In arid and semiarid ecosystems, invasion by exotic grasses may be driving state changes in vegetation defined by losses of native shrub communities. Changes in wildfire regimes and fall precipitation timing related to climate change may promote fluctuations in resource availability that reinforces invasion and state changes in vegetation. The objective of this study was to investigate how earlier fall precipitation timing and fire affected the germination, establishment, and growth of the dominant native shrub Wyoming big sagebrush (Artemisia tridentata subsp. wyomingensis), and one of the most problematic invaders, cheatgrass (Bromus tectorum L.) in the Great Basin. We extracted soil cores from Rush Valley, Utah (UT), USA, on the eastern side of the Great Basin ecoregion and placed them in a common garden in Provo, UT, and planted seeds of sagebrush and cheatgrass in individual cores. We measured the response of sagebrush and cheatgrass to experimental fire and two fall precipitation timing pulses in a full factorial design. Water was added for two weeks in early September (early fall treatment) and mid-October (late fall treatment). We measured seedling emergence, plant height, biomass, density, seed production, and survival. Early fall precipitation did not significantly affect the amount of cheatgrass or sagebrush seedling emergence. Early fall precipitation significantly increased cheatgrass density, height, biomass, and seed production, and sagebrush height and biomass, but not density. Surprisingly, cheatgrass did not respond positively to fire. In contrast, fire increased sagebrush density (twofold) and survival. These findings indicate that fire can have positive impacts on sagebrush establishment. The data suggest that projected increases in fall moisture in the Great Basin due to climate change are likely to have positive impacts on both cheatgrass and sagebrush. However, additional studies are needed to identify how fall precipitation timing and fire might impact competitive interactions between sagebrush and cheatgrass and the bearing on invasion success at influencing state changes in the Great Basin.

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.