Using an ecosystem model linked to GCM-derived local weather scenarios to analyse effects of climate change and elevated CO2 on dry matter productionand partitioning, and water use in temperate managed grasslands

Local effects of climate change (CC) and elevated CO2 (2 x CO2, 660 mu mol mol(-1)) on managed temperate grasslands were assessed by forcing a dynamic ecosystem model with weather scenarios. The aims of the study were to comp are the relative importance of individual and combined effects of CC, 2 x C O2, and photosynthetic acclimation, and to assess the importance of local s ite conditions. The model was driven by hourly means for temperature (T), p recipitation (P), global radiation (G), vapour pressure (VP), and wind spee d (U). Local climate scenarios were derived by statistical downscaling tech niques from a 2 x CO2 simulation with the General Circulation Model of the Canadian Climate Centre (CCC-GCMII). Simulations over 14 growing seasons to account for year-to-year variability of climate were carried out for a low , relatively dry site, and a high, more humid site. At both sites, shoot dry matter responded positively to 2 x CO2 with the si te at low elevation being more sensitive than the higher site. The effect o f assumed changes in climate was negative at the lower, but positive at the higher site. Shoot dry matter was more sensitive to the effects of 2 x CO2 than to CC. Both effects combined increased shoot dry matter by up to 20%. This was attributed to direct effects of 2 x CO2 and increased T,and indir ect stimulation via increased soil N availability. Biomass partitioning to roots increased with 2 x CO2 but decreased with CC, while an intermediate r esponse resulted from the combination. Leaf area index (LAI) increased unde r 2 x CO2, but not enough to compensate fully for a decrease in leaf conduc tance. Under the 2 x CO2 scenario evapotranspiration (ET) decreased, but in creased under CC. Photosynthetic acclimation reduced the effect of 2 x CO2 on shoot growth, but had little effect on ET. The seasonal water use effici ency (WUE) was improved under 2 x CO2, and reduced under CC. With the combi nation of both factors, the change was small but still positive, especially at the high elevation site with more favourable soil water conditions. Thi s reflects the stronger positive yield response in combination with a small er increase in ET under cooler, more humid conditions. The results for the combination of factors suggest that except for shoot gr owth, effects of 2 x CO2 and CC tend to offset each other. While CC determi nes the sign of the ET response, the sign of the biomass response is determ ined by 2 x CO2. The results highlight the importance of a site-specific an alysis of ecosystem responses by using a flexible approach based on a combi nation of state-of-the-art downscaling, spatially resolved data sets, and a mechanistic model to obtain quantitative and reproducible assessments of c limate change impacts at the ecosystem level.