Wednesday, August 8, 2007: 9:20 AM
C3&4, San Jose McEnery Convention Center
Temperature and rainfall are critical environmental drivers for grasslands. In the U.S. Central Plains mean temperatures are expected to increase and rainfall patterns are predicted to become more variable and extreme, with increased frequency of large rainfall events and extended inter-rainfall droughts. Changes in the timing of rainfall events will alter soil moisture dynamics, and increased temperatures may intensify or moderate the impacts of altered rainfall timing on specific ecosystem processes. We are assessing the ecosystem consequences of these climate changes, using field-scale rainfall manipulation plots to alter the timing of growing season rainfall events (since 1998) and infrared heaters to increase mean soil and canopy temperature (since 2003) in intact grassland plots. Results to date indicate significant reductions in aboveground net primary productivity with altered rainfall timing, although this response has diminished with time. Across all years, annual ANPP was positively related to mean soil water content (altered rainfall timing treatment only), and negatively related to within season soil moisture variability (all treatments), suggesting that more extreme rainfall regimes will decrease productivity and increase the sensitivity of ANPP to interannual variation in precipitation amounts. We have observed only modest reductions in the productivity of C3 forbs with warming. Soil CO2 flux was generally reduced by both altered rainfall timing and warming. Our results suggest that elevated temperatures decrease growing season soil CO2 flux in these grasslands as a result of increased water stress. Increased rainfall variability doubled resin-bag collected nitrate, while warming had no effect, indicating that soil N fluxes are more sensitive to soil moisture than temperature. These results suggest important interactive effects of altered precipitation and warming on grassland structure and function.
