Historical wildfires do not promote cheatgrass invasion in a western Great Plains steppe

Abstract

Plant invasion and wildfire are often tightly linked. Invasive grasses, in particular, can severely alter ecosystems by increasing fire frequency and intensity. In western North America, positive feedbacks between wildfire and Bromus tectorum (cheatgrass) invasion have contributed to widespread plant community conversion. Impacts of conversion include reduced biodiversity, wildlife habitat, and livestock weight gains, as well as increased costs associated with fire-fighting and ecosystem restoration. While B. tectorum has been studied intensively in the Intermountain West, it is unclear whether fire-invasion feedback cycles observed in the Great Basin operate similarly in the western Great Plains, where annual bromes coexist with fire-adapted native species. In a shrub–grass ecotone in northeastern Wyoming, we asked how wildfires have influenced B. tectorum and its congener, B. arvensis, and whether the effects of wildfire on annual bromes varied based on landscape context. We sampled annual bromes along 142 transects associated with 28 historical wildfires (2–26 years since fire). Both brome species were equally likely to occur in burned and unburned sites. Cover of B. tectorum was lower in burned sites. Soil texture, cover of other plant species, slope, and aspect were strongly associated with annual brome abundance. In the western Great Plains, single wildfires do not appear to promote B. tectorum invasion. However, the effects of repeated fires on invasion in this system remain unclear. Our findings stress that relationships between fire and plant invasion are governed not by invader identity alone but by ecosystem-specific interactions among invaders, fire regimes, and resident species.