Patterns in Cheatgrass Abundance in Foothills Grasslands in Montana

Cheatgrass (Bromus tectorum L.) is an invasive, exotic annual grass that exerts substantial negative ecological and economic influence in many of the ecosystems it invades. Cheatgrass has been extensively studied in the Great Basin region of North America where most precipitation comes in winter and early spring and the vegetation consists primarily of cool-season species and cespitose graminoid growth forms. However, much less research has been performed in the northern Great Plains region where precipitation comes primarily in spring and summer, supporting a mixture of cool and warm season plant species and both sod-forming and cespitose graminoid growth forms. In order to better understand cheatgrass ecology in the northern Great Plains region, we modeled cheatgrass abundance in relation to disturbance, vegetation, and site characteristics in two grassland locations in Montana. Multimodel inferences based on large generalized linear mixed-effects regression were used to identify variables important in predicting cheatgrass abundance. Our results suggest that cheatgrass appears to favor droughty site conditions associated with either coarser soil textures, shallower soils, or south-facing aspects. However, cheatgrass can exhibit extremely high abundances on more productive sites if disturbance creates an opportunity for invasion. Across all sites, it appears that soil disturbance can generate increased cheatgrass abundance and land management that promotes robust and vigorous vegetation and maximizes spatial and temporal niche occupancy should be encouraged to limit cheatgrass invasion and expansion.