Downy brome (Bromus tectorum L.), an invasive winter annual grass, may be increasing in extent and abundance at high elevations in the western United States. This would pose a great threat to high-elevation plant communities and resources. However, data to track this species in high-elevation environments are limited. To address changes in the distribution and abundance of downy brome and the factors most associated with its occurrence, we used field sampling and statistical methods, and niche modeling. In 2007, we resampled plots from two vegetation surveys in Rocky Mountain National Park for presence and cover of downy brome. One survey was established in 1993 and had been resampled in 1999. The other survey was established in 1996 and had not been resampled until our study. Although not all comparisons between years demonstrated significant changes in downy brome abundance, its mean cover increased nearly fivefold from 1993 (0.7%) to 2007 (3.6%) in one of the two vegetation surveys (P  =  0.06). Although the average cover of downy brome within the second survey appeared to be increasing from 1996 to 2007, this slight change from 0.5% to 1.2% was not statistically significant (P  =  0.24). Downy brome was present in 50% more plots in 1999 than in 1993 (P  =  0.02) in the first survey. In the second survey, downy brome was present in 30% more plots in 2007 than in 1996 (P  =  0.08). Maxent, a species–environmental matching model, was generally able to predict occurrences of downy brome, as new locations were in the ranges predicted by earlier generated models. The model found that distance to roads, elevation, and vegetation community influenced the predictions most. The strong response of downy brome to interannual environmental variability makes detecting change challenging, especially with small sample sizes. However, our results suggest that the area in which downy brome occurs is likely increasing in Rocky Mountain National Park through increased frequency and cover. Field surveys along with predictive modeling will be vital in directing efforts to manage this highly invasive species.

Nomenclature: Downy brome, Bromus tectorum L

Interpretive Summary: The data from field study plots sampled over the last 15 yr suggest that downy brome is spreading at high elevations in Rocky Mountain National Park. We applied a model that used environmental conditions where downy brome is found in the Park to predict where the plant was likely to spread. All three generated models showed a similar predicted distribution that was greater than the current distribution of downy brome in Rocky Mountain National Park. Many of the new locations of downy brome occurred where the earlier models predicted they would occur. This indicates that downy brome may likely continue spreading within this high-elevation region.

The models suggest that areas close to roads and trails, at lower elevations, and in shrubland plant communities in the park are most likely to be invaded by downy brome. Knowledge of both the high-risk areas and environmental factors that support the growth and spread of downy brome can greatly increase the efficiency of management efforts. Maxent is a geospatial model, so its mapping images can help focus weed management and control on areas with a high probability of spread as well as areas of high resource value. Using this or similar models can reduce the time and effort managers spend searching for weeds in areas of high priority but low probability of occurrence.