UC Santa Cruz

Bromus tectorum L. is an annual grass native to Eurasia that was introduced into the U.S.A. in the late 1800s and has become dominant in large parts of the Intermountain West. At high density, it can alter the fire regime and subsequently act to displace native shrub and bunchgrass communities. In the eastern Sierra Nevada, California, B. tectorum exists in low-density populations and its impacts have been minimal. However, agents of global change, such as changing climate and increased anthropogenic nitrogen (N) deposition, will likely affect plant species distributions and may facilitate the spread of B. tectorum. In this dissertation, I examine: (1) how climate change and increased N deposition might affect B. tectorum spread, (2) how increased N deposition might affect species diversity and B. tectorum dominance, (3) the ecological effectiveness of B. tectorum control techniques, and (4) the feasibility of B. tectorum control in the eastern Sierra Nevada, CA, considering social logistical, and regulatory factors.

Through a series of in-situ experiments, I found that a shift from snow to rain could increase B. tectorum dominance in the region, depending on timing, frequency, and magnitude of rain events. Effects of increased N on B. tectorum were dependent on rainfall. Through a four-year study monitoring species composition, I found that B. tectorum dominance has increased significantly at the site at the expense of native forb species richness. In disturbed areas, B. tectorum cover is approaching the threshold for increased fire risk, which could result in more significant impacts for the region. Several methods of control were effective in reducing B. tectorum and increasing the dominance of native species, including hand pulling, soil solarization, and mulching. However, through interviews with local land managers, I identified a number of regulatory, social, and logistical obstacles to active B. tectorum control in the region. Future work should be focused on overcoming these barriers to prevent increased impacts of B. tectorum at its high elevation range margin.