Herbicide application in Mojave Desert shrublands: Invasive grass reduction, native forb resilience, and shrub mortality

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

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2025
Publication Date: 9/19/2025
Citation: Sullivan, R., Newingham, B.A. 2025. Herbicide application in Mojave Desert shrublands: Invasive grass reduction, native forb resilience, and shrub mortality. Rangeland Ecology and Management. 103:184-195. https://doi.org/10.1016/j.rama.2025.08.012.
DOI: https://doi.org/10.1016/j.rama.2025.08.012

Interpretive Summary: Non-native grasses like red brome and Mediterranean grass are widespread in the Mojave Desert, harming native plants and wildlife. These grasses disrupt fire cycles and reduce biodiversity. Efforts to control them have typically focused on already disturbed areas. This study looked at the effectiveness of using herbicides proactively in intact areas. The herbicide imazapic was applied at four sites, with some areas receiving a second application a year later. We monitored plant communities over three years and found that initial treatments reduced red brome cover but the effects faded over time. Twice-applied treatments only showed further reduction in the second year after the second application was completed. Native plants were initially affected, but recovered. The herbicide also increased mortality in a key shrub species during the drought. This study suggests that the timing of herbicide treatments is crucial, needing adequate rainfall to be effective, and highlights the balance needed in managing invasive species while protecting native ecosystems.

Technical Abstract: Non-native grasses like red brome (Bromus rubens) and Mediterranean grass (Schismus spp.) are widespread in the Mojave Desert, causing significant harm to native flora and wildlife habitats. These invasive grasses alter fire cycles, degrade habitats, and reduce biodiversity. Typically, control efforts are reactive, targeting already disturbed areas. We evaluated the effectiveness of proactive herbicide treatments in reducing non-native grasses in otherwise intact plant communities. Using a split-plot design, we examined how single and repeated herbicide applications affect native and non-native plant communities. Pre-emergent herbicide (imazapic) was aerially applied at four sites in Gold Butte National Monument in fall 2019, with an additional application on a portion of each plot in fall 2020, resulting in once- and twice-treated sections. Vegetation was monitored during the 2020, 2021, and 2022 growing seasons for changes in non-native grass cover, annual forb cover, and perennial shrub cover. Initial treatments reduced Bromus rubens cover in 2020 by as much as 6.3% ± 1.2%, but effects diminished in subsequent years. Twice-applied treatments did not show further reduction until the second year. Native forbs decreased by as much as 2.9 % ± 0.6% after the first treatment but recovered in following years. Perennial shrub cover was unaffected overall, though drought induced Ambrosia dumosa mortality was greater in herbicide treated plots. While imazapic initially reduced red brome cover, severe drought that began in 2020 reduced the emergence of annual plants, potentially negating the herbicide's benefits. Furthermore, increased mortality of a foundational shrub suggests a significant tradeoff with treatments intended to reduce invasive grasses. This study highlights the need for timing herbicide treatments with adequate precipitation to enhance their efficacy. Effective management of invasive grasses requires a proactive approach, considering both treatment timing and environmental conditions to sustain native plant communities and prevent habitat degradation.