Factors influencing big sagebrush cover in Southwest Montana

Abstract

The sagebrush steppe provides important habitat to many wildlife species. Conifer expansion of Douglas-fir and Rocky Mountain juniper into this area is a growing concern. Currently management focuses on the removal of Rocky Mountain juniper. The primary objective of this study was to determine the relationship between live sagebrush cover and aspect, slope, elevation, soil texture, soil depth and rock outcroppings. The second objective was to compare Douglas-fir and Rocky Mountain juniper's canopy areas at comparable ages. Percent cover of shrubs and conifers as well as the abiotic factors were recorded at forty Wyoming and forty mountain big sagebrush plots at each of three study sites in southwest Montana (Pipestone, Elkhorn, and Whitetail) (n = 240). The best-fit model utilizes the terms site, sagebrush subspecies, square root of conifer cover and site by sagebrush subspecies as the independent variables, with square roots of sagebrush cover as the dependent variable. The model found that abiotic factors had no correlation (p>0.05) with live sagebrush cover, but conifer cover had a negative effect on sagebrush cover. Equations were developed to predict canopy area of Douglas-fir and Rocky Mountain juniper thru time. Our findings suggest conifers should not be removed to increase sagebrush cover due to low response of sagebrush cover. Douglas-fir is the dominant conifer on these sites and has three times more canopy than Rocky Mountain juniper at similar ages (p<0.001). Rocky Mountain juniper is generally thought to have higher food and cover values for deer and elk and upland game birds compared to Douglas-fir (Gunderson 1990, Kufeld 1973). Therefore, the current practice of removing Rocky Mountain juniper to increase sagebrush cover should not be continued.