Effects of herbicide use on Bromus rubens and soil seed banks in open and shrub microsites of the Mojave Desert

Introduction Soil seed banks are reservoirs of future vegetation in deserts, supporting plant diversity and resilience. Annual plants adapted to fluctuating climates help maintain ecosystem function. In the Mojave Desert, invasive grasses such as Bromus madritensis ssp. rubens (“ B. rubens ”) outcompete native species by reducing their growth, reproduction, and seed bank inputs. Shrubs influence plant dynamics by creating microsites that affect native and invasive species' spatial patterns. Imazapic, a pre‐emergent herbicide, is used to control non‐native grasses but may affect native flora. Objectives We evaluated how imazapic applications aimed at reducing B. rubens affect native and non‐native seed banks in invaded but otherwise intact plant communities. We examined the effects of single and repeated applications across interspace and shrub canopy microsites during a wet and drought year. Methods Imazapic was aerially applied (418 g/ha) at four sites in Gold Butte National Monument (NV, U.S.A.) in fall 2019 with partial retreatment in fall 2020. Seed banks from shrub and open microsites in treated and untreated areas were quantified via emergence assay. Results Imazapic reduced B. rubens seed density both years in interspaces; reductions under shrubs varied by species. Native forb seed density was unaffected overall, though species‐specific impacts occurred. A single application reduced B. rubens seeds; a second application during drought provided no further reduction. Reductions were greater in open spaces than under shrubs. Conclusions Imazapic reduced B. rubens seed densities with minimal impacts on natives. However, imazapic efficacy was diminished beneath shrubs.