According to the latest IPCC report, global climate change models are predicting an increase in the variability and intensity of extreme weather events, such as drought, in semi-arid regions.  Semi-arid grasslands, or shortgrass-steppe, are among the most responsive ecosystems to global climate change. Therefore it is critical to determine the underlying mechanisms of their responses to scenarios like drought and how these mechanisms vary across space and time.  Annual net primary productivity (ANPP) is strongly influenced by precipitation and increases along spatial precipitation gradients; however, time series analyses of grasslands have revealed temporal lags in recovery following drought.  These time lags may create a "window of opportunity" for invasive species to colonize shortgrass ecosystems because of the reduction in plant cover and meristem density, and the accumulation of soil nitrogen.   The objective of this project is to better understand the effects of severe drought on the invasibility of semi-arid grassland ecosystems across a broad geographic scale.  To simulate drought, rainout shelters were used at three sites across the shortgrass-steppe ecoregion of the United States for two years.  To determine if a resource opportunity is created, which may increase invasibility in a drought-impacted community, we measured changes in plant cover and soil nitrogen at sites in shortgrass steppe sites from northern Colorado to northern New Mexico.

Results/Conclusions

Results show the rainout shelters effectively created drought at all three sites by reducing daily average soil moisture up to 70% of ambient levels.  Soil nitrogen significantly increased in plots experiencing two years of severe drought (p < 0.05).  Furthermore, significant shifts in plant community composition occurred similarly between sites. In general, total plant cover decreased by as much as 15%, with most reductions occurring with native forbs, and relative cover of native grasses increased by as much as 25% (p < 0.001).  

We conclude that severe drought will influence plant community composition and soil nitrogen in semi-arid grasslands in ways that are consistent with the creation of a resource opportunity that facilitates invasion.  Continuation of this study will determine whether invasibility is affected by changes in vegetation and soil nutrients specifically resulting from drought.