Disturbance and site characteristics relate to cheatgrass (Bromus tectorum) abundance on ranches in Montana foothills ecosystems

Abstract

Cheatgrass (Bromus tectorum L.), also known as downy brome, is an invasive, exotic annual grass found throughout North America. Cheatgrass has been extensively studied in the Great Basin region of North America, where the majority of precipitation comes in winter and early spring, and the vegetation consists primarily of cool-season species. However, little research has been done in northern foothills grasslands in and near Montana, where most precipitation comes in spring and summer, supporting a mixture of cool- and warm-season plant species. Climate and vegetation differ between the northern foothills grasslands and the Great Basin, so the ecological impacts of cheatgrass in the northern foothills grasslands are unknown. In order to better understand cheatgrass ecology in this region, we assessed cheatgrass abundance in foothills ecosystems in Montana. Fifteen study plots were established at each of two ranches in Montana, both of which are owned and operated by the Montana Agricultural Experiment Station: Thackeray Ranch, southeast of Havre, Montana, and Red Bluff Ranch, east of Norris, Montana. At these plots we examined relationships among cheatgrass abundance, biotic and abiotic site characteristics (e.g. species diversity, soil depth), and disturbance indicators (e.g. livestock fecal counts) in 2014 and 2015. Large generalized linear mixed-effects regression models for each ranch were reduced to simpler models by comparing the Aikake's Information Criterion (AIC) for each fitted model, and selecting for lower AIC to best describe cheatgrass abundance. Multimodel inference based on the best models for each ranch identified important variables to predict cheatgrass abundance. Disturbance and site characteristics such as aspect and soil texture are important predictors that land managers in the northern foothills grasslands could monitor to estimate the risk of cheatgrass invasion or dominance on the landscapes they manage.