Abstract

Historically, desert ecosystems in the southwestern United States have been characterized by low incidence of fire. However, widespread wildfires in these systems have been recently observed. Large areas of creosote bush (Larrea tridentata) scrub scattered in the Mojave and Sonoran deserts were impacted by wildfires in 2005, after a series of years with above-average precipitation. It has been hypothesized that exotic invasive grasses, for example, Schismus arabicus, are responsible as they are able to grow in the open and may produce sufficient biomass to carry fires during high-rainfall years. However, there has been little experimental evidence to support this hypothesis. We conducted a field experiment exploring the response of the annual plant community to increased rainfall in two creosote-dominated sites in the Sonoran and Mojave deserts by manipulating precipitation levels, fire history, soil disturbance, and exotic-annual-plant seed availability. We examined how these treatments impacted fuel loads (biomass) and species distributions as a function of distance from shrubs. We found that enhanced rainfall produces a general increase in biomass and density of annual plants. At the Sonoran site, rainfall tends to increase the density of exotic annuals. At the Mojave site, it is the opposite, where native annuals benefit more from higher rainfall. However, it is important to note that in the Mojave site, native annuals produce higher biomass under shrubs and exotic annuals produce higher biomass in the open in response to increased rainfall. The introduction of fire at both sites slightly increased biomass production as well. Soil disturbance and seed addition had only subtle effects. From our results, it is dear that increased rainfall in southwestern U.S. deserts is conducive to increasing biomass of annual plants, especially in the inter-shrub areas, which in turn can lead to enhanced fire risk. The potential to shift toward higher dominance by exotic annuals with fire at the Sonoran site could further enhance this risk. With several consecutive years of high rainfall, increased seed production by exotics would potentially amplify this effect.