Bromus tectorum invasion and global change:Likelihood of spread and feasibility of control
Concilio, A. L.. 2012. UC Santa Cruz PhD
Abstract
Bromus tectorum L. is an annual grass native to Eurasia that was introduced
into the U.S.A. in the late 1800s and has since 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.
