Sagebrush Response to Conifer Cover

Abstract

Sagebrush habitat is declining throughout the United States. This can have negative impacts for big game as well as other wildlife species. The purpose of our research was to analyze the relationship between several abiotic factors and the cover of two conifer species, Douglas-fir (Pseudotsuga menziesii) and Rocky Mountain juniper (Juniperus scopulorum), on the cover of Wyoming (Artemisia tridentata spp. wyomingensis) and mountain (Artemisia tridentata spp. Vaseyana) big sagebrush. Abiotic factors, percent cover of sagebrush and conifers, and individual conifer age and canopy area were recorded at 40 Wyoming and 40 mountain big sagebrush plots at each of three study sites in southwest Montana (n = 240). No correlation was found between any abiotic factor and live sagebrush cover over all sites (p>0.05). A model of the relationship between the combined cover of the two conifer species and the two subspecies of big sagebrush was developed. The best-fit model included the terms: study site, sagebrush subspecies,  and the interaction, study site by sagebrush subspecies as the independent variables, with as the dependent variable (  = Intercept_i– 0.401 ; R² = 0.61). There was a negative relationship between conifer cover and sagebrush cover and no difference in the effects of conifer cover on the two sagebrush subspecies. Validation trials were successful at one of three locations outside the study area and suggested that the model is better suited to lower elevation, less productive sites. Individual Douglas-fir have a 3 fold larger canopy area than Rocky Mountain juniper at comparable ages (p<0.001). Controlling conifers to increase Wyoming big sagebrush does not appear to be effective due to the low level of sagebrush cover. If conifer control is desired, Douglas-fir should be targeted over Rocky Mountain juniper on mountain big sagebrush sites, but not on Wyoming big sagebrush sites.