MODELED RESPONSES OF TERRESTRIAL ECOSYSTEMS TO ELEVATED ATMOSPHERIC CO2 - A COMPARISON OF SIMULATIONS BY THE BIOGEOCHEMISTRY MODELS OF THE VEGETATION ECOSYSTEM MODELING AND ANALYSIS PROJECT (VEMAP)/

Although there is a great deal of information concerning responses to increases in atmospheric CO2 at the tissue and plant levels, there are substantially fewer studies that have investigated ecosystem-level re sponses in the context of integrated carbon, water, and nutrient cycle s. Because our understanding of ecosystem responses to elevated CO2 is incomplete, modeling is a tool that can be used to investigate the ro le of plant and soil interactions in the response of terrestrial ecosy stems to elevated CO2. In this study, we analyze the responses of net primary production (NPP) to doubled CO2 from 355 to 710 ppmv among thr ee biogeochemistry models in the Vegetation/Ecosystem Modeling and Ana lysis Project (VEMAP): BIOME-BGC (BioGeochemical Cycles), Century, and the Terrestrial Ecosystem Model (TEM). For the conterminous United St ates, doubled atmospheric CO2 causes NPP to increase by 5% in Century, 8% in TEM, and 11% in BIOME-BGC. Multiple regression analyses between the NPP response to doubled CO2 and the mean annual temperature aid a nnual precipitation of biomes or grid cells indicate that there are ne gative relationships between precipitation and the response of NPP to doubled CO2 for all three models. In contrast, there are different rel ationships between temperature and the response of NPP to doubled CO2 for the three models: there is a negative relationship in the response s of BIOME-BGC, no relationship in the responses of Century, and a pos itive relationship in the responses of TEM. In BIOME-BGC, the NPP resp onse to doubled CO2 is controlled by the change in transpiration assoc iated with reduced leaf conductance to water vapor. This change affect s soil water, then leaf area development and, finally, NPP. In Century , the response of NPP to doubled CO2 is controlled by changes in decom position rates associated with increased soil moisture that results fr om reduced evapotranspiration. This change affects nitrogen availabili ty for plants, which influences NPP. In TEM, the NPP response to doubl ed CO2 is controlled by increased carboxylation which is modified by c anopy conductance and the degree to which nitrogen constraints cause d own-regulation of photosynthesis. The implementation of these differen t mechanisms has consequences for the spatial pattern of NPP responses , and represents, in part, conceptual uncertainly about controls over NPP responses. Progress in reducing these uncertainties requires resea rch focused at the ecosystem level to understand how interactions betw een the carbon, nitrogen, and water cycles influence the response of N PP to elevated atmospheric CO2.