Terrestrial ecosystems respond to climate conditions on multiple scales: gene expression, species, and ecosystem. We are studying California annual grassland mesocosms exposed to altered precipitation (315, 570 (ambient), and 885 mm y^-1) and soil and air temperature (ambient and +5^oC) in replicated greenhouses. This presentation will combine plant and ecosystem level results with mRNA and activity measurements for key enzymes, such as rubisco and nitrate reductase, currently underway. As expected, aboveground biomass and net ecosystem carbon uptake were correlated with soil moisture and most species stayed active later in the season under high rainfall. However, warming reduced the rainfall effects on season length and net CO₂ uptake by drying soil and stimulating soil respiration. Higher precipitation reduced Avena biomass but increased Hordeum biomass and total aboveground productivity (1400 vs. 800-1000 g/m²). Important compensations and interactions were observed, with the effect of warming depending on precipitation and species. For some processes, warming with low rainfall had no effect but warming with high rainfall created large, species-specific responses. For example, warming at high rainfall advanced flowering of Amsinckia and Erodium forbs by a week, and delayed flowering of drought-adapted Aegilops grass by three days. Similarly, warming alone had little effect on trace gas flux, while warmer, wetter conditions stimulated 30-70% increases in soil N₂O and CO₂ fluxes. For other processes, such as light-saturated leaf photosynthetic capacity, the driest treatment (warming, low rainfall) resulted in 80% reductions from the low temperature, high rainfall case. These results suggest that ecosystem responses to future warming depend strongly on soil moisture. While warming of 3-6^oC is fairly certain this century, precipitation is much less certain, with some forecasts drier and others wetter for a given location, highlighting a current limitation in predicting ecosystem response to climate change or even the influence of warming alone.