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An ecosystem process model was used to assess likely ecosystem responses to natural climatic variation and GCM-predicted climatic change in the semiarid north-western U.S.A. Simulated equilibrium conditions between soil water availability and leaf area were compared to long-term natural variations for annual grass and mixed sagebrush/bunchgrass communities subjected to 2 degrees C increases in daily temperature, coupled with 10% increases and decreases in precipitation. Equilibrium simulations suggested that a less productive, invasive grass community would tolerate climate change, whereas a native sagebrush community would not survive the increased temperatures predicted by GCMs. High air temperatures, and the subsequent increases in vapor pressure deficit, caused decreased stomatal conductance, and hence decreased net photosynthesis. High air temperatures also increased maintenance respiration, leading to decreases in net primary production. As the productivity of this community declined, substantial increases in soil water storage occurred. When natural variation of annual weather is incorporated into simulations, both community types were able to survive by adjusting levels of biomass production. Soil water storage was not significantly affected when the vegetation adapted to increased or decreased precipitation through proportional adjustments to leaf area, which in nun dictates evapotranspiration and son water drawdown. (C) 1996 Academic Press Limited