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.
Resource availability and biotic interactions control opportunities for the establishment and expansion of invasive species. Studies on biotic resistance to plant invasions have typically focused on competition and occasionally on herbivory, while resource-oriented studies have focused on water or nutrient pulses. Through synthesizing these approaches, we identify conditions that create invasion opportunities. In a nested fully factorial experiment, we examined how chronic alterations in water availability and rodent density influenced the density of invasive species in both the Mojave Desert and the Great Basin Desert after fire. We used structural equation modeling to examine the direct and mediated effects controlling the density of invasives in both deserts. In the first 2 years after our controlled burn in the Great Basin, we observed that fire had a direct effect on increasing the invasive forb Halogeton glomeratus as well as a mediated effect through reducing rodent densities and herbivory. 4 years after the burn, the invasive annual grass Bromus tectorum was suppressing Halogeton glomeratus in mammal exclusion plots. There was a clear transition from years where invasives were controlled by disturbance and trophic interactions to years were resource availability and competition controlled invasive density. Similarly, in the Mojave Desert we observed a strong early influence of trophic processes on invasives, with Schismus arabicus benefitted by rodents and Bromus rubens negatively influenced by rodents. In the Mojave Desert, post-fire conditions became less important in controlling the abundance of invasives over time, while Bromus rubens was consistently benefitted by increases in fall rainfall.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.