Fire frequency effects on plant community characteristics in the Great Basin and Mojave Deserts of North America

Abstract Background: Wildfire regimes are changing dramatically across North American deserts with the spread of invasive grasses that perpetuate invasive grass fire cycles, resulting in larger and more frequent fires. This study experimentally compared how single and repeat fires, a characteristic of invasive grass fire cycles, influence plant fuels dominated by invasive grasses, in the Great Basin and Mojave Deserts. Both study sites had an identical study design. In the summer of 2011, we experimentally burned half of each experimental block the other half remaining as an unburned control. Half of the burned plots were reburned five years later to simulate a repeat burn characteristic of increasing burn frequency. We estimated non-woody plant biomass, cover and density in plots from 2017-2020. Results: Biomass did not vary between sites, but there was higher plant cover but lower plant density at the Mojave site than the Great Basin site. Plant biomass, density and cover varied significantly across years, with stronger annual fluctuations in the Great Basin. At both desert sites, fire increased plant density and biomass compared to unburned plots but had no effect on biomass. The effect of fire on plant cover varied significantly between years for both deserts but was greater in the Great Basin than the Mojave site. Repeat fires did not amplify the initial fire effects. Conclusions: The results suggest that annual fluctuations in fine fuel production generally, and in response to fire was more responsive at the Great Basin site than the Mojave site, with no compounding effect of repeat fires.