Climate change-mediated expansion of Utah Juniper across the Bighorn Canyon National Recreation Area: Implications for bighorn sheep

Abstract

Plant distribution is changing with a warming climate, with some species expanding into adjacent areas that were formerly unsuitable. Since the late 1990s, curl-leaf mountain mahogany (Cercocarpus ledifolius; CLMM) has shown an increased mortality rate concurrent with an expansion of Utah juniper (Juniperus osteosperma) within Bighorn Canyon NRA, which caused concern about impacts for the resident bighorn sheep (Ovis canadensis) population. To understand some of the mechanisms responsible for the progression of the change to a new climax community, the following objective was addressed: estimate the density, percent mortality, and recruitment density of curl-leaf mountain mahogany and Utah juniper on the Bighorn Canyon National Recreation Area across a precipitation gradient. The density of CLMM (34.4-1744.4 plants/ha) and juniper (30.0-454.4 plants/ha) varied by stand type and area. The percent mortality of CLMM differed by stand type (F = 18.06, P < 0.0001) and area (F = 4.11, P = 0.0189), whereas there was an interaction (F = 3.33, P = 0.0122) between stand type and area in the percent mortality of juniper. Recruitment of CLMM ranged from 0 to 86.6 plants/ha and juniper recruitment ranged from 0 to 14.6 plants/ha. There was an interaction between area and stand type (F = 6.91, P < 0.0001) in density of juvenile CLMM. Our results did not definitively show an effect of juniper upon CLMM mortality or recruitment. Relationships varied dependent on stand type, area, and location.