Site-Specific Effects of Exotic Annual Grass Control Integrated with Revegetation

Abstract

Invasion by exotic annual grass species such as downy brome and Japanese brome has been implicated in ecosystem degradation in much of the western United States and strategies to restore lands dominated by these species are needed. We evaluated integration of herbicide and revegetation to restore old-field and rangeland sites dominated by annual brome species. In split-plot field studies, we compared three imazapic herbicide rates and eight seeding treatments at an old-field and a rangeland site. We evaluated brome, seeded species, and existing vegetation response to our treatments. Herbicide controlled annual brome at both sites, with > 95% decreases in brome biomass in plots treated with the high rate compared to non-sprayed controls. Control of annual brome persisted for two growing seasons at the old-field site and one season at the rangeland site. Annual brome abundance in non-treated control plots at both sites decreased by 80% between the first and second growing season. No seeded individuals established at the old-field site, but they persisted through two growing seasons at the rangeland site. Seeded species establishment at the rangeland site was impacted by both herbicide rate and seed mix, but no clear patterns emerged. Finally, while we controlled brome at both sites, existing desired vegetation increased only at the old-field site, where perennial grass biomass increased by about 30% in plots sprayed with both rates of herbicide. The differing responses of the existing and seeded plant communities across the two sites highlight the importance of integrating site-specific knowledge into restoration plans.