The invasive annual grass downy brome is the most ubiquitous weed in sagebrush systems of western North America. The center of invasion has largely been the Great Basin region, but there is an increasing abundance and distribution in the Rocky Mountain States. We evaluated postfire vegetation change using very large–scale aerial (VLSA) and near-earth imagery in an area where six different fires occurred over a 4-yr period at elevations ranging from 1,900 to over 2,700 m. The frequency of downy brome increased from 8% in 2003 to 44% in 2008 and downy brome canopy cover increased from < 1% in 2003 to 6% in 2008 across the entire study area. Principal component analyses of vegetation cover indicate a shift from plant communities characterized by high bare soil and forbs immediately postfire to communities with increasing downy brome cover with time after fire. The highest-elevation sampling area exhibited the least downy brome cover, but cover at some midelevation locations approached 100%. We postulate that the loss of ground-level shade beneath shrubs and conifers, accompanied by diminished perennial vegetative cover, created conditions suitable for downy brome establishment and dominance. Without a cost-effective means of landscape-scale downy brome control, and with infestation levels and climate warming increasing, we predict there will be continued encroachment of downy brome at higher elevations and latitudes where disturbance creates suitable conditions.
Nomenclature: Downy brome, Bromus tectorum L
Management Implications: The annual grass known as downy brome or cheatgrass is one of the worst weeds of western North America infesting nearly 23 million hectares, reducing rangeland forage and habitat value and increasing wildfire risk. Downy brome is not only expanding across the landscape, but is also expanding to higher elevations, spreading the fire risk from sagebrush lowlands into higher-elevation rangeland systems. Downy brome cover in the foothills of the southern Wind River Mountains at elevations of 1,900 to 1,700 m averaged < 1% in 2002, but expanded to > 6% cover by 2008, approaching 100% cover at some individual sites. Local spring temperatures have increased into the range that supports early downy brome growth. We speculate that fire removed shading overstory, further increasing spring soil temperatures to allow downy brome to rapidly expand. Concurrently, longer, drier growing seasons are reducing native plant growth and competitiveness. We recommend that land managers be aware of the risk of downy brome expansion at higher elevations and adjust their management to address postfire downy brome infestations.