No abstract

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.

No abstract

The spread of invasive grasses across Earth are modifying fire cycles resulting in state changes in arid ecosystems. Disturbance, biotic resistance of native biological communities and propagule pressure, are likely the important factors influencing the spread of invasive grasses and their influence on changing fire regimes. Over a 5‐year period (2011–2016), we tested how the potential loss of biotic resistance of native plant and native rodent communities related to fire and rodent exclusion treatments, in concert with increased propagule pressure affected the establishment of Bromus tectorum L. (cheatgrass) and the spread of secondary fires. Our study results suggest that native plant and native rodent communities contribute to biotic resistance against cheatgrass invasion and that fire and high propagule pressure act to diminish biotic resistance by native communities. Five years into the study, cheatgrass establishment was 11‐fold greater in burned plots than in unburned plots (with native plant communities still intact), 2.4‐fold greater in rodent exclusion plots than rodent access plots and 1.8‐fold greater with increased propagule pressure. At the start of the experiment in 2011 cheatgrass was present in the experimental blocks but at very low density (<1 plant/m2). However, by 2016, burned‐rodent excluded plots were fully invaded (1625 stems/m2). High propagule pressure released cheatgrass from biotic resistance of rodent communities in post‐fire conditions but had minimal effects on the biotic resistance of native plant communities in unburned plots. Fire in combination with either rodent exclusion or high cheatgrass propagule pressure produced higher density cheatgrass stands that were positively correlated with the spread of secondary fires that are characteristic of invasive grass‐fire cycles. Synthesis. Loss of native plant cover or reduction in rodent populations due to fire, extreme climatic events or disease outbreaks, which are increasing with human activity, may provide windows of opportunity for invasive grasses to escape biotic resistance and reinforce invasive grass‐fire cycles.

Human activities are increasing the size, frequency, and severity of disturbance across earth's ecosystems including deserts. Ants are important drivers of ecosystem function and are good bioindicators of ecosystem sensitivity to disturbance and change. Rodents also play an important role in ecosystem response to disturbance and often compete with ants for resources. The purpose of our study was to test the main and interactive effects of fire, rodent activity, and time on ant forager abundance, species richness, and diversity, as well as changes in ant mound density and disk area in the Great Basin Desert. We experimentally applied burn and rodent exclusion treatments and used pitfall traps to collect ants each month from April through October from 2014 to 2016. Over the three‐year period, burned areas had lower richness and diversity than unburned areas. Rodent exclusion had minimal effects on the ant community, and there was not a significant rodent exclusion interaction with fire. Treatment effects varied by month and year. The western harvester ant, Pogonomyrmex occidentalis, was the most abundant ant species, comprising about 70% of the total ants captured. Shifts in ant diversity following fire were driven by positive responses of harvester ants to burned habitat conditions. In contrast, all other ant species when analyzed together had lower forager abundance in burned plots, which drove lower ant diversity in burned plots. Ant forager abundance, richness, and diversity increased each year of the study in all plots; however, richness and diversity remained lower in burned areas than in unburned areas each year. Structural equation modeling shows that the effects of fire on ant community diversity are partially mediated through the plant community. While rodents affected the plant community, those effects do not seem to transfer over to the ant community. Pogonomyrmex occidentalis mound density was higher in burned areas, but disk area was smaller. Our results suggest that fire has adverse effects on ant community diversity. This could have long‐lasting effects on ecosystem function in the face of a changing fire regime in deserts of North America caused by invasive annual plants.

No abstract

Human activities are changing patterns of ecological disturbance globally. In North American deserts, wildfire is increasing in size and frequency due to fuel characteristics of invasive annual grasses. Fire reduces the abundance and cover of native vegetation in desert ecosystems. In this study, we sought to characterize stem growth and reproductive output of a dominant native shrub in the Mojave Desert, creosote bush (Larrea tridentata (DC.) Coville) following wildfires that occurred in 2005. We sampled 55 shrubs along burned and unburned transects 12 years after the fires (2017) and quantified age, stem diameter, stem number, radial and vertical growth rates, and fruit production for each shrub. The shrubs on the burn transects were most likely postfire resprouts based on stem age while stems from unburn transects dated from before the fire. Stem and vertical growth rates for shrubs on burned transects were 2.6 and 1.7 times higher than that observed for shrubs on unburned transects. Fruit production of shrubs along burned transects was 4.7‐fold more than shrubs along paired unburned transects. Growth rates and fruit production of shrubs in burned areas did not differ with increasing distance from the burn perimeter. Positive growth and reproduction responses of creosote following wildfires could be critical for soil stabilization and re‐establishment of native plant communities in this desert system. Additional research is needed to assess if repeat fires that are characteristic of invasive grass‐fire cycles may limit these benefits.