UC Riverside

Desert ecosystems in California have been negatively impacted by the invasion of exotic plant species and increased nitrogen inputs due to anthropogenic nitrogen deposition. Anthropogenic nitrogen deposition acts as an artificial fertilizer in nutrient-poor desert soil, often increasing the growth of invasive species. This can alter species composition, soil nutrient cycling, and fire risk. Although several studies have focused on the impacts of increased nitrogen deposition on invasive species, fewer studies have examined its impact on native annual species and how nitrogen affects their interactions with invasive species. The purpose of this dissertation research was to understand how nitrogen deposition affects native and invasive annual species in the desert from the level of the seed to the community. A seed bank study demonstrated that, although aboveground percent cover of invasive plants can increase under nitrogen fertilization, the soil seed bank was not affected. Rather, invasive plant seeds overwhelmed the seed bank in both unfertilized and fertilized plots. A follow-up study explored seed germination responses to a range of soil nitrogen concentrations and found that species identity may be more important in determining germination differences than soil nitrogen concentration. A greenhouse study measured the growth of native and invasive annuals under a combination of nitrogen and watering treatments. This study showed that both native and invasive species could benefit from increased soil water and nitrogen availability and highlighted the importance of sufficient water in order for natives to take advantage of excess nitrogen. The invasive forb, Erodium cicutarium, was able to utilize increased nitrogen with less sensitivity to water availability. Finally, a field-fertilization and invasive removal study was used to elicit the effects of increased nitrogen on native annuals in the field and their competitive interactions with invasive annuals. The results demonstrated that native annuals could make use of increased nitrogen in years with sufficient precipitation and that natives can coexist with invasive species at low densities, even with high nitrogen inputs. Understanding how the annual plant community is affected by nitrogen deposition and exotic invasion is essential for the conservation and restoration of ecosystem function in California's deserts.