Contrasting effects of precipitation manipulations on species composition and community structure at two sites within the central grassland region, USA

Background/Question/Methods

Changes in climate, including increased temperatures and alterations to precipitation patterns, are occurring in grasslands worldwide; modifying soil water dynamics. As grasslands are primarily water limited, these predicted changes in climate will likely have dramatic impacts on ecosystem function and community structure, yet the magnitude of change and the types of species favored may differ among ecosystems. We conducted a four year (2008-2011) water manipulation experiment in the driest portions of the central grassland region of North America, USA, to understand how changes in soil water content will affect species composition and community structure. We altered growing season soil water dynamics at our sites, located in the semiarid shortgrass steppe and subhumid mixed grass prairie, by constructing rainfall shelters and subjecting plots to three different precipitation regimes (drought, ambient, and water addition). We monitored daily soil water content and measured annual changes in species composition, diversity, evenness, and cover by functional groups.

Results/Conclusions

Our water manipulation treatments were successful in changing the soil water content in the treatments. On average, the drought treatment decreased soil water content by 20% and the water addition treatment increased soil water content by 21% during the growing season. These changes in soil water resulted in different compositional changes at each site. At the shortgrass steppe, neither the dominant species, Bouteloua gracilis, nor any of the functional groups showed large responses to changes in soil water content. By contrast, the dominant species at the mixed grass prairie site, Schizachyrium scoparium, responded rapidly to water addition and drought treatments, resulting in an increase in percentage cover of 5% in the water addition treatment and an average decrease in percentage cover of 19% in the drought treatment. Likewise, there were significant changes in percentage cover by the graminoid and dwarf-shrub functional groups. The differing sensitivities of the dominant species and functional groups to similar changes in soil water at two sites demonstrates that both abiotic and biotic factors play important roles in regulating responses of plant community structure and composition to climate change in the central grassland region of North America.