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Contrasting effects of precipitation manipulations in two Great Plains plant communities

Question 
Anthropogenic climate change is altering temperature and precipitation in grasslands worldwide. As grasslands are primarily water-limited, these changes in climate will likely have dramatic impacts on ecosystem function and community structure, yet the magnitude of change and the types of species favoured may differ among ecosystems or even among local communities within ecosystems. How much might plant community responses to altered precipitation vary at two sites within one grassland region? 
 
Location 
Semi-arid and sub-humid natural grasslands in the US Great Plains. 
 
Methods 
At two sites we used rainfall shelters and irrigation to create irrigation, drought and control treatments. We measured changes in species composition, richness, Shannon's diversity, evenness and cover by plant functional groups across 4 yr (2008–2011). 
 
Results 
The semi-arid grassland community was relatively insensitive to precipitation manipulations, and in the few cases where there was a significant relationship between treatment precipitation and the response variable, the slope of the relationship was weak. In contrast, the sub-humid grassland community was very sensitive to changes in treatment precipitation throughout the experiment, and responded more strongly, and negatively, to drought compared to irrigation. 
 
Conclusions 
The differing sensitivity of the dominant species to altered precipitation amount appears to largely determine the overall differences in community response at these sites. The variable responses we observed within a single grassland region highlight the challenge of forecasting community impacts of climate change.