We are examining how a California annual grassland ecosystem and its component parts respond to differences in cumulative season soil moisture (315, 675, 1245 mm) and repeated within season soil moisture recharge and dry down events.  Here we report plant responses including growth, species composition and leaf and canopy level CO₂ fluxes. At midseason, the high and low precipitation plots were 70% and 10% reduced in biomass production relative to the ambient control (675 mm).  By the end of the season, low and high water treatments reversed trends resulting in a 70% and 15% decrease in standing biomass relative to the ambient control.   The ambient and high precipitation plots supported six grass and two forb species, while the low precipitation plots only supported four grass species.  Seed production was greatest and most diverse in the ambient plots, with low and high precipitation plots producing 60% and 20% less seed.  Avena barbata produced twice as much seed as any other species in the high precipitation plots.  Under ambient and low precipitation conditions, Hordeum murinum produced twice as much seed as Avena.  High and ambient soil moisture conditions differed little in leaf and canopy level gas exchange and were equally sensitive to soil moisture dry down (8 days without water) which decreased whole ecosystem gas exchange by 50%.  Low precipitation plots had the lowest leaf and canopy fluxes during both soil moisture recharge and dry down periods but had a lower magnitude in flux shifts in the transition from soil water recharge to dry down.