Ecosystem Responses to Gradual and Extreme Changes in Precipitation: What Have We Learned From Experiments in Grasslands?.

Precipitation patterns and amounts are critical environmental drivers for most ecosystems and .rainfall patterns are forecast to become more variable, with increased frequency of large rainfall events and extended inter-rainfall droughts as well as more extreme.  We have been assessing the ecosystem consequences of these climate changes, using field-scale rainfall manipulation experiments to alter the timing of growing season rainfall events and to impose severe drought in intact grassland plots. Results indicate that there can be significant reductions in aboveground net primary productivity (ANPP) with altered rainfall patterns that included fewer but larger rain events and extended periods between rain events in more mesic grasslands. But this response has been variable among years.  Moreover, in more arid grasslands experimental manipulations of precipitation regime resulted in responses in ANPP opposite those in mesic grassland. The size of individual rain events had a disproportionate effect on ANPP in these drier grasslands, with the positive effects of a few large events potentially offsetting the negative effects of reductions in precipitation amount. Results from extreme drought experiments in semi-arid grasslands were consistent with the conclusion that grassland responses to alterations in precipitation inputs may vary dramatically depending on the long-term hydrologic regime.