Net primary production and carbon stocks in differently managed grasslands: simulation of site-specific sensitivity to an increase in atmospheric CO2and to climate change

Elevated atmospheric CO2 and climate changes are expected to influence mana ged grassland ecosystems. The mechanistic pasture simulation model (PaSim) was used to quantify effects on net primary productivity (NPP) and carbon ( C) stocks at three locations differing in climate and soil type. An earlier model version was modified to enable long-term simulations at different al titudes, and to consider management in the form of either cutting or grazin g by lactating cows. Results from simulations under current conditions agre ed favourably with measured data for yield and C stocks, and model behaviou r appeared to be plausible. Elevated CO2 alone or in combination with incre ased temperature stimulated NPP at all sites. The stimulation was positivel y related to increasing precipitation at dry sites, but negatively at cool sites. Climate change scenarios in combination with elevated CO2 led to inc rease C stocks. The sensitivity of C stocks to changes in temperature and p recipitation was similar, and much larger than to management. Grazing led t o higher C stocks compared with cutting, depending mainly on the difference in NPP between the management options. Grazing had a positive effect on C stocks under cool conditions, but the effect tended to become negative with increasing temperature. Comparing different sites revealed that local cond itions affect system behaviour qualitatively. In quantitative terms, the re sults confirm that the combination of elevated CO2 and climate change affec ts NPP and C stocks, and that the influence of management is site-specific. (C) 2000 Elsevier Science B.V. All rights reserved.