Does fire promote non-native annuals in the American deserts?

Background/Question/Methods

The deserts of North America have recently become fire-prone due to invasion of non-native plants (chiefly grasses) that provide fuel sources, a novel component in desert ecosystems. As many of the native desert perennials, especially shrubs and trees, lack adaptations for fire resistance, now frequently occurring fires have changed desert vegetation drastically. Fires are now to be expected in years of abundant precipitation and are postulated to increase the amount of invasive plants thereby causing positive feedbacks between invader performance and fire frequency.  Such invasive grass/fire cycles have been demonstrated in other water-limited ecosystems. It is not clear whether such feedbacks indeed exists in the hot deserts of the arid Southwest as published literature reports contrasting results on this.  We intended to test whether invasive annual plants increase after fires.  For this we investigated the short-term annual vegetation change of experimentally burned shrubs in the first two years after fire and longer-term changes by comparing burned and unburned sites for natural fires that occurred seven and eight years ago. We conducted these comparisons in creosote bush (Larrea tridentata) dominated desert scrublands in army training areas in the lower Sonoran Desert and the Mojave Desert. 

Results/Conclusions

The percentage of non-natives increased under canopies of burned shrubs both in the Mojave and the Sonoran Desert in the two years after experimental fires indicating a short-term, overall fire dependant increase in non-native plant dominance. This is in contrast to areas that have been burned a longer time ago (7 years) where native plants did not increase in comparison to unburned areas. Schismus spp., non-native grasses abundant in both desert sites, that contribute to fire fuels in some years, decreased in density the first two years after experimental fire but appears to increase slightly in the Mojave Desert in burned areas seven years post fire. The same species increased in density in the Sonoran Desert immediately after fire but was reduced after seven years post fire. These desert specific, contrasting results for the dominant invasive annual and the long-term decreases of non-native species seem to indicate that non-native plants in general do not promote fire by positive feedback mechanisms alone.