The influence of Douglas-fir and Rocky Mountain juniper on Wyoming and mountain big sagebrush cover in Southwest Montana

Abstract

Expansion of conifers into sagebrush steppe is a management concern, since conifers reduce sagebrush cover for wildlife. The primary objective of this research was to examine the relationship between the conifers, Douglas-fir (Pseudotsuga menziesii) and Rocky Mountain juniper (Juniperus scopulorum), and two subspecies of big sagebrush, Wyoming big sagebrush (Artemisia tridentata spp. wyomingensis) and mountain big sagebrush (Artemisia tridentata spp. vaseyana), in southwest Montana using a best-fit model. A secondary objective was to determine whether either of the two conifer species have a greater influence on sagebrush cover. Percent cover of both conifers and sagebrush was recorded at 40 Wyoming and 40 mountain big sagebrush plots at each of three study sites in southwest Montana (n = 240). The best-fit model utilizes the terms site, sagebrush subspecies, the square root of conifer cover, and site by sagebrush subspecies as the independent variables, with the square root of sagebrush cover as the dependent variable. The model (the square root of sagebrush cover = a i - 0.401 x the square root of conifer cover; r ² = 0.61) found a negative relationship between conifer cover and sagebrush cover and indicates that there is no difference between the two sagebrush subspecies and across all three sites in the study area. The best-fit model was validated within the 95 % confidence interval at all three study sites. Validation trials with data from three sites outside the study area were successful for one site, suggesting that the model is better suited to lower elevation, less productive sites. There was no difference in the influence of Douglas-fir or Rocky Mountain juniper cover on live sagebrush cover, indicating that sagebrush responds similarly to competition from both species. Controlling both conifers may increase sagebrush cover slightly, but responses will be small due to low levels of initial sagebrush cover. If maintenance of sagebrush cover is desired, conifer control should be initiated before conifer cover reaches 10 %, since the rate of sagebrush decline is highest at low levels of conifer cover. However, conifer control is not recommended because both conifers and sagebrush are important components of big game winter range.