Influences of prolonged drought and high temperature on carbon exchange of an oak/grass savanna in California

We present a long-term study on carbon flux over an oak/grass savanna, located at the lower foothill of Sierra Nevada in California. The ecosystem is consisted of overstory of blue oak (Quercus douglasii) and understory of C₃ annual grass. It has a Mediterranean climate with wet and cold winter, dry and hot summer. Strong seasonality in soil moisture and large day-to-day temperature and VPD changes make the site a unique laboratory to investigate the roles of drought, high temperature, and plant functional type on the carbon exchange between ecosystems and atmosphere. Continuous measurements of carbon fluxes over and under the oak/grass savanna ecosystem started in April 2001. Meteorological and soil parameters (air temperature, radiation, VPD, precipitation, soil moisture etc.) were also measured. Grass leaf area index (LAI), predawn water potential were measured at 1-2 week intervals. Gas exchange (A/Ci curve) of oak leaf were also measured at weekly intervals to quantify the seasonal variations in stomatal conductance (g_s) and maximum photosynthetic capacity (V_cmax). The growing season for the understory grass is limited in the wet season, which is from late Oct to early May. Oak leaf out at the end of March and quickly reach its maximum LAI of 0.6. Results show that daytime carbon uptake for the understory grass followed closely its LAI, which in turn was controlled by soil moisture content. For oak trees, besides soil moisture, changes in V_cmax need to be taken into account to explain its seasonal canopy carbon uptake. Results also show a strong VPD effect on NEE. We observed large pulse ecosystem respiration after rain events, especially those during dry seasons. In our ecosystems with very low NEE, such rain events could be one of major environment forcings that cause larger inter-annual variability in NEE. Severe water stress didn't alter water-use efficiency (WUE) of the savanna. WUE was much higher when compared with data from other AmeriFlux forest sites. Despite the prolonged drought and low LAI, the annual net carbon exchange in 2002 was -193 gC m^-2.