CARBON ASSIMILATION AND WATER-USE EFFICIENCY BY NEIGHBORING MEDITERRANEAN-CLIMATE OAKS THAT DIFFER IN WATER ACCESS

Quercus agrifolia Nee and Quercus durata Jeps. are sclerophyllous ever greens that co-occur in regions of northern California that have a ser pentine-derived soil. The species are of similar stature and density w here they grow together, even though late-drought xylem pressure poten tials indicate that Q. durata has access to more soil water than Q. ag rifolia. To assess how carbon uptake and water-use efficiency (WUE) ar e affected by water access, I monitored leaf and canopy gas exchange o f neighboring Q. durata and Q. agrifolia trees over a 15-month period. Transpiration and photosynthesis by Q. agrifolia peaked in spring and declined through the summer, whereas transpiration and photosynthesis by Q. durata continued at a moderate rate year round. When summed ove r the study, Q. agrifolia transpired 25% less water on a ground-area b asis than Q. durata, but assimilated 25% more carbon. Quercus agrifoli a achieved a greater integrated WUE by: (1) maintaining a 20% advantag e in instantaneous WUE as a result of lower leaf intercellular CO2 con centrations; (2) responding rapidly to increased soil water following rain; and (3) assimilating carbon at high rates during periods of low evaporative demand.