The elevational ascent and spread of exotic annual grass dominance in the Great Basin, USA

Abstract

Aim:In the western United States, sagebrush (Artemisia spp.) and salt desert shrublands are rapidly transitioning to communities dominated by exotic annual grasses, a novel and self-reinforcing state that threatens the economic sustainability and conservation value of rangelands. Climate change is predicted to favour annual grasses, potentially pushing transitions to annual grass dominance into higher elevations and north-facing aspects. We sought to quantify expansion of annual grass-dominated vegetation communities along topographic gradients over the past several decades. Location: Our analysis focused on rangelands among three ecoregions in the GreatBasin of the western United States, where several species of exotic annual grasses are widespread among shrub and perennial grass-dominated vegetation communities. Methods:We used recently developed remote sensing-based rangeland vegetation data to produce yearly maps of annual grass-dominated vegetation communities spanning 1990-2020. With these maps, we quantified the rate of spread and characterized changes in the topographic distribution (i.e. elevation and aspect) of areas transitioning to annual grass dominance. Results:We documented more than an eightfold increase in annual grass-dominated area since 1990, occurring at an average rate of >2,300 km 2 year −1 (0.6% of the area of Great Basin rangelands). In 2020, annual grasses dominated approximately one-fifth (>77,000 km 2 ) of Great Basin rangelands. This rapid expansion was associated with a broadening topographic niche, with widespread movement into higher elevations and north-facing aspects consistent with predicted effects of a warming climate.Main conclusions: More than a century after first appearing in the region, exotic annual grasses continue to proliferate and establish dominance in new environments across the Great Basin. Accelerated, strategic intervention is critically needed to conserve vulnerable sagebrush and salt desert shrub communities not yet heavily invaded. In this era of warming, future climate provides important context for selecting from among alternative management actions and judging long-term prospects of success.

Temporal mismatch in space use by a sagebrush obligate species after large‐scale wildfire

“…Our findings suggest that some portions of Trout Creek Mountains are less resistant to invasive annual grasses and forbs, which highlights the importance of mitigating the spread of invasive grasses following a fire. The elevational ascent of invasive grasses across the Great Basin now threatens higher elevation sagebrush communities that were previously believed to be resistant to invasion due to impacts of climate change, which will have a negative effect on sage-grouse habitat (Smith et al, 2021). Management efforts that directly affect the control of the spread of annual grasses following a wildfire can positively influence sage-grouse habitat selection (Poessel et al, 2022).…”

“…Greater sage‐grouse ( Centrocercus urophasianus ; hereafter sage‐grouse) are an umbrella species for sagebrush ecosystems at large spatial scales because they require a mosaic of patch types throughout their life history (Rowland et al, 2006). While leks, or breeding display sites, typically occur in open areas surrounded by sagebrush, nest sites are located within sagebrush stands with an herbaceous component (Connelly, et al, 2000; Smith et al, 2021). Optimal brood‐rearing areas occur in upland sagebrush plant communities, and broods thrive in mesic areas with high perennial forb and invertebrate diversity during summer (Dumroese et al, 2015).…”

Indaziflam reduces downy brome (Bromus tectorum) density and cover five years after treatment in sagebrush-grasslands with no impact on perennial grass cover

“…Downy brome (Bromus tectorum L.) invasion into the sagebrush-grasslands of western North America is one of the most critical threats facing these important rangelands (Clark 2020;DiTomaso et al 2010;Smith et al 2021). Variable germination timing (Beck 2009;Knapp 1996;Mack 1981), rapid growth (Arredondo et al 1998), high seed production (Young et al 1987), and acquisitive root morphology (Aguirre and Johnson 1991;Arredondo and Johnson 1999), help B. tectorum exploit important soil resources sooner than native plants and promote successful B. tectorum establishment.…”

Where there’s smoke, there’s fuel: dynamic vegetation data improve predictions of wildfire hazard in the Great Basin

“…Among the shrublands and grasslands of the Great Basin, however, the upturn appears to be driven largely by increasing fine fuels—in particular, exotic annual grasses. While perennial forb and grass production has remained stationary to slightly decreasing, cover and production of annuals has increased rapidly (Pastick et al 2021, Smith et al 2022), with an attendant rise in average fire probability (Fig. S6).…”