Mooney 9727060 Global changes over the coming decades are almost certain to be multi-factorial, with altered levels of atmospheric CO2, temperature, nitrogen deposition, and precipitation. To date, no experimental studies address this full suite of interacting global changes in the field. Despite a profound lack of consensus on the nature of the interactions among these factors, the foundation for interpretation and prediction is built largely from observations on natural patterns and data from single-factor experiments. While these can, at least in principle, yield access to the full suite of potentially important mechanisms, single-factor experiments and observations along natural gradients do not provide a context for evaluating our ability to predict the consequences of several, simultaneously operating mechanisms. To address this lack of information, this project will use field experiments with a factorial combination of altered CO2, temperature, water inputs, and nitrogen inputs. Focusing on four integrated response variables - primary production, soil carbon storage, soil nutrient availability, and species or functional group composition - this research will test the null hypothesis that responses to warming and elevated CO2 in combination are essentially additive, the sum of the responses to the individual factors. The experiments will employ mini-FACE and infrared heater technology in a California annual grassland, where it is feasible to execute a design combining several generations of the dominant organisms, a range of treatments, and reasonable replication. To facilitate the comparison with model output, experiments with three levels of CO2 and three levels of temperature will be used.
