Biological invasion, the spread of non-native organisms, is occurring rapidly worldwide, and even New World desert areas currently show a dramatic increase in the arrival and spread of non-native Old World plant species. Among the detrimental effects are alterations in fire regimes and direct negative impacts on native plant species. Prior invasions of annual plants were mostly restricted to nutrient-rich areas under desert shrubs, and mostly avoided open areas between shrubs. We are examining the factors by which some of the non-native annuals, now dominant and problematic, are able to spread into the areas between the shrubs and thereby can greatly increase the fuel load in the matrix, which has historically produced a natural firebreak between shrubs. We conducted factorial experiments to determine the effects of fire, rainfall change, seed limitation and disturbance on the populations of native and non-native desert annuals. For this we burned individual shrubs, installed rain-out shelters and irrigated plots to mimic changing rainfall amount, experimentally disturbed soils, and added seeds of invasive species (currently present at the research area) to our experimental sites in the Sonoran Desert (Barry M. Goldwater Range) and the Mojave Desert (Fort Irwin).
Results/Conclusions
The short-term responses of annual plants populations and communities to our treatments show that native and non-native species in both deserts react differentially in the two deserts. As expected, annual biomass increased in both deserts with increasing rainfall and fire increased biomass in the Sonoran more than in the Mojave. Disturbance had little effect on biomass in both deserts. In the Mojave Desert disturbance and increasing rainfall favored non-native species, while burns did not cause a relative increase of non-natives. In contrast, non-native annuals in the Sonoran Desert did not become more abundant with increasing rainfall - most likely due to a strong reaction of native species - but increased with disturbance and fire. This suggests that the invasion processes differ between the deserts and that the impacts of fire and disturbance are regionally very different. We are currently investigating the possible reasons for these striking differences by focusing on soil nutrient and water distribution in relations to microhabitats that markedly differ between shaded shrub sub canopies and open intershrub areas and exposition.