Abstract

Californian annual grassland on sandstone and serpentine soils at the Jasper Ridge Biological Preserve, Stanford, California, were exposed to ambient or elevated atmospheric CO2 in open-top chambers since December 1991. We measured ecosystem evapotranspiration with open gas-exchange systems, and sell moisture with time-domain reflectometry over 0-15 cm and 0-30 cm depths, at times of peak above ground physiological activity. Evapotranspiration decreased by 12 to 63 percent under elevated CO2 in three consecutive years in the sandstone ecosystem. In correspondence with decreased evapotranspiration, late-season sell moisture reserves in the sandstone were extended temporally by 10 ± 3 days in 1993 and by 28 ± 11 days in 1994. The effect of elevated CO2 on soil moisture was greater in the drier spring of 1994 than in 1993. In the serpentine ecosystem, evapotranspiration and soil moisture reserves were not clearly affected by elevated CO2. Soil water may be conserved in drought-affected ecosystems exposed to elevated CO2, hut the amount of conservation appears to depend on the relative importance of transpiration and soil evaporation in controlling water flux.