Prescribed fire drives divergent invasive annual grass responses in a dry forest-mosaic landscape

Abstract Background The reintroduction of fire, with the goal of reducing fuels and restoring forest resilience, is a critical management practice in dry conifer forests of the western United States. In dry forest landscapes, invasion by non-native annual grasses could complicate restoration goals if fire exacerbates invasion and associated impacts, particularly in areas with low tree cover and in adjacent open areas. Although annual grasses are becoming increasingly pervasive across the West, little is known about how species differ in their response to burning, especially in historically invasion-resistant forest-mosaic ecosystems. We use field data collected before and after a prescribed burn in eastern Oregon, USA, to uncover how prescribed fire impacts three problematic invasive annual grass species, cheatgrass ( Bromus tectorum ), ventenata ( Ventenata dubia ), and field brome ( B. arvensis ) and understory plant communities for 3 years after burning in a dry forest-mosaic landscape. Results Plant communities shifted towards invasive dominance over time regardless of burning. All three invasive annual grass species recovered within 2 years of burning to at or above pre-treatment levels and continued to increase in cover over time. Ventenata increased at the fastest rate across both burned and unburned areas, averaging less than 10% in 2016 to nearly 20% in 2020. This increase was slightly slower in burned areas than in unburned areas. Conclusions Our study demonstrates that invasive annual grasses are rapidly spreading across the study area regardless of prescribed fire. Low severity prescribed fire may not exacerbate the invasion of some annual grasses in forested areas and sagebrush patches where the canopy cover is already open. However, burning in sagebrush patches may have negative effects on important non-forest ecosystems when burning results in the loss of fire-sensitive species. This information can help aid decision making and improve species-specific management and treatment effectiveness in dry forest landscapes.