Human land uses are a primary cause of introduction and spread of exotic annual Bromus species (hereafter Bromus). Initial introductions were likely linked to contaminated seeds used by homesteading farmers in the late 1880s through early 1900s. Transportation routes aided their spread. Ecosystems with cooler and moister soils, especially when most precipitation falls during the growing season, tend to have greater potential to recover from disturbances (resilience) and to be more resistant to Bromus invasion and dominance. Warmer and drier ecosystems are less resistant to Bromus and are threatened by altered fire regimes which can lead to Bromus dominance, impacts to wildlife, and alternative stable states. Native Americans used fire for manipulating plant communities throughout the West, but to our knowledge did not contribute to the dominance of Bromus except potentially in portions of California during Spanish control.
Results/Conclusions
Fire as a tool is now limited to site preparation for revegetation in most ecosystems where Bromus is a significant problem, but remains a risk for increases in Bromus. Once Bromus dominates, breaking annual grass/fire cycles requires restoring fire tolerant perennial grasses and forbs, which can compete with Bromus and resist its dominance. In the Great Basin, cover of perennial grasses between 15 and 20 % appears to keep Bromus as a subdominant even after disturbances. Historic homestead policies likely contributed to the spread Bromus as lands were often tilled, but many were later abandoned allowing Bromus to increase unchecked. Similarly, unchecked livestock grazing led to land degradation during the time of Bromus expansion. The Taylor Grazing Act of 1936 and several land policies in the late 1960s and 1970s curbed this degradation, but many areas may require restoration before they can improve. Since Bromus is rarely listed as a state noxious weed, current weed management policies often lack regulations to prevent further expansion of Bromus and adjustments in this policy might be considered. Roads, vehicles, and energy infrastructure become potential conduits of Bromus spread through their regular maintenance and continued vehicle traffic. Research is needed on how and where livestock grazing might help increase perennial grass and forb abundance to create ecosystems that are more resistant to Bromus. Adjustments in livestock management to sustain perennial grasses may aid ecosystem resistance to Bromus. Also, studies are needed to ascertain the role, if any, of oil and gas development in contributing to the spread of Bromus.