TRANSIENT RESPONSES OF NORTH-AMERICAN GRASSLANDS TO CHANGES IN CLIMATE

Our objective was to evaluate the transient responses of grasslands in the central grassland region of North America to changes in climate. We used an individual plant-based gap dynamics simulation model (STEPP E-GP) linked with a soil water model (SOILWAT) to evaluate the effects of changes in climate on the composition and structure of grassland v egetation. Five functional types of plants were simulated based upon l ifeform, physiology, and rooting distribution with depth. C-3 and C-4 perennial grasses with either a shallow or deep rooting distribution, and deeply rooted C-3 shrubs were simulated under current climatic con ditions and under a GFDL climate change scenario for nine sites repres entative of the temperature and precipitation regimes in the grassland region. Although vegetation at the sites responded differently to cli mate change, shifts in functional types occurred within 40 years of th e start of the climate change. C-4 grasses increased in dominance or i mportance at all sites with a change in climate, primarily as a result of increases in temperature in all months at all sites. The coolest s ites that are currently dominated by C-3 grasses were predicted to shi ft to a dominance by C-4 grasses, whereas sites that are currently dom inated by C-4 grasses had an increase in importance of this functional type with a change in climate. Current annual temperature was the bes t predictor of changes in C-3 biomass, and C-3 and C-4 biomass combine d; current annual precipitation was the best predictor of changes in C -4 biomass. These predicted shifts in dominance and importance of C-3 versus C-4 grasses would have important implications for the managemen t of natural grasslands as well as the cultivation of crops in the cen tral grassland region.