Mojave Desert restoration: drought and facilitation influences on plant communities and soil seed banks in annual grass invaded shrublands
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Authors
Sullivan, Ranae Michelle
Issue Date
2024
Type
Dissertation
Language
en_US
Keywords
Bromus , drought , facilitation , invasive species , Mojave Desert , seed bank
Alternative Title
Abstract
Non-native grasses, like Bromus rubens, have become widespread in arid ecosystems across the western United States, including the Mojave Desert. Their prolific seed production and rapid spread pose significant threats to native flora and wildlife habitats. These invasive grasses alter fire cycles, degrade habitats, and reduce biodiversity. In desert ecosystems, soil seed banks are critical reservoirs for future vegetation, supporting plant diversity and ecosystem resilience. In particular, annual plants rely on seed banks to buffer against fluctuating climatic conditions, which helps sustain population stability and ecosystem function. Native annuals often use bet-hedging strategies like delayed germination to persist through unfavorable conditions. In contrast, invasive species like B. rubens lack a persistent seed bank, relying on early germination, rapid growth, and high reproductive output, allowing them to quickly dominate soil seed banks. As climate change intensifies with shifting precipitation patterns and increasing drought episodes, native and invasive species plant and seed bank dynamics become more complex. In addition to seed banks, desert shrubs play a crucial role in shaping plant communities by creating microsites that buffer harsh climate conditions and facilitate plant germination and establishment. Shrub microsites often enhance native and invasive plant abundance, but it remains unclear whether these patterns persist in seed banks under changing climate conditions. Additionally, efforts to control non-native grasses with pre-emergent herbicides like imazapic can unintentionally affect native species. Historically, invasive grass management has been reactive, targeting areas post-disturbance, such as after wildfire events that remove established vegetation.
Here, I evaluated the effectiveness of imazapic applications in reducing B. rubens cover and seed banks in invaded, but otherwise intact, plant communities. Using a split-plot design, I examined how single and repeated herbicide applications affect native and non-native plant communities and soil seed banks. Imazapic was aerially applied at four sites in Gold Butte National Monument in fall 2019, with a second application applied to a portion of each plot in fall 2020, creating once- and twice-treated sections. Vegetation was monitored from 2020 to 2022 to assess changes in non-native grass cover, annual forb cover, and perennial shrub cover. I also examined imazapic’s effects on native and invasive seed banks across different microsites (under shrub canopies and in open spaces) during a wet year (2020) and a drought year (2021). An additional drought year (2022) of seed bank data was collected in untreated plots to investigate how shrub facilitation and drought, as measured by the Palmer Drought Severity Index (PDSI), influenced seed bank density and composition. Soil samples were taken from beneath shrub canopies and interspaces and the seed bank was quantified using an emergence assay.
Initial herbicide treatments reduced B. rubens cover by 6.3% ± 1.2% in 2020, but this effect diminished in subsequent years. Twice-applied treatments did not show additional reductions until the second year after the second application. In interspaces, B. rubens seed density was reduced by once-applied treatments in both years, while reductions under shrub canopies varied by shrub species. Twice-applied treatments did not lead to further seed density reductions, likely due to minimal B. rubens growth during the application and concomitant drought years. Native forb cover decreased by as much as 2.9 % ± 0.6% after the first treatment but recovered in the following years. Herbicide treatments did not significantly affect the overall seed density of native forbs, though some species were negatively impacted. Perennial shrub cover remained stable, though drought induced mortality of the shrub Ambrosia dumosa was higher in herbicide treated plots.
While imazapic initially reduced B. rubens cover, the drought beginning in 2020 likely counteracted these benefits by suppressing annual plant emergence. Additionally, increased mortality of key shrub species suggested a tradeoff between reducing invasive grasses and maintaining native vegetation. Analyzing an additional year of seed bank data revealed that B. rubens seed density declined during drought but remained higher under shrub canopies compared to open spaces, suggesting that shrub facilitation of seed banks intensified during drought. Native forb density remained stable or increased despite drought, although their response to shrub facilitation was less pronounced. Species richness and diversity fluctuated, primarily driven by declines in B. rubens seed density.
These findings underscore the importance of aligning herbicide treatments with favorable precipitation conditions to enhance treatment efficacy. Effective management of invasive annual grasses requires proactive approaches that account for environmental conditions and microsite variability to safeguard native plant communities and prevent further habitat degradation. This study highlights the critical role of microsites and species-specific responses in shaping seed bank dynamics, offering valuable insights for restoration efforts aimed at managing invasive species and conserving native biodiversity in arid regions under the pressures of climate change.