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 (STEPPE-GP) linked with a soil water model (SOILWAT) to evaluate the effects of changes in climate on the composition and structure of grassland vegetation. Five functional types of plants were simulated based upon lifeform, physiology, and rooting distribution with depth. C₃ and C₄ perennial grasses with either a shallow or deep rooting distribution, and deeply rooted C₃ shrubs were simulated under current climatic conditions and under a GFDL climate change scenario for nine sites representative of the temperature and precipitation regimes in the grassland region.Although vegetation at the sites responded differently to climate change, shifts in functional types occurred within 40 years of the start of the climate change. C₄ grasses increased in dominance or importance at all sites with a change in climate, primarily as a result of increases in temperature in all months at all sites. The coolest sites that arc currently dominated by C₃ grasses were predicted to shift to a dominance by C₄ grasses, whereas sites that are currently dominated by C₄ grasses had an increase in importance of this functional type with a change in climate. Current annual temperature was the best predictor of changes in C₃ biomass, and C₃ and C₄ biomass combined; current annual precipitation was the best predictor of changes in C₄ biomass. These predicted shifts in dominance and importance of C₃ versus C₄ grasses would have important implications for the management of natural grasslands as well as the cultivation of crops in the central grassland region.