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Mountain big sagebrush (Artemisia tridentata subsp. vaseyana) in southwest Idaho suffered extensive overwinter foliagemortality during 1976-77 where the normally deep snow cover was lacking. Mortality was 75 to 100% in areas where snow usually covers dense stands of sagebrush; however, winterkill was slight in areas of usually shallow snow cover. Winter-induced physiologic drought caused by frozen soils, low soil water content, and above average air temperature was the apparent cause of sagebrush foliage mortality. In the spring of 1977, extensive foliage mortality of mountain big sagebrush (Artemisia tridentata subsp. vaseyana) was evident in the mountainous terrain of southwestern Idaho above the 1700 m elevation. The affected area is part of the Owyhee Plateau (Tisdale et al. 1969), which includes parts of Idaho, Oregon, and Nevada. Infestations of aroga moth (Aroga websteri) (Gates l964), longtailed voles (Microtus longicaudus latus) (Frischknecht and Baker 1972), and gall midges (Dipteru cecidomyiidae) (Jones 1971) have been documented as causes of sagebrush kills. Upon careful examination of the affected areas, infestation by insects or rodents was considered unlikely. Other investigators have attributed kills of big sagebrush to severe droughts (Allred 1941, Ellison and Woolfolk 1937, Ferguson 1964, Pechanec et al. 1937, and Reed and Peterson 1961), even though studies have shown that big sagebrush can survive during long periods of low soil water (Branson et al. 1970, 1976; Branson and Shown 1975). Winterkill of big sagebrush has seldom, if ever, been reported in the literature. However, winterkill has been associated with the mortality of other shrub species, such as fourwing saltbush (Atriplex canescens) (Van Epps 1975), bitterbrush (Purshia tridentata) (Jensen and Urness 1979) and snowbrush (Ceanothus velutinus) (Stickney 1965). These species are found with big sagebrush in southwest Idaho, which suggests that winterkill was a possible cause of mountain big sagebrush mortality during 1976-77. We also noted extensive top kill of snowberry (Symphoricarpos oreophilus) associated with areas of sagebrush mortality. This study was conducted to determine the extent of mountain big sagebrush overwinter foliage mortality and its relationship to winter drought. Study Area and Procedures The study area included 2710 ha in the southeast part of the Reynolds Creek Experimental Watershed in southwest Idaho (Fig. 1). This watershed was selected for hydrologic studies in 1960 to Authors are agricultural engineer, research hydraulic engineer, and range scientist. Northwest Watershed Research Center, U.S. Dep. Agr., Agr. Res. Serv., Suite 116, Boise, Idaho 83705. This research is a contribution from the Northwest Watershed Research Center, U.S. Dep. Agr,, Agr. Res. Serv., Bureau of Land MaFagement, U.S. Dep. Interior; and Idaho Agrndtural Experiment Station, cooperatmg. Manuscript received June 30, 1980. 142 Fig. 1. Reynolds Creek Experimenial Watershed. represent extensive tangeland areas in Idaho and surrounding states (Robins et al. 1965). The study area ranges in elevation from about 1490 m to 2208 m. The four Soil Associations in the study area are Bakeoven-Reywat-Babbington, Harmehl-GabicaDemast, Searla-Bullrey, and Takeuchi-Kanlee-Ola (Stephenson 1977). The major plant species of the study area include: Idaho fescue (Festuca idahoensis), Sandberg bluegrass (Poa sandbergii), bearded bluebunch wheatgrass (Agropyron spicatum), needleand-thread (Stipa comata), mountain brome (Bromus marginatus), squirreltail (Sitanion hystrix), sedges (Carex spp.), mountain big sagebrush (Artemisia tridentata subsp. vaseyana), low sagebrush (A. arbuscula), Douglas fir (Pseudotsuga menziesii), quaking aspen (Populus tremuloides). and snowberry (Symphoricarpos oreophilus). In this study, we did not survey the low sagebrush (A. arbuscula) communities because this species was not killed in 1976-77. Line transects were established on each vegetation mapping unit, as described by Stephenson (1977). During July and August JOURNAL OF RANGE MANAGEMENT 35(2), March 1982 1977, we assessed any mountain big sagebrush plants if 20% or more of their canopy was intercepted by the transect lines. The percentage of dead foliage was visually estimated, and each plant was classified into one of the following categories: (1) no dead foliage due to winterkill (normal); (2) <25% dead foliage; (3) 25 to 50% dead foliage; (4) 50 to 75% dead foliage, and; (5)>75% dead foliage. Surveyed plants were further classified as being less than or greater than 30 cm high. Plants or plant canopy that had died before the 1976-77 winter season were not included in the survey. Total herbage production, including annual production of sagebrush plants, was measured on two plots at the Upper Sheep Creek study site and two plots at the Reynolds Mountain study site (Fig.

Foliage Mortality of Mountain Big Sagebrush [Artemisia tridentata subsp. vaseyana] in Southwestern Idaho during the Winter of 1976-77