CO2 EFFECTS ON THE WATER-BUDGET OF GRASSLAND MICROCOSM COMMUNITIES

Experimental grassland ecosystems, in microcosms 0.2 m in diameter and with a 0.95 m soil column, varied in their responses to elevated part ial pressure of CO2 (pCO(2)) and altered moisture inputs. Ecosystems o n moderately fertile sandstone soil and with a typical mix of moderate ly fast-growing sandstone species, responded to elevated pCO(2) with d ecreases in mid-season evapotranspiration of nearly 50%. This pattern reversed at the end of the growing season, and sandstone ecosystems un der elevated pCO(2) continued active transpiration farther into the su mmer drought. The sandstone ecosystems appeared to convert mid-season water conservation into increased late-season growth. Effects of incre ased pCO(2) on ecosystem evapotranspiration were much smaller in ecosy stems with very infertile serpentine soil and a diverse mixture of slo w-growing serpentine species.