Plant community response to Bromus tectorum management in the high-elevation sagebrush steppe of southwest Montana

Abstract

Bromus tectorum (cheatgrass) invasion is a management concern across the western United States. Herbicide is the most common B. tectorum control method, but integrated management is becoming more common. Previous research indicates that the high-elevation sagebrush steppe is resilient to B. tectorum invasion due to a combination of native perennial grass abundance, topography, and cooler and wetter climate. Additionally, plant-soil feedbacks, the interaction between plants and their soil microbial community, are proposed to play a role in ecosystem invasibility. We explored the response of B. tectorum invaded plant communities to herbicide, grazing, restoration seeding, and plant soil feedbacks (PSF). In the high-elevation sagebrush steppe of southwest Montana, we found that herbicide reduced B. tectorum for three to four years. Importantly, the paired "reference" communities did not observe an increase in B. tectorum. The herbicide control was not associated with an increase in species richness nor perennial grass biomass. A community analysis found the sprayed communities were resilient to B. tectorum management. Both sprayed and reference community assemblages shifted due to slight increases in B. tectorum and native perennial forb cover and a reduction in native perennial grass cover. These changes were correlated with warmer and drier seasons, which could impact the future resiliency of this system. Integrated non-native plant management is a successful alternative to singular methods. Through a combination of grazing, herbicide and restoration seeding we found that intensive fall grazing was the most effective form of B. tectorum management. Unexpectedly, herbicide application resulted in a nutrient pulse, possibly altering soil nutrient cycling and soil microbial communities. Indeed, our PSF study found that B. tectorum received a strong positive feedback that was not affected by competition; though native soil reduced the competitive effect of B. tectorum. We propose that PSFs are a mechanism by which plant communities confer resilience to non-native plant invasion. This research demonstrates that where B. tectorum has invaded south-facing slopes of sagebrush steppe in southwest Montana, it can be controlled by herbicide though without, increased diversity or productivity, and that both invaded and uninvaded communities are resilient.