the interactive effects of changes in CO2 and other resources at a variety of scales, ranging from physiological processes to whole ecosystems, merit strong support (IGBP, 1990a). Such research programs should emphasize whole- ecosystem experiments and associated process-based modeling in both managed and unmanaged ecosystems to clarify the importance of ecosystem-level feed- backs and indirect CO2 effects. Research should be designed to test conceptual understanding of ecosystem processes and their response to environmental forc- ing. Carefully controlled greenhouse and laboratory experiments should focus on controls over specific processes that are shown to be critical in ecosystem re- sponses. The field experiments should be accompanied by efforts to improve geographically explicit regional models of ecosystem function and biogeochemis- try, using remote-sensing data to extend experimental results to large areas. Research Program The research program outlined here is discussed in greater detail in interna- tional documents (IGBP, 1990a, 1992). 1. GLOBAL CARBON POOLS AND FLUXES. Objective: To develop an inventory of dynamic terrestrial carbon pools and fluxes that is linked to global carbon models. Current estimates of terrestrial carbon pools should be used as input to process-based models that predict fluxes over broad regional areas. These flux estimates should then be linked to tracer models based on general circulation model (GCM) runs, using inverse modeling. (Inverse modeling is the process by which known outputs of a modelâfluxes between the atmosphere and the bio- sphere in this caseâare used as constraints on model behavior.) A remote-sens- ing program should be developed that is capable of estimating plant production on the basis of satellite observations validated by adequate field studies. Initially, this will require development and verification of relationships between vegetation reflectivity and production for different ecosystems due to regional and seasonal variations in leaf display. Carbon fluxes from land to freshwater and near-shore marine ecosystems should be quantified (see the section on land-water interac- tions at the end of this chapter). The global inventory should be maintained as a data bank available to the scientific public. The offices that house these data banks could serve as centers for ecosystem synthesis and modeling. 2. WHOLE-ECOSYSTEM RESPONSES. Objective: To determine the mecha- nisms by which linked plant-soil systems respond to simultaneous changes in COy temperature, water, and nutrientsâthat is, critical environmental factors that may constrain ecosystem responses to rising atmospheric CO2. This experi- ment will involve factorial field manipulations of these factors conducted for at
16 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE least a decade, using free-air CO2 enrichment (FACE) technology or large field greenhouses. Critical measurements include gross primary production, net pri- mary production, changes in soil and plant carbon storage, litter quality, carbon and nitrogen mineralization by soil microbes, nitrogen loss to groundwater, trace- gas fluxes, evapotranspiration, water and nutrient use efficiencies, reproductive output, phenology, and changes in species composition of plants and animals. To explain the mechanisms by which species composition changes in whole-ecosys- tem experiments, supplementary experiments examining species interactions such as competition, disease, herbivory, and plant-microbe interactions may be neces- sary. Particular attention to below-ground processes is warranted. Experimental work should begin in ecosystems where nutrients, water, or temperature constrain CO2 response (e.g., semiarid grasslands or dryland wheat, where CO2-water-nu- trient interactions are probably important, or temperate forests, where changes in wood or woody litter could affect carbon storage and nitrogen availability). Other critical ecosystems with greater logistical challenges that should be addressed include tundra and boreal forest, which may respond sensitively to climate change, and whose large carbon stores are potentially strong positive feedbacks to global climate. A range of approaches should be considered to alter CO2 concentration, particularly FACE arrays and large field-installed greenhouses. The FACE arrays provide more natural conditions of wind, humidity, animal movement and the like, but make it difficult to manipulate air temperature. 3. BELOW-GROUND RESPONSES. Objective: To determine how interactions between elevated CO2 and temperature, water, and nutrients affect below- ground processes. Experimental work and process-based models should clarify mechanisms and conditions under which elevated CO2 increases carbon flux to soils, and the consequences of this increase for nutrient availability and carbon storage, and for interactions among plants, mycorrhizal fungi, pathogens, below- ground herbivores, and decomposer organisms. 4. WHOLE-PLANT RESPONSES. Objective: To determine how simultaneous changes in COy temperature, water, and nutrients affect plant growth and yield. Experiments should be undertaken where necessary to explain the results of the whole-ecosystem experiments described above. Experimental work should be closely integrated with process-based modeling to predict plant growth and com- petitive balance on the basis of whole-plant carbon and nutrient allocation and water use. The use of isotopes, both stable and radioactive, may be required in these studies. Where necessary to explain whole-plant performance, controlled- environment experiments and associated models should explore organ and cellu- lar processes (e.g., stomatal conductance, respiration, root exudation). In man- aged systems these experiments should be conducted on critical crop and forest species (IBSNAT, 1989; IGBP, 1989). Similar lists should be developed for repre- sentative functional groups of wild plants, as mentioned in the section on ecosys- tem distribution later in this chapter.
RESEARCH PROGRAM 17 Status and Priorities There has been considerable research on single-factor CO2 effects on plants under controlled-environment conditions. At present, the greatest uncertainty in predicting the role of the terrestrial biosphere in the global carbon cycle is know- ing to what extent low nutrient or water availability constrains the response of ecosystems to elevated CO2 and temperature. Therefore, the highest research priority should be accorded to several whole-ecosystem experiments in which CO2 is manipulated in combination with temperature, nutrients, and water in both managed and unmanaged ecosystems. Research in managed ecosystems is criti- cal to predicting and sustaining future yields of food and fiber. Because un- managed or little-managed systems comprise most of the terrestrial biosphere, their response to elevated CO2 and temperature is critical to global predictions of terrestrial CO2 responses. Ecosystem experiments should be supplemented by controlled-environment experiments to address specific questions raised by the whole-ecosystem experiments. Knowledge and technical expertise are currently sufficient for the initiation of such experiments immediately in logistically trac- table managed and unmanaged ecosystems. Whole-ecosystem manipulations in other ecosystems could be phased to build on experience gained initially. Such research efforts will be expensive; each experiment will require about $2 million annually (IGBP, 1992). Within 10 years this research program should provide the information necessary to predict the conditions under which elevated CO2 alters (1) production of food and fiber and (2) the role of the biosphere as a feedback to atmospheric CO2. An additional area of high priority is the development and validation of re- mote-sensing procedures to estimate productivity and soil processes on a regional scale. These measurements could provide validation for the substantial data on terrestrial carbon pools that are currently being synthesized, mapped and used in process models to predict global carbon fluxes. They would also provide a direct measure of the extent to which the productivity of the biosphere is being sus- tained. Together these programs should determine the extent to which terrestrial ecosystems constitute the missing sink for CO2. CONTROLS OVER TRACE-GAS FLUX TO THE ATMOSPHERE The Problem During the past decade, there has been increasing awareness of how strongly the physical and chemical properties of the earth's atmosphere are influenced by biologically mediated trace-gas exchanges between the land and the atmosphere. Microbial processes in soils play a major role in the generation and/or consump- tion of CH4 and N2O, greenhouse gases that are accumulating in the atmosphere at
18 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE annual rates of 0.9 percent and 0.25 percent, respectively (Houghton et al., 1990). These microbial processes respond to a variety of environmental factors, includ- ing soil temperature, moisture, redox, and nutrient status. At present, the re- sponses of trace-gas fluxes to simultaneous changes in these variables cannot be adequately predicted. For this reason, the major causes of the increases in atmo- spheric concentrations of biogenic trace gases are not known, and the isotopic budget of methane, a major trace gas, is severely out of balance given current estimates of sources and sinks (Cicerone and Oremland, 1988). Other Current Efforts The International Global Atmospheric Chemistry (IGAC) project of the IGBP has developed a comprehensive research agenda that includes research on the biological controls of trace-gas fluxes in high-latitude, temperate, and tropical ecosystems (IGBP, 1990b). Research on ecological controls over trace-gas flux is being planned by the biogeochemistry activity of GCTE. Research planned at high latitudes includes surveys, environmental correlation studies, and mechanis- tic process studies, all emphasizing CH4. In the temperate zone, the IGBP pro- grams focus on exchanges of N2O, CH4, CO, and CO2 between natural and man- aged ecosystems and the atmosphere. The IGBP tropical program focuses on the effects of land-use change and rice agriculture on exchanges of NO, N2O, CH4, CO, and CO2. The information available on patterns of trace-gas flux between terrestrial ecosystems and the atmosphere has expanded dramatically in the past 5 years, but much less is known about the controls over these fluxes under natural conditions. In high-latitude, temperate, and tropical regions, the GCTE project of the IGBP is recommending whole-ecosystem manipulations to study the conse- quences of changing environmental variables on trace-gas fluxes. Research Questions The major research question is: What is causing the rapid increase in trace- gas flux from terrestrial ecosystems to the atmosphere? In tundra and boreal forests, where soils contain large stocks of potentially decomposable carbon, the most critical question is: How does soil warming, soil drainage, and their interac- tion affect CO2 and CH4 fluxes from soils? A recent National Science Founda- tion-sponsored workshop evaluated experimental approaches and recommended sites for soil-warming experiments. In temperate forest and grassland soils and upland boreal soils, which are CH4 sinks and sources of N2O (Steudler et al., 1989; Mosier et al., 1991; Whalen et al., 1991), a key research question is: How will increases in temperature and precipitation, as well as increases in nitrogen deposition from acid rain and agricultural fertilization, affect the uptake of CH4 and the release of N2O? How are these processes affected by conversion from
RESEARCH PROGRAM 19 unmanaged to agricultural systems or by management practices? In the tropics, where land-use change causes major changes in trace-gas flux (Matson and Vitousek, 1990), critical questions are: How does temperature, moisture, and nu- trient availability change at various times following forest clearing and pasture abandonment? How do these changes affect fluxes of CO2, CO, CH4, N2O, and NO? General Strategy The research program developed internationally by the IGAC and GCTE (IGBP, 1990a, 1990b) merits strong support. This program emphasizes (1) ex- perimental and modeling studies of biotic controls over trace-gas fluxes in natural and managed systems and (2) inverse modeling, which links information from terrestrial ecosystems with global atmospheric budgets. Research Program The research program outlined here is discussed in greater detail in interna- tional documents (IGBP, 1990a, 1990b). 1. LINKAGE TO GLOBAL ATMOSPHERIC MODELS. Objective: To determine the major terrestrial sources and sinks for trace-gas exchange with the atmo- sphere. Inverse modeling using tracer models based on GCM runs should be done to estimate the general location and magnitude of terrestrial sources and sinks for trace-gases. This modeling would provide a basis for allocating efforts in regional experimental studies and studies that measure trace-gas fluxes at a variety of scales (e.g., the National Aeronautics and Space Administration's Bo- real Ecosystem-Atmosphere Study [BOREAS] project). Better information on the isotopic composition of terrestrial trace-gas sources will be required to fully implement a program in inverse modeling. 2. CH4/CO2 FLUXES FROM TUNDRA AND BOREAL WETLANDS. Objective: To determine how changes in temperature and hydrology affect CH4 and CO2 fluxes from tundra and boreal wetlands to the atmosphere. Laboratory micro- cosms, field manipulations, and latitudinal transects should be used to determine how changes in temperature and hydrology interact to affect CH4 and CO2 ex- changes between wetlands or lakes and the atmosphere. For example, a northern wetlands ecosystem should be drained and warmed in factorial combination; mea- surements of controlling variables, including temperature, moisture, redox, pH, and nitrogen availability, would be made over time, along with measurement of C02 and CH4 fluxes. Studies should be designed to consider spatial heterogeneity at the watershed and regional scales, as well as seasonal variations in environ- ment. For high-latitude ecosystems and for the regional studies described below, the results should be synthesized into process-based models of trace-gas produc-
20 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE tion, consumption, and transport, and these models should be coupled with data bases organized in a geographic information system (GIS) to estimate regional trace-gas fluxes under various scenarios of future climate. Particular attention should be paid to nonlinearities and thresholds in the response of trace-gas fluxes to the environment because these responses cannot be predicted from observa- tions over a limited range of conditions. 3. CH4/N2O PRODUCTION/CONSUMPTION IN UPLAND TEMPERATE ECOSYSTEMS. Objective: To determine how changes in soil temperature, moisture, and nitro- gen input affect CH4 uptake and N2O production by temperate and boreal for- ests, grasslands, and agricultural ecosystems. Possible field experiments in- clude soil warming, precipitation exclusion, wet-up/dry-down plots, and nitrogen fertilization. Results from these studies should be used to develop process models of CH4 and N2O fluxes. Ultimately, these models should be linked with a regional GIS so that estimates of current regional CH4 and N2O fluxes, and the changes in these fluxes as a result of future climatic change and past agricultural conversion, reversion to forest, and acid deposition can be made. 4. TROPICAL LAND-USE CHANGE. Objective: To determine how clearing of tropical forests for crop and pasture, management of the latter systems and their abandonment affect the fluxes of COy CO, CHf N2O, and NO between soils and the atmosphere. Changes in soil fluxes of these gases and their controls should be linked to factors regulating carbon and nitrogen cycling in replicated managed sites (including grazing systems, dryland agriculture and rice paddies), fallow systems, and undisturbed forests and savannahs. Plots of different ages (i.e., a chronosequence) should be used to determine the time course of trace-gas flux following land clearing and pasture abandonment. Transects across precipi- tation and temperature gradients should be used to estimate patterns of environ- mental control. Examination of plots with different management regimes (e.g., for rice cultivation) will allow evaluation of different management practices. The results should be synthesized with process models and GISs to develop regional trace-gas exchange estimates for future regional land-use scenarios. Status and Priorities Most of the research areas described above are being actively investigated or plans to do so exist. Research to date has documented patterns of trace-gas flux and has led to hypotheses about environmental and biotic controls. The highest priority should be given to field manipulations and associated process-based mod- eling to test these hypotheses; and to inverse modeling, linking terrestrial sources and sinks to patterns of atmospheric changes in trace-gas concentrations. Field manipulation experiments and process-based models are essential to delineate controls over processes, which must be the basis of meaningful scenarios of future trace-gas flux. Inverse modeling provides constraints on current flux estimates.
RESEARCH PROGRAM 21 A clearly focused experimental and modeling program could determine the major causes of increased flux of trace gases to the atmosphere within 10 years. Programs that document patterns of trace-gas flux have been useful in devel- oping hypotheses about which regions and environmental controls are critical to global patterns of trace-gas flux. These programs should continue, now focusing on environmental gradients of factors that are expected to change with altered climate (e.g., latitudinal temperature and moisture gradients in northern regions, moisture gradients in semiarid grasslands, successional changes following land clearing in the tropics). These measurements should be closely tied to measure- ments of ecosystem processes so that controls over trace-gas flux can be linked to understanding of biogeochemical and carbon cycling (see the section on interac- tive effects of CO2 and climate above). FUTURE DISTRIBUTION AND STRUCTURE OF ECOSYSTEMS The Problem It is a major challenge to predict how the present ecosystems of the world will respond to global change. However, if predictions of GCMs are correct, the cli- mate will change at least an order of magnitude more rapidly than at any time in the Pleistocene, probably more rapidly than many species can migrate (Houghton et al., 1990). As a result, future ecosystems will consist of new combinations of species, structured by new patterns of species interactions. Moreover, the world's future ecosystems will experience novel combinations of environmental factors, an altered disturbance regime, and exploitation by a larger human population. Patterns and intensity of land and water management for food, fuel, and fiber are highly responsive to human demand as well as to climate, but the ecological im- pact of a given rate of population growth will differ dramatically between societ- ies (Stern et al., 1992). A critical challenge, therefore, is to predict the future distribution and structure of ecosystems and the resulting changes in energy bal- ance, trace-gas flux, and hydrologic balance (Mooney et al., 1987; IGBP, 1990a). Other Current Efforts A three-tiered hierarchical modeling approach has been the major basis of the research planned by GCTE to predict the future structure and distribution of eco- systems (Prentice et al., 1989; NRC, 1990; IGBP, 1990a). In brief, the strategy consists of developing models of processes operating at patch, landscape, and regional scales. Patch-scale models describe the response of a relatively homoge- neous community (e.g., a farmer's field or a forest stand) to changes in climate, atmosphere and land use. Patches interact through exchanges of soil, water and biological propagules, and through contagious disturbances such as fire and pest
22 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE outbreaks. Finally, regional-scale models that predict the distribution of broadly defined vegetation types can be coupled to GCMs, allowing feedback between vegetation properties and atmospheric processes. Functional groups (i.e., a group of species with similar effects on ecosystem processes), have been characterized in aquatic and terrestrial ecosystems (Minshall et al., 1985; IGBP, 1990a), allow- ing generalized predictions of conditions necessary for successful establishment, growth, and reproduction. Research in the social sciences has begun to quantify the magnitude of the ecological impact of a given population size, technology, and growth rate (Meyer and Turner, 1992; Stern et al., 1992), and the changes in water use and agricultural output that result from altered climate (Rosenberg and Crosson, 1991). However, there has been little attempt to link socioeconomic predictions with ecological projections. Research Questions The major research questions in this section are: How can the future structure and distribution of terrestrial ecosystems be predicted as they change in response to global change? How will increasing CO2 and climatic change alter the rate and extent of disturbances to influence the interaction and distribution of currently widespread species and of general types (functional groups) of organisms? What are the important biological factors (e.g., soil properties, seed banks, long-lived individuals) that respond slowly to global change and may modify the predicted changes in distribution of functional groups? What will be the rate and distribu- tion of land-use change in the future? What social, cultural and economic factors govern the ecological impact of a given rate of human population growth? What are the impacts of land-use changes on climate, disturbance regime, and ecosys- tem structure and distribution? General Strategy Predictions of changes in ecosystem distribution should begin with species that are currently widespread dominants of major ecosystem types (e.g., trout, white-tailed deer, Douglas fir) and/or are of critical economic importance (e.g., major food or fiber sources), because a great deal is already known about these species and changes in their distribution will have profound biotic and economic consequences. Boundaries between major ecosystems are places where changes in species distributions may be detected most readily. More general predictions should focus on "functional groups" of wild and cultivated species, defined by properties that have predictable responses to soil resources, disturbance regime and land use (Hobbie et al., 1992). Models must be developed that predict changes in disturbance regime and land use as functions of human population growth, per capita ecological impact, and other aspects of global change (Rosenberg and Crosson, 1991). Because of the uncertainty of these predictions, it is more reason-
RESEARCH PROGRAM 23 able to develop alternative scenarios based on explicit assumptions than to at- tempt a single prediction based on many uncertainties. Research Program Human Impact on Ecosystems 1. PATTERNS OF LAND-USE CHANGE. Objective: To predict the rate and dis- tribution of land-use changes. Socioeconomic models that predict these changes should be validated with historical data and satellite observations of current trends and used to predict ecosystem structure, landscape structure, and dispersal of or- ganisms. These efforts should be focused in areas where land-use changes (in- cluding altered management practices) are causing the greatest change in ecosys- tem structure (e.g., tropical forests, arid grasslands, and temperate agricultural fields that are reverting to forests) and where land-use change might cause the greatest atmospheric feedbacks (e.g., tropical wet forest, boreal forest). 2. INTERACTIONS BETWEEN CO2-INDUCED ENVIRONMENTAL CHANGE AND LAND-USE CHANGE. Objective: To predict how CO2-induced environmental change may alter rates or patterns of land-use change. Models that predict the biological consequences of changes in climate and CO2 should be linked to socio- economic models in order to predict changes in human population growth, water use, and land use (Rosenberg and Crosson, 1991). For this linkage to be success- ful, the ecological models must include probabilities of critical ecological or agri- cultural catastrophes as a function of climate variability and land use. The feed- backs of these changes on climate and atmospheric composition similarly require attention. Climatic Effects on Ecosystems 1. FORECASTS OF FUTURE DISTRIBUTION AND PRODUCTIVITY OF CURRENTLY DOMINANT SPECIES. Objective: To develop empirically based models that pre- dict changes in distribution and productivity of species that are currently wide- spread community dominants or are critical as sources of food or fiber. Obser- vations at current limits of distribution and transplant experiments within and beyond the current ranges of these species can suggest the climatic and biotic controls over establishment, growth, reproduction, and mortality. Such baseline information can be used to design controlled-environment or field transplant ex- periments in which species' responses to temperature, water, CO2 and nutrients, as well as the interactions of these factors with pathogens and herbivores, are quantified (see the carbon-cycle section at the beginning of the chapter). For crop species those changes in productivity and allocation that determine economic yield should be emphasized. Experimental results should be synthesized by modeling to predict (1) to what
24 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE extent genetic and physiological plasticity can allow species to sustain productiv- ity despite altered climate and (2) how the distribution of these species and the nature of species interactions might respond to changes in climate, water supplies, and other environmental factors such as ozone, UV-B, and nitrogen deposition. These species-specific scenarios provide a basis for monitoring at community boundaries those parameters that would be the most sensitive indicators of biotic response to climatic change. In the case of crop species, the scenarios will pro- vide critical information relevant to the sustainability of human populations. 2. CLIMATIC CONTROLS OF FUNCTIONAL GROUPS. Objective: To predict how functional groups of soils and organisms respond to a changing environment. Beginning with information on the widespread and/or critical species described above, species should be combined into functional groups according to general patterns of response to environmental resources (e.g., water and nutrients) and life-history traits determining migration. The predictive value of grouping spe- cies or soils into functional groups should be tested, and models incorporating climate responses to predict the future distribution of these types should be vali- dated against the paleoecological record. Predictions of changes in energy and water balance and in CO2 and trace-gas flux resulting from altered vegetation structure due to novel combinations of species should be incorporated into GCM projections. The functional group approach is intended to complement the indi- vidual species approach (described in paragraph one above), in situations where the physiological and population traits of individual species are poorly known or when computational constraints limit the number of taxa that can be considered in integrated vegetation/climate models. The criteria used to define functional groups must be flexible and will change depending on the specific goals, scales, and geographic location pertinent to the research question. Rates and Patterns of Community Change 1. DISTURBANCE EFFECTS ON ECOSYSTEM STRUCTURE AND DISTRIBUTION. Objective: To predict the relationship between climate or resource management and the type, extent, and ecological consequences of disturbance. Paleoecologi- cal data (e.g., fire scars in trees, charcoal in lake sediments) and historical data should be the basis of models that predict the type, extent, frequency, and conse- quences of disturbance from information on climate, resource management prac- tice (e.g., fire suppression, thinning), and human modification of the landscape (e.g., urban encroachment, road networks). Similarly, floods, droughts and species invasions can be critical disturbances to aquatic systems. These models should incorporate climatic and land-use forcing functions, ecosystem feedbacks and spe- cies interactions (e.g., insect outbreaks) that modify the disturbance regime; and processes governing the abundance of both juvenile and mature individuals. 2. MIGRATION RATES. Objective: To determine factors that govern rates of species movement. Rates of species movement in response to past changes in
RESEARCH PROGRAM 25 climate can be deduced from fossil pollen data. Historical data on rates of move- ment of invading exotic species can be used to deduce spread rates under condi- tions of human disturbance. Such data should be combined with general consider- ations of life-history traits, germination and seedbed requirements (water-flow requirements for aquatic species), dispersal mechanisms, and successional status to formulate general models of the potential migration rates of species and func- tional groups under changing climate regimes and human use of the landscape. Both changes in mean conditions and changing frequencies of extreme events need to be considered in modeling climatic constraints on species movement. 3. BIOLOGICAL LEGACIES. Objective: To determine the importance of bio- logical legacies (e.g., soil properties, seed storage in soils, long-lived individu- als, mycorrhizal inocula) in altering rates of ecological change. Field experi- ments and modeling should be combined to predict the importance of biological legacies. For example, the patterns and rates of change in soil properties follow- ing transplant into different climatic zones should be determined. Status and Priorities There has been considerable research in all of the above areas, particularly in forestry and aquatic systems, but it has not been generalized in a fashion that allows broad predictions for the globe. Priority should be given to synthesis and modeling focused on processes likely to cause large functional or structural changes in ecosystems. The following areas are particularly critical: ⢠predicting the types and magnitude of human impact on terrestrial ecosys- tems; ⢠determining the role of CO2 concentration and climate, as mediated by species interactions, in governing future distribution of currently widespread spe- cies and of generalized functional groups; and ⢠understanding the role of landscape-scale processes, especially distur- bance and land-use change, in governing ecosystem change. Modeling should be supplemented by monitoring of regions thought to be particularly sensitive to changes induced by increased CO2 concentrations (particularly at community boundaries) and regions in which changes may feedback to climatic change. With 2 to 5 years of research focused on the development of a strong theoreti- cal framework and links to the social sciences, it should be clear what types of research are necessary to predict the future distribution and structure of terrestrial ecosystems. Within 10 to 20 years, models that project future productivity and the role of the biosphere in the earth system should be sufficiently realistic to serve as a solid basis for management decisions affecting the rate of global change.
26 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE GLOBAL CHANGE AND ECOLOGICAL COMPLEXITY The Problem Ongoing global changes in ecological diversity resulting from land-use change and species introductions will probably be exacerbated by future atmo- spheric and climatic changes. Loss of biological diversity is the least reversible of the many ongoing and anticipated global changes to terrestrial ecosystems. While changes in the atmosphere and climate are reversible on the time scale of centu- ries, and land degradation may be reversed on a similar scale, the loss of species is truly permanent. However, as yet there is neither a clear understanding of the interaction between global change and biological diversity in terrestrial ecosys- tems nor a coherent program to develop such an understanding. Diversity is di- rectly important to people for aesthetic and recreational reasons and because it provides a wide range of plant and animal products critical to human societies. Other Current Efforts The effects of global change on ecological diversity have received consider- able attention, notably, but not wholly focusing on loss of species diversity. Agronomists are concerned that the genetic base of major crops is being narrowed dangerously. Taxonomists and population ecologists have proposed urgent na- tional and international efforts to address the loss of genetically distinct popula- tions, subspecies, and speciesâmany of which have not been and may never be cataloged. Community and ecosystem ecologists are concerned about changes in landscape diversity and their consequences. Aquatic ecologists have analyzed the interactions among eutrophication, toxic contaminants, and species alterations in regulating the structure and functioning of freshwater and intertidal ecosystems. All of these changes have been driven primarily by human-caused changes in land use and industrialization, and this is likely to remain the major component of global change for some decades. However, climatic and atmospheric changes loom as increasingly important factors. This is true because (1) while the present species have persisted through glacial/interglacial cycles of climate and atmo- spheric change, the predicted changes are both more rapid and outside the bounds of glacial/interglacial cycles (Houghton et al., 1990) and (2) the migration of species in a human-dominated landscape will differ qualitatively from that in ear- lier times of rapid climatic change. The other area of concern, the effects of ecological complexity (i.e., genetic diversity in populations, species diversity, landscape diversity) on ecosystem function, has received less attention. Global change and ecological complexity is a focus within GCTE that will consider both the effects of global change on com- plexity and the effects of complexity on ecosystem function (IGBP, 1990a). A Scientific Committee on Problems of the Environment (SCOPE) project
RESEARCH PROGRAM 27 has been established to address the effects of ecological complexity on ecosystem function. It is designed to review and synthesize knowledge in the field. Follow- ing an initial meeting to define issues and set directions, this project will organize 12 system-based meetings and syntheses, followed by an overall synthesis. The IGBP-GCTE steering committee will use results from the SCOPE process as it develops to define research needs in its core program. Research Questions Although it is well established that human activity is causing a loss of diver- sity within and among species as well as an increase in landscape diversity, there is little ability to predict the patterns and consequences of these changes. Several major questions arise: Are there predictable patterns of change in ecological di- versityâby level of diversity, functional or taxonomic group, or region? Are there components of global change that are particularly important to ecological diversity (e.g., the breakdown of barriers to species dispersal caused by human travel, or the erection of such barriers by land use or dams)? Once ecological and genetic diversity has been lost, can it be restored? What controls the rates at which diversity is generated? To answer these questions it is critical to integrate scenarios of land-use change with ecological predictions. There is even less basis for predicting how changes in ecological diversity will affect the functioning of ecosystems. Does there exist a threshold for diver- sity within functional groups (defined here as groups of organisms that perform an ecosystem function), above which diversity has no further detectable effects on ecosystem function? If so, do managed ecosystems exhibit these effects? General Strategy The two-pronged approach to this problem involves (1) determining how glo- bal change (particularly land-use change) affects ecological and genetic diversity at population, species, and landscape levels and (2) identifying the thresholds at which changes in ecological diversity feedback to alter patterns of human activity and climate and the nature of the feedback. Research Program 1. LOSS OF GENETIC DIVERSITY. Objective: To determine the causes and consequences of the loss of genetic diversity associated with global change. In- formation from managed systems should be synthesized to establish the effects of management practice on genetic diversity of critical crops, fisheries, and timber species over their geographic ranges to determine what causes such loss. The relationship of the genetic diversity of a crop species to the range of conditions under which it can grow should be examined, with emphasis on important food
28 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE crops in Third World countries. The relationship between genetic diversity and climatic tolerance should then be examined in long-lived wild species to predict how much climatic change a given species might tolerate without migration or extinction. Genetic models should seek to identify the factors governing rates of evolutionary change as well as management approaches that will reduce prob- abilities of extinction for different types of species. 2. PATTERNS OF CHANGE IN SPECIES DIVERSITY. Objective: To predict future patterns of change in species diversity. Information on past and present patterns of species extinctions and endangerment should be synthesized to rules and mod- els that predict (1) what kinds of species are most prone to extinction resulting from global change and (2) what types of habitat fragmentation and change are most likely to cause species extinctions or speciation. Because one can never hope to catalog all the species that are being lost, it is essential that the factors controlling species extinction be determined so that extinction rates can be mini- mized. Traits that might influence extinction are as diverse as trophic level, hu- man utility, population size, home range, habitat complexity, habitat or biome type, or number of species in a functional group or community. At the same time, political and economic scenarios of future land-use patterns should be linked with ecological models of populations and communities to predict those community types that are particularly vulnerable to species extinction. 3. ECOSYSTEM CONSEQUENCES OF SPECIES DIVERSITY. Objective: To deter- mine the ecosystem consequences of changes in species diversity. The ecosys- tem consequences of changes in species diversity can be addressed using species addition/removal experiments (including simplified managed systems) in con- junction with modern ecosystem-level measurements of functions. Such studies should emphasize those ecosystem functions that will alter terrestrial feedbacks to the atmosphere or interactions among landscape units, or cause further changes in ecological diversity. Particular attention should be paid to the impact of diversity within as well as between functional groups and to thresholds of diversity below which ecosystem function is seriously impaired. The purpose here is to identify the types of species that are critical to ecosystem function and the extent of spe- cies diversity required to ensure that these functions can be maintained. 4. CHANGES IN SPECIES INTRODUCTION/MIGRATION. Objective: To predict patterns of genetic and species diversity in a world with fewer barriers to species introduction but more barriers to migration. Paleoecological studies can now be integrated with studies of current population processes to determine the controls over species migration into new communities. While it has proved difficult to predict which species will establish new populations, models should be developed to predict which types of species are most likely to establish populations beyond their current ranges and the ecological and human cultural factors that most strongly influence these probabilities. Historical, observational, and experimental studies should examine the effects of migration and exotic introductions on eco- system function.
RESEARCH PROGRAM 29 5. CHANGES CM PATTERN OF LANDSCAPE DIVERSITY. Objective: To predict future patterns of landscape diversity and their significance for the functioning of landscape units. Observational studies can now take advantage of past human alterations of landscape diversity to examine what types of landscape units are most sensitive to changes in the size and arrangement of patch types within a landscape or barriers such as chains of rivers and lakes. Predictions of future patterns of landscape diversity will need to make use of socioeconomic models of human populations and land use, as well as the developing understanding of patch interactions on a landscape scale. These models can be compared with satellite data on habitat fragmentation. 6. RECOVERY AND RESTORATION OF ECOSYSTEM DIVERSITY. Objective: To develop principles leading to practical programs to restore ecological diversity. Principles developed in forestry and aquatic ecology (NRC, 1991) should be gen- eralized to determine how management practice, abandonment of managed sys- tems, species interactions, and natural succession alter ecological complexity at all levels (genetic diversity to landscape diversity). Innovative programs will be needed that use basic ecological principles to speed the recovery of ecological complexity. These studies are critical if the ecological impacts of global change are to be mitigated. Status and Priorities There are programs and scientific consortia actively concerned with most factors governing ecological diversity. However, many of these studies have been oriented toward a basic understanding of ecological diversity rather than toward predictions of how it will respond to changing climate and land use or how changes in ecological diversity influence ecosystem function. The most reward- ing approach would be a program designed to determine causes of loss in diver- sity and its consequences for feedbacks to land use, water quality, biogeochemis- try, and atmospheric chemistry. This program should begin with workshops to synthesize the current state of knowledge and to define critical experimental and modeling approaches (an effort already initiated by SCOPE) that can be rapidly implemented. These workshops should emphasize species diversity because this is the level at which ecological diversity is being lost most irrevocably. A second high-priority area is the development of protocols to maintain and restore ecological diversity that are based on existing expertise in forestry and aquaculture. Understanding of the importance of genetic and landscape diversity is less mature and will require a phase of workshops to develop critical research agendas. Within 2 to 5 years a theoretical framework could be constructed through workshops and research initiated that could, in the long term (10 to 20 years),
30 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE explain the major ecosystem consequences of losses of biotic diversity and pro- vide programs to maintain or restore effective functioning of damaged ecosystems. BIOTIC EFFECTS ON WATER AND ENERGY BALANCE The Problem Global environmental change introduces new uncertainties in predicting the future water supply, a parameter that is critically important in determining how ecosystems and human societies function. Although GCMs predict that precipita- tion patterns will change (Houghton et al., 1990), there is wide disagreement about the direction and degree of change. A change in precipitation would have far- ranging impacts for streamflow, groundwater recharge, and the characteristics of terrestrial ecosystems. These changes in ecosystem properties feedback to affect the partitioning of energy at the earth's surface, altering sensible heat flux and evapotranspiration. These changes in water and energy balance, if extensive enough, can affect regional and possibly global climate (IGBP, 1990b; Shukla et al., 1990; Schlesinger et al., 1990). Soil-vegetation-atmosphere transfer (SVAT) models predict water flux to the atmosphere from large land areas, using relatively crude assumptions about the role of vegetation. General hydrologic models (GHMs) incorporate greater detail of vegetation characteristics (e.g., canopy structure and rooting depth) and can predict partitioning of water for selected ecosystems under current conditions. At present however, how the current veg- etation will partition water and energy under novel combinations of CO2, water, and nutrition cannot be predicted accurately, much less how future communities with different canopy structures would regulate water and energy flux. Under what conditions do physiological controls over stomatal conductance exert strong effects on canopy conductance (Jarvis and McNaughton, 1986; Rosenberg et al., 1989)? Although our understanding of the processes that regulate the partitioning of water and energy is incomplete, an even more pressing problem is how to inte- grate our understanding of canopy-level processes at small spatial and temporal scales with (1) climate models whose grid-cell resolution lumps together many ecosystems and blurs topographic controls and (2) successional models that pre- dict vegetation change on time scales of decades to centuries but ignore the effects of vegetation on regional water and energy balance. Even when geographically explicit simulation models are used to predict evapotranspiration and production across broad spatial scales, the appropriate satellite-based technology to validate such broad-scale predictions has not yet been developed (Running et al., 1989). Other Current Efforts There is a long history of study of the exchange of water and energy between individual leaves and the atmosphere. More recently, this work has been extended
RESEARCH PROGRAM 31 to estimates of water vapor conductances by canopies of selected ecosystems us- ing SVAT and GHM models (Running and Couglin, 1988). Efforts are under way to develop remote-sensing technology that can be used in combination with ground-based data for determination of evapotranspiration over large areas (Moran et al., 1989). At the international level, these studies are being organized within IGBP from the atmospheric perspective by the Biospheric Aspects of the Hydrologic Cycle program and from the vegetation perspective by the GCTE program (focus 1, activity 3; IGBP, 1990a, 1990b). Several agencies within the United States have developed global change research programs with a hydrology component (CEES, 1990; see Appendix, this volume). Research Questions The major question motivating this research is: How do changes in vegeta- tion structure and physiology alter regional climate and hydrologic budgets? How do changes in cropping patterns, decertification from overgrazing, land-use change, or wholesale species changes in unmanaged ecosystems affect the surface energy balance and the various components of the hydrologic cycle? The ecosys- tem questions are important because changes in surface energy and water balance may feed back to alter the rate of global change. If the albedo, architecture, or canopy conductance changes, how are the patterns of global warming and precipi- tation affected, as predicted by the GCMs? Similarly, at the mesoscale, how do changes in evapotranspiration affect regional precipitation? Changes in terrestrial ecosystems can affect the hydrologic cycle through several mechanisms, and it is important to know the sensitivity of water flux to each. At the leaf level, how sensitive is stomatal conductance of water vapor and foliage temperature to interactive changes in solar radiation, CO2, air temperature, water supply, and the nutrients in different vegetation types? Similarly, how do longer-term changes in growth, which alter canopy architecture and rooting pat- terns, affect canopy conductance, interception, runoff, soil water storage, ground- water recharge, and long-term water flux to the atmosphere? Will these factors change crop water requirements? General Strategy The major issue to be resolved is how to incorporate our current understand- ing of vegetation effects on water and energy budgets, which is well developed at the levels of leaves and canopies, into GCM models so as to predict vegetation effects on climate at regional and global scales. The research program outlined in focus 1, activity 3, of the GCTE in the international program should be supported. Briefly, the general approach is to determine the role of canopy conductance in mediating plant response to CO2 and climate and the effect of vegetation structure on canopy conductance and energy balance, so that successional models of veg- etation structure can be used to predict water partitioning and energy balance.
32 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE Finally, these patch-level models must be aggregated to the scale of GCM grids, incorporating the interactions of vegetation patches and topography in order to predict the effects of vegetation on regional and global climate. Research Program 1. DETERMINATION OF EVAPOTRANSPIRATION OVER LARGE AREAS. Objec- tive: To determine water and energy exchange rates over land areas from patch scales up to GCM grid scales. Regional models of atmospheric energy and water transport are needed to determine whether changes in canopy conductance and evapotranspiration affect regional precipitation patterns. These models require better hydrologic interfaces between the lower boundary of GCMs and terrestrial ecosystems. To validate these regional models, methods should be developed to estimate water and energy flux over large areas. Remote-sensing techniques need to be developed and improved to estimate leaf area index (LAI) and canopy tem- perature, which can then be combined with standard meteorological measure- ments to infer water and energy flux at a regional scale. Critical problems to be resolved include time scale, spatial resolution, and methods of estimating aerody- namic resistance. These models and methods must be tested by field campaigns that measure input parameters and fluxes at a variety of temporal and spatial scales (e.g., First International Satellite Land Surface Climatology Project Field Experi- ment, BOREAS). Combined with measurements of streamflow, these large-scale measures of evapotranspiration should be developed until they give better esti- mates of soil water storage, a sensitive but poorly known component of GCMs. Combining data on LAI and evapotranspiration may allow improved estimates of ecosystem productivity, a goal of carbon balance studies (see the first section in this chapter, on CO2 and interactive effects). 2. MANIPULATION AND MONITORING OF WATERSHEDS. Objective: To deter- mine the effects of changing vegetation on streamflow, evapotranspiration, and groundwater storage of a watershed. Mechanistic distributed hydrologic models need to be developed or modified to predict the effects of changing canopy con- ductance, canopy architecture, and rooting patterns on evapotranspiration, runoff, soil water storage, snowpack, groundwater recharge, and the watershed supplies of surface water and groundwater, using data from watershed manipulations. Monitoring of streamflow in these watersheds should be continued because streamflow may be one of the more sensitive parameters of global change. These experiments should be closely integrated with biogeochemical studies of nutrient and material transport from land to streams and lakes (see the section below on land-water interaction). 3. CANOPY WATER AND ENERGY EXCHANGE. Objective: To determine the effects of increasing CO2 and changing climate on energy and water balance at the patch scale. Process-based modeling, supplemented by field plot experiments where necessary, are required to determine the effects of changing CO2, tempera-
RESEARCH PROGRAM 33 ture, water supply, nutrients, and light on canopy conductance, canopy architec- ture, canopy temperature, albedo, evapotranspiration, soil water storage, ground- water recharge, and runoff for representative ecosystems. These experiments should be an integral part of the CO2 enrichment and trace-gas experiments de- scribed in earlier sections because they require the same experimental design. Because of the strong effect of enclosure walls on wind speed and evapotranspira- tion, CO2 enrichment experiments that incorporate studies of water and energy balance should be made using the FACE approach. Status and Priorities SVAT and GHM models that predict which and energy partitioning for se- lected ecosystems have been developed and parameterized. Measurement of canopy conductance as a function of CO2, temperature, water supply, nutrients, and growth form is required before the current models can predict water budgets under future climatic scenarios. The highest priorities should be given to regional hydrologic modeling, de- velopment of remote-sensing methods to measure model inputs, and the field mea- surements needed to validate these models in order to link the relatively sophisti- cated current understanding of patch-scale water and energy balance (SVAT and GHM models) with GCMs. This is essential to all questions of biospheric feed- backs to regional and global climate. Continuation of watershed research and of integration of the information from these experiments with regional hydrologic models merits strong support. At the patch level, the highest priorities are to validate the effectiveness of canopy models in simulating the effects of CO2, cli- mate, and canopy structure on water flux and partitioning, and to link patch-level measurements to regional models. In general, research is well advanced in tackling these objectives. Within 5 to 10 years models are expected to be sufficiently realistic to describe how changes in the physiology and structure of vegetation affect regional climate. ECOLOGICAL CONTROLS OVER LAND-WATER INTERACTION The Problem Global change is likely to alter the location and flux of major freshwater resources around the globe. In addition, sea-level rise will alter coastal ecosys- tems and the extent of tidal rivers and streams. Associated with changes in pools and fluxes of water will be changes in the nature of linkages among land, freshwa- ter, and coastal marine ecosystems. The most pressing scientific need is to under- stand the controls over fluxes of water, carbon, and nutrients among these ecosys- tems.
34 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE Transport of water and materials from land to freshwater ecosystems is strongly influenced by vegetation, particularly by vegetation at the margins of lakes and streams, and by nutrient loading through atmospheric deposition and fertilizer inputs. However, the role of vegetation in controlling these fluxes is poorly known. The magnitude and timing of inputs of water and associated nutri- ents and sediments to aquatic systems are critical to their functioning (Schindler et al., 1990; Grimm and Fisher, 1991) and affect both the processing of materials of terrestrial origin and the magnitude of additional carbon inputs through aquatic primary production. Reductions of water inputs are likely to exacerbate the ef- fects of contaminants in lake (Schindler et al., 1990) and groundwater systems. With global changes in the transfer of water and materials from land to aquatic systems, changes in species composition will probably occur, with as-yet-un- known consequences for ecosystem function. Certain fisheries are likely to ex- pand (Magnuson et al., 1990) while others collapse (Schindler et al., 1990). Range shifts of nuisance species, parasites, and pathogens may be expected. However, relatively little is known about the consequences of these species changes for the aquatic systems or their coupling to the terrestrial landscape, including consump- tion by humans. Little is known too about how these changes in community structure affect the transport of carbon and other materials through lakes, streams, rivers, and estuaries to the coastal marine zone. A landscape view integrating lakes, streams, rivers, and coastal zones with terrestrial systems can now be devel- oped. Critical ecosystem couplings occur through hydrologic fluxes, movements of nutrients and organic detritus, and harvest of aquatic resources by species, in- cluding humans, that play important roles in terrestrial ecosystems. Other Current Efforts A major reason for highlighting aquatic systems in this report is that they have not received focused attention in IGBP planning, with the exception of the coastal zone, in which case research is being organized in the Land-Ocean Inter- actions in the Coastal Zone program. At the national level there are several pro- grams that focus on aquatic ecosystems (see Appendix). Research Questions The most critical questions relate to coupling among terrestrial, stream, lake, river, estuarine, and marine ecosystems and the role of aquatic community struc- ture in controlling the strength of these linkages. The role of terrestrial vegetation in controlling water inputs is covered in the previous section. How do vegetation type and land use affect the capacity of terrestrial vegetation to filter nutrients from groundwater, prevent sediment input to aquatic systems, and add carbon and nutrients through litterfall? How do terrestrial inputs (or removal of water for irrigation) affect the species composition, production, and biogeochemical cy-
RESEARCH PROGRAM 35 cling in lakes and rivers and, therefore, the role of these ecosystems in processing and transporting carbon to the ocean? How will these changes in aquatic systems affect water quality and the capacity of lakes and rivers to support higher trophic levels, including humans, and how does this affect the carrying capacity and pat- terns of land use of adjoining terrestrial ecosystems? How do changes in freshwa- ter aquatic systems affect the coastal marine ecosystems? How will changes in sea level alter estuarine ecosystems? How does processing of carbon and nutri- ents by freshwater and marine organisms affect their rates of deposition in sedi- ments? General Strategy The general approach is to examine the controls over fluxes of carbon and nutrients from terrestrial to aquatic systems, fluxes among aquatic systems, and fluxes back to the terrestrial system (e.g., harvest by humans and other predators). Controls that may be expected to be critical and deserving of study are: 1. the role of terrestrial vegetation in governing hydrologic budgets (see the section on filtering nutrients), 2. the role of aquatic community structure in governing the processing of carbon and nutrients (and therefore their rates of longitudinal transport to the ocean and extent of deposition in sediments), and 3. factors governing the harvest of aquatic resources by people and other animals and the impact of this harvest on the carrying capacity and land use of adjoining terrestrial ecosystems. Research Program 1. IMPACT OF TERRESTRIAL VEGETATION ON TRANSPORT OF CARBON, NUTRI- ENTS, AND SEDIMENTS TO AQUATIC SYSTEMS. Objective: To determine how veg- etation structure and land use influence material transport to aquatic systems. Information from watershed studies should be synthesized to develop hypotheses about how the biomass and structure of vegetation influence the magnitude and seasonality of inputs of water (see the previous section on water/energy balance), nutrients, and sediments to aquatic systems. Particular attention should be paid to watersheds exhibiting different degrees of nutrient loading from agriculture to acid rain. This information should be used to develop process-based models of nutrient and sediment transport to aquatic systems. These models should be vali- dated where land management practices (e.g., logging, wetland restoration) are modifying the presumed controls over this coupling. 2. THE HYDROLOGIC CYCLE AND ECOSYSTEM FUNCTION. Objective: To de- termine how changes in the hydrologic cycle will alter productivity and bio- geochemical cycling and transport in aquatic systems and associated riparian
36 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE and near-shore marine areas. There is a need to establish sites, in a range of landscape types, where biotic-hydrologic interactions among upland, stream, and lake systems can be studied. Activity at each site would have three foci. The centerpiece would be ecosystem-scale experiments aimed at large-scale effects of changes in water flux (through vegetation modification or removal of irrigation water), nutrients, contaminants, and community structure (i.e., species composi- tion). Where appropriate, these ecosystem experiments would be coordinated with field mesocosms and laboratory experiments. The supporting foci would be modeling (aimed at integration and synthesis at the landscape level) and back- ground measurement of key structural components, fluxes, inputs, and outputs. Also required are large-scale, long-term studies of the effects of hydrologic change on the linkages between freshwater systems and adjoining riparian and coastal marine areas. 3. EFFECTS OF AQUATIC PRODUCTION AND COMMUNITY STRUCTURE ON HU- MAN HARVEST AND LAND USE. Objective: To determine the feedback between aquatic productivity and human land-use change. Information from Third World countries should be synthesized to determine the effect that harvest of freshwater and marine resources has on patterns of human land use and harvest of terrestrial and aquatic resources. 4. EFFECTS OF AQUATIC COMMUNITIES ON SEQUESTRATION OF CARBON AND NUTRIENTS IN SEDIMENTS. Objective: To determine the factors that control the fate of organic and inorganic carbon that enters aquatic systems. Information on rates of carbon flux to the atmosphere as CO2 or CH4 versus its burial in sedi- ments of lakes, estuaries, and near-shore marine ecosystems should be summarized to determine what biotic factors are correlated with these sequestration rates. This information should be incorporated into models that predict carbon sequestration in sediments and should be validated with appropriate observations and experiments. Status and Priorities There have been few large-scale, long-term studies that have linked freshwa- ter and marine ecosystem studies with terrestrial hydrologic patterns. The techni- cal capabilities and expertise needed for these studies exist, but they have usually been applied to smaller-scale questions delimited by disciplinary boundaries. High priority should be assigned to achieving the necessary synthesis at appropri- ately large scales. Key elements are (1) integrated analyses of streams, lakes, and their catchments; (2) interdisciplinary teams of aquatic ecologists, terrestrial ecologists, hydrologists, and climatologists; (3) integration of basic and applied research objectives; and (4) long-term observation, experimentation, and model- ing to determine feedbacks between global environmental change and fresh wa- ters at the scale of landscapes. At this time, workshops and discussions are needed to define the scientific issues and approaches in more detail and develop the inter- disciplinary linkages needed to carry the research forward.
RESEARCH PROGRAM 37 The conceptual basis for studies on the role of vegetation as a filter for mate- rials entering aquatic systems, the impact of human harvest from aquatic systems, and controls over carbon sequestration in sediments are less developed than is the area of aquatic community structure. Workshops and literature syntheses in these areas will also be required prior to the development of an experimental program. Within 2 to 5 years a research plan that identifies the role of aquatic ecosys- tems in the global carbon cycle may be expected. Within 10 to 20 years our understanding should be sufficiently advanced to predict how global change will affect the flux of materials through aquatic systems to the ocean and the impacts these changed fluxes will have on the global carbon cycle, aquatic species compo- sition, the productivity of fisheries, and the quality of water for drinking, industry, and agriculture, all of which affect the human carrying capacity of the earth.
