Drs. Field, Chapin and Mooney will conduct integrated studies of the responses of California annual grassland to altered resource balance, focusing on factors controlling the response of the ecosystem to elevated CO2. Their research involves coordinated empirical and modeling programs to test a series of hypotheses that potentially account for the mechanisms and feedbacks governing system-level behavior of aboveground and belowground production, decomposition, and nutrient cycling in response to altered resources. The conceptual model for the system-level effects of elevated CO2 is based on two hypotheses concerning the mechanisms of the system level responses and the loci of the direct effects. First, they hypothesize that elevated CO2 may generate novel resource balance, but the response of plant growth can be understood and predicted from responses of other variations in resource balance that change the availability of carbon, relative to other resources. Second, they hypothesize that system-level responses to elevated CO2 are driven primarily through direct effects on plants. This research program consists of five components: 1) a study of the responses of natural ecosystems to elevated CO2 in open top chambers. 2) a probe of ecosystem responses to a range of modifications, including elevated CO2, altered nutrient and water availability, and altered species composition, using artificial microecosystems in open top chambers. 3) a study of controls on decomposition and nutrient cycling with laboratory, field and microecosystem experiments using plant litter from the other experiments as well as isotopic tracers. 4) an exploration of the resource effects on plant growth, allocation, and tissue chemistry with a series of greenhouse experiments. 5) a modeling effort to integrate the experimental results and to link and extend existing ecosystem, community, and plant models. This program has the potential to advance several aspects of our understanding of system-level responses to global climate change. First, studies with elevated CO2 in the California annual grassland will complement CO2 studies in other grasslands, with results from a system that is unique in terms of low carbon storage and rapid equilibration. Second, the small stature of the plants, combined with the small time constant for system equilibration, will allow the investigators to explore consequences of a broad range of resource availabilities and plant species characteristics. Finally, the emphasis on mechanisms of system-level responses, integrated through linked models, should make the results of this study generalizable to other ecosystems.
