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

Authors: SULLIVAN, RANAE - University Of Nevada; Newingham, Beth

Submitted to: Restoration Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2025
Publication Date: 9/3/2025
Citation: Sullivan, R., Newingham, B.A. 2025. Effects of herbicide use on Bromus rubens and soil seed banks in open and shrub microsites in the Mojave Desert. Restoration Ecology. 2025, 1061-2971. https://doi.org/10.1111/rec.70189.
DOI: https://doi.org/10.1111/rec.70189

Interpretive Summary: Soil seed banks are vital for supporting plant diversity and ecosystem resilence, especially in desert environments were annual plants buffer against flucuating climate conditions. Invasive annual grasses such as red brome in the Mojave Desert can outcompete native vegetation and change soil seed bank composition. This study examined the impact of the herbicide imazapic on seed banks of native and invasive plants in the Mojave Desert. Imazapic was applied at four sites with some of the treated areas receiving a second application of imazpic in the following year. We evaluated the seed bank for two years in areas treated with one application of herbicide, areas with a second treatment of herbicide applied the following year, and untreated control areas both under shrubs and in open spaces. We found that imazapic reduced red brome seed density, but the amount of reduction depended on the presence of shrubs and percipitation. While a single herbicide treatment reduced red brome seeds in interspaces, repeated treatments during drought conditions were less effective. The herbicide treatments did not have a significant effefct on overall native seed density, but it did have species-specific impacts on native seed banks. This study underscores the importance of careful timing in herbicide applications to reduce invasive species while minimizing harm to mative plants and highlights the need for targeted management strategies that consider environmental conditions.

Technical Abstract: Soil seed banks are critical reservoirs of future vegetation in desert ecosystems, supporting plant diversity and ecosystem resilience. Annual plants adapted to persist through fluctuating climatic conditions serve as a buffer against population declines and ensuring ecosystem function. Invasive annual grasses, such as Bromus rubens in the Mojave Desert, disrupt this balance by outcompeting native species and reducing their ability to contribute to the soil seed bank. Shrubs further influence desert ecosystems by creating microsites that facilitate germination and establishment, which shapes the spatial patterns of native and invasive species. Imazapic, a pre-emergent herbicide, is commonly used to control non-native grasses, though herbicide can unintentionally affect native flora. We evaluated the effects of imazapic applications aimed at reducing B. rubens cover and seed banks in invaded, but otherwise intact, plant communities. We investigated how single and repeated herbicide applications influenced native and non-native seed banks in interspace and under shrub canopy microsites during a wet and drought year. Seed banks were quantified using an emergence assay. Imazapic reduced B. rubens seed density in both years in interspaces; reductions under shrub canopies varied by shrub species. Neither herbicide treatment significantly affected native forb seed density, but there were species-specific impacts on native forb and shrub species. There was an initial reduction in B. rubens seed density with a single herbicide application, but a second application during a drought year provided no further reduction. These results highlight the importance of timing herbicide treatments with favorable precipitation conditions to maximize efficacy of seed bank reduction and support the need for precise management strategies that target seed banks of invasive species while minimizing impacts on native flora.