Post-fire Downy Brome (Bromus tectorum) invasion at high elevation in Wyoming

Authors: MEALOR, B - University Of Wyoming; COX, SAMUEL - Bureau Of Land Management; Booth, D

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2012
Publication Date: N/A
Citation: N/A

Interpretive Summary: The annual grass known as downy brome or cheatgrass is one of the worst weeds of western North America infesting nearly 56 million acres, 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 high-elevation conifer stands. Downy brome cover in the foothills of the southern Wind River Mountains at elevations of 6234 to 8858 feet was less than 1% in 2002, but expanded to 6.1% cover on a per-plot basis by 2008. Local temperature data show early spring warming has brought air temperatures into the range to support early downy brome growth and we speculate that wildfire and controlled burns removed shading shrubs and trees increasing spring soil temperatures to allow downy brome to rapidly expand and concurrently producing longer, drier growing seasons that are reducing native plant growth and competitiveness. We recommend that land managers be aware of the risk of downy brome at these higher elevations and adjust their management to prevent post-fire downy brome infestations.

Technical Abstract: 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 post-fire vegetation change using very large-scale aerial (VLSA) and near-earth imagery in an area which experienced six different fires over a 4 year period at elevations ranging from 1900 to over 2700 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 post-fire to communities with increasing downy brome cover with time after fire. The highest elevation sampling area exhibited the least downy brome cover, but some mid-elevation locations were approaching 100%. The mean March air temperature of our study area rose above freezing between 1960 and 1970 and was above 2oC by 2000. We postulate the loss of ground-level shade beneath shrubs and conifers allowed soil temperatures to mimic air temperatures, thus creating 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 conditions suitable for downy brome.