UC Santa Cruz

Bromus tectorum L. (Poaceae, ‘cheatgrass’) is a flammable, non-native, annual grass from Eurasia that has spread throughout the western United States, exerting serious impacts on ecosystems by fueling frequent fires and displacing native vegetation. Over the last decade, B. tectorum has begun expanding into higher elevations, where it currently coexists with native vegetation and has not yet altered fire regimes. In this dissertation, I examine the impacts of climate change and management activities on high-elevation B. tectorum populations in the eastern Sierra Nevada, California. In Chapter 1, I explore the impacts of changing patterns of snow and rain on B. tectorum as compared to native species. I found that B. tectorum responses to precipitation change (snow and rain) were of greater magnitude than the responses of native species at the University of California Valentine Eastern Sierra Reserve. Bromus tectorum density followed the declining trend in winter snowpack over three years, and within-year snowpack reductions advanced the timing of B. tectorum phenology. Species-specific responses to spring rainfall showed some variation in different years of the study, highlighting the importance of both winter and spring precipitation and suggesting that inter-annual variability will generate different impacts on different species depending on specific conditions. Chapter 2 presents a greenhouse experiment to study the impacts and interactions of temperature, competition, and facilitation on native species and B. tectorum. Higher temperatures in the greenhouse dramatically decreased B. tectorum reproductive allocation and increased vegetative biomass. Bromus tectorum tended to have negative impacts on neighboring species by reducing their biomass, reproductive allocation, stomatal conductance, and photosynthetic rates (suggesting possible competition), while a native nitrogen-fixing forb tended to have positive impacts on these variables for B. tectorum and other native species (suggesting possible facilitation). Chapter 3 is an applied research project conducted at Devils Postpile National Monument to assess the impacts of extreme disturbance events and fuels management activities on fuel loads and B. tectorum invasion. An extreme windstorm substantially elevated fuel loads, which were reduced in areas that were treated for hazardous fuel reduction. Anthropogenic disturbance from fuel reduction projects was not creating establishment sites for B. tectorum, but areas that were subjected to a high-severity wildfire burn about twenty years ago provided loci of B. tectorum invasion. Prioritizing monitoring efforts in high-severity burned areas, especially when they coincide with high levels of disturbance from recreational activities, such as along hiking trails and near pack stations, would strengthen active management of B. tectorum infestations at Devils Postpile National Monument. Overall, this dissertation shows that various climatic changes and anthropogenic impacts could influence mechanisms for future high-elevation B. tectorum expansion in different ways.