Dynamic Relationships: Annual Plant Growth in an Urban Ecosystem

The productivity and diversity of desert annual plants is regulated by multiple factors such as precipitation, soil inorganic nitrogen (iN), and consumption by herbivores. In this study, we examined the relative importance of regulating factors of annual plants across an urban-rural gradient in metro Phoenix, AZ. We measured precipitation, soil iN pools, total plant production, and community composition at sites in two landscape regions: desert remnant parks (i.e. urban) and the surrounding Sonoran desert (i.e. non-urban). Total plant growth and community composition were quantified within exclosure plots (excluding small mammals and birds) and control plots. Results from our study indicate that precipitation is the most significant predictor of total plant growth across the city. However, soil iN is not significantly related to plant growth. Total annual plant growth and diversity were found to be significantly greater outside of the city. We also found that total plant growth was greater in plots that excluded herbivores. Consequently, we concluded that while consumption by herbivores has a significant impact on total plant biomass, this impact does not change across the city nor do herbivores cause changes in annual plant diversity. Annual plants provide vital ecosystem services. This and other studies can help urban planners and conservation biologists determine how urbanization may be altering an ecosystem’s capacity to provide these services for society.