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

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 Great Basin 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 km² year⁻¹ (0.6% of the area of Great Basin rangelands). In 2020, annual grasses dominated approximately one-fifth (>77,000 km²) 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.