Control of establishing conifers to lessen their impacts on understory is a contentious habitat issue throughout the western United States. Our objective was to model the effect of increasing Douglas-fir (Pseudotsuga menziesii) canopy cover on mountain big sagebrush (Artemisia tridentata vaseyana) canopy cover. We sampled 25 sites, representing different levels of Douglas-fir establishment, within each of 3 southwestern Montana mountain big sagebrush communities. Over all sites, Douglas-fir canopy cover had a significant correlation (r = −0.86; P < 0.001) to mountain big sagebrush canopy cover. Regression models were variable among study areas as linear and curve-linear declines in mountain big sagebrush canopy cover were found. While model validation results indicated that individual study-area models were marginal for predicting relationships for the other study areas, the overall model developed from all sites combined generally predicted individual sites well, especially when mountain big sagebrush cover was <15%. Mountain big sagebrush canopy cover declined from >20% to <15% and <5% as Douglas-fir canopy cover increased beyond 20% and 35%, respectively. At least 92 species of mammals and 93 species of birds having either an obligatory or a facultative relationship with sagebrush are affected to differing degrees by loss of sagebrush cover.