Modelling grassland responses to climate change and elevated CO 2

Abstract

A mechanistic model for productive grassland was used to simulate the annual production of above- and below ground plant biomass in relation to fluxes of C, N, and water, and to test the sensitivity of yield, shoot/root ratio, evapotranspiration, and water use efficiency (WUE) to climate change scenarios (CC) and to elevated CO 2 (2×CO 2) with or without consideration of photosynthetic acclimation of the plants. Validation with data from two Swiss sites revealed satisfactory agreement between simulation and measurement for yield, energy fluxes, and N-dynamics. Local weather scenarios were derived from the results of two General Circulation Models (GCM) for 2×CO 2 by a statistical down-scaling procedure. Biomass production changed by a maximum of 8% in response to CC without 2×CO 2 effects, by 1–17% in response to 2×CO 2 alone, and by 6–20% in response to the combination of CC and 2×CO 2. With plant acclimation, biomass production increased only up to 8% with elevated CO 2, as compared to a maximum increase of 20% in the absence of plant acclimation. Reduced yield with CC was obtained for sites with low soil water holding capacity. Decreased evapotranspiration and increased WUE with 2×CO 2 were partially offset by CC. The simulations indicated that productivity of managed grassland is sensitive to different assumptions about changes in climate, CO 2 concentration, and photosynthetic acclimation, and that the effects of elevated CO 2 are modified by CC and depend on local soil conditions.