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1 Introduction Terrestrial ecosystems provide most of the food, fodder and fiber that support human populations. The freshwater ecosystems embedded in terrestrial land- scapes provide water for drinking, industry and agriculture, and the coastal zones of the ocean provide important fisheries. Human-induced global environmental changesâencompassing changes in climate, land use, water, soil, and atmo- spheric chemistryâchallenge the ability of the biosphere to support the world's growing human population. Changes in land use, for example, will increase the proportion of land area overtly managed by humans, with implications for the long-term ability of the soil to sustain productivity. Direct CO2 effects on vegeta- tion and indirect effects due to changes in temperature and precipitation have both obvious and subtle implications for agriculture and forestry. These changes in ecosystems have important consequences for the earth systemâthrough changes in fluxes of water, energy, CO2, and trace gases to the atmosphere. The rising human population, with its doubling time of 40 years, advances in technology, and the increasing per capita impacts of human populations on natural systems are the basic causes of most recent large-scale changes in terrestrial ecosystems. It is imperative, therefore, that our understanding of the nature and consequences of these changes be improved rapidly in order to generate meaningful policy options and to ensure human persistence in a sustainable biosphere (Lubchenco et al., 1991 ;NRC, 1991). Several efforts are under way nationally and internationally to coordinate research aimed at understanding how ecosystems will respond to global environ- mental changes over the coming decades and centuries (see Appendix). The U.S. Global Change Research Program (USGCRP) includes "Ecological Systems and
6 THE ROLE OF TERRESTRIAL ECOSYSTEMS IN GLOBAL CHANGE Dynamics" as one of the seven science elements (CEES, 1990). The International Geosphere-Biosphere Program (IGBP) includes a core project on Global Change and Terrestrial Ecosystems (GCTE) whose objectives are "to develop the capabil- ity to predict the effects of changes in climate, atmospheric CO2, and land use on terrestrial ecosystems, and how these effects can lead to feedbacks to the physical climate systems" (IGBP, 1990a). The Human Dimensions of Global Environ- mental Change Programme, the social science counterpart to IGBP at the interna- tional level, includes research on economics, societal development and land use. In 1990 the Committee on Global Change, predecessor group to our board, initiated an effort to identify those aspects of research on terrestrial ecosystems that contribute to an understanding of global change and to define scientific ap- proaches to the development of research plans for this element of the USGCRP. Specifically, the group assembled to address these tasks was charged to: 1. develop scientific priorities for U.S. contributions to the IGBP projects related to terrestrial ecosystems (i.e., core projects on GCTE and Global Change and Ecological Complexity); 2. identify specific priorities and research strategies for the USGCRP science element on "ecological systems and dynamics," taking full account of existing plans for the program; and 3. ensure involvement of the ecological community in defining the scientific priorities for global change research related to terrestrial ecosystems. In view of this charge, this report identifies major research initiatives for the USGCRP where substantial progress could be made within the next decade in reducing uncertainty about the responses of terrestrial ecosystems to global changes in climate, land use, and the feedbacks of ecosystem charges to climate. These initiatives, which represent the initial U.S. contribution to the IGBP core projects, would be a step toward acquiring the ecological knowledge needed to ensure the sustainability of the biosphere, which is a research priority of the Eco- logical Society of America (Lubchenco et al., 1991). A secondary objective of this report is to outline ways in which the agricultural and ecological scientific communities can contribute to improved prediction of the role of terrestrial eco- systems in global change. Selection of research approaches was guided by several themes: â¢ Traditional distinctions between studies of managed and unmanaged eco- systems must be overcome. Methods and theories used to study managed ecosys- tems must be applied to unmanaged systems, and vice versa. â¢ An understanding of species interactions and interactions among ecosys- tems in a landscape is critical to prediction of the role of terrestrial ecosystems in global change. Thus, methods and theories from specialties ranging from agron- omy to population, community, and landscape ecology need to be integrated. â¢ Freshwater, estuarine, and coastal marine ecosystems are explicitly in-
INTRODUCTION 7 eluded in the definition of "terrestrial" ecosystems because they are integrally linked through biogeochemical and hydrological fluxes and have not been in- cluded in IGBP programs developed to study the oceans or the atmosphereâthe other major components of the earth system. â¢ The ecological, economic and cultural forces causing human population growth and land-use change must be included in models designed to predict ter- restrial responses to global change. Many previous reports have described extensively the rational and potential designs for global change research in terrestrial ecosystems (e.g., IGBP, 1989, 1990a, 1990b; Schimel et al., 1989; Stern et al., 1992). The present report builds on this research planning and sets priorities among this large range of important topics by focusing on those research efforts that will contribute most within the next decade to improved understanding of the role of terrestrial ecosystems in global changes of the earth system. Many of these research levels are currently progressing at both national and international levels, but others will require new research initiatives. A central theme of this report is the development and use of comprehensive models of ecological and physical systems. Current general circulation models, some of which include the surface biota in a highly parameterized fashion, have widely acknowledged deficiencies, and their ability to predict responses to un- precedented long-term changes is as yet untested. Nevertheless, these models represent the best available methodology for linking our small-scale understand- ing of ecological processes with our understanding of the large-scale processes determining climate.