Herbicide Application in Mojave Desert Shrublands: Invasive Grass Reduction, Native Forb Resilience, and Shrub Mortality

Abstract

Nonnative grasses such as red brome (Bromus rubens) 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. We evaluated the effectiveness of proactive herbicide treatments in reducing nonnative grasses in otherwise intact plant communities. Using a split-plot design, we examined how single and twice-applied herbicide applications affect native and nonnative 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 2020–2022 growing seasons to track changes in nonnative grass cover, annual forb cover, and shrub cover. Initial treatments reduced B. 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. Shrub cover was unaffected overall, though drought-induced Ambrosia dumosa mortality was greater in imazapic treated plots. Although imazapic initially reduced B. rubens cover, severe drought that began in 2020 reduced the emergence of annual plants, potentially negating or masking the herbicide’s benefits. Furthermore, increased drought-induced mortality of a foundational shrub in twice-applied treatments suggests a significant tradeoff with treatments intended to reduce invasive grasses. This study highlights the need for timing pre-emergent 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.