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

Authors: SMITH, JOSEPH - University Of Montana; ALLRED, BRADY - University Of Montana; Boyd, Chad; Davies, Kirk; JONES, MATT - University Of Montana; KLEINHESSELINK, ANDREW - University Of Montana; MAESTAS, JEREMY - Natural Resources Conservation Service (NRCS, USDA); MORFORD, SCOTT - University Of Montana; NAUGLE, DAVID - University Of Montana

Submitted to: Diversity and Distributions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2021
Publication Date: 11/17/2021
Citation: Smith, J.T., Allred, B.W., Boyd, C.S., Davies, K.W., Jones, M.O., Kleinhesselink, A.R., Maestas, J.D., Morford, S.L., Naugle, D.E. 2021. The elevational ascent and spread of exotic annual grass dominance in the Great Basin, USA. Diversity and Distributions. 28(1):83-96. https://doi.org/10.1111/ddi.13440.
DOI: https://doi.org/10.1111/ddi.13440

Interpretive Summary: Large areas of the Great Basin are transitioning to exotic annual grasslands. Climate change is predicted to directly and indirectly favor annual grasses and potentially allow annual grassland transitions in higher elevations. We used recently developed remote sensing-based rangeland vegetation data to retrospectively quantify expansion and elevational range shift of annual grassland transitions in the Great Basin from 1986–2019. We document an alarming six-fold increase in annual grassland area (to >28,900 square miles) occurring at a rate of 481,855 ac yr-1. This rapid expansion has been in part facilitated by exotic annual grasslands developing in higher elevations, with the leading edge of annual grassland transitions moving upslope at 60-110 m decade-1. This information will be used by scientists, land and wildlife managers, and policy makers.

Technical 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 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.