Precipitation Change in a Semi-Arid Grassland: Plant Community Responses and Management Strategies

Shifts in precipitation patterns can alter the composition and function of plant communities. My dissertation research examines the effects of changes in the amount and timing of precipitation on plant species composition in a semi-arid grassland in the foothills of Colorado, USA. I also investigated possible management strategies to promote native plant communities in the face of global change. I established a manipulative study to examine how changes in the seasonal distribution of precipitation may affect the abundance of historically dominant (native) and recently-introduced (non-native) plant species, and the resulting impacts on the function of the ecosystem. My results showed that non-native grasses, especially Bromus tectorum, responded positively to increases in winter precipitation. In contrast, native species were least abundant in treatments with increased winter precipitation and most abundant in treatments with reduced winter precipitation and increased summer precipitation. Plots with higher abundance of the non-native grass B. tectorum had lower available soil moisture and plant species diversity. Although B. tectorum was most successful with additional winter precipitation, Ustilago bullata, a pathogen that infects B. tectorum, was also more prevalent in treatments that received increased winter precipitation. In a separate experiment, I tested a possible management strategy to address future changes in species composition in grasslands. I used timed mowing applications designed to reduce the abundance of non-native winter-active species and increase abundance of native species. Spring and summer mowing reduced cover of non-native grasses, but increased cover of non-native forbs. Spring mowing also increased abundance of native plant species. If yearly precipitation shifts to a more winter-wet pattern, non-native winter-active grasses could become more invasive in Colorado grasslands and reduce abundance of native plants and associated ecosystem services. However, management strategies that target the temporal niche of non-native grasses may reduce their abundance, thereby promoting more desirable grasslands in the future