Bromus tectorum in the Intermountain West and Great Plains (USA): Population Variation and Regional Environment Influence the Course of an Invasion
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Biological invasions are an important component of anthropogenic global change and are a major source of environmental and economic damage. Future distributions of invasive species are expected to shift as a result of climate change and an understanding of the factors that determine range limits will be critical to the management of invasions. In addition, biological invasions provide case studies to evaluate the role of multiple factors in ecological and evolutionary dynamics. The annual grass Bromus tectorum (cheatgrass) is a widespread invader across North America, but the damage from this invasion is concentrated in the Intermountain West. I evaluated the role of population variation and regional environments for cheatgrass invasion in the Intermountain West and Great Plains using field, greenhouse, and controlled environment experiments. Phenotypic traits vary among populations from different regions and both stabilizing and divergent selection appear to contribute to the evolution of cheatgrass in North America. Regional environment, genetic variation, and phenotypic plasticity all affected cheatgrass survival and growth in reciprocal garden studies. Potential local adaptation was detected in populations from the Great Plains but in not populations from the Intermountain West. In field trials that examined the relationship between cheatgrass and summer precipitation, populations in eastern Washington were not directly affected by increased summer precipitation; this result suggests that cheatgrass invasion may not be reduced under such a future climate scenario but the indirect effects from biotic factors also need to be assessed. Cheatgrass populations from the Great Plains have greater freezing tolerance compared to Intermountain West populations, which may indicate local adaptation that could contribute to range expansion. Genetic variation and environmental factors both contribute to the distribution and potential invasion of cheatgrass, and local adaptation may contribute to further range expansion especially in combination with climate change. Additional assessment of the biophysical thresholds and biotic factors that affect cheatgrass range would further quantify the potential for future invasion and to provide insight into the evolution and ecology of species' range limits.