INTRODUCTION 



The Pryor Mountain Desert region of southern Carbon County, Montana has one 

 of the most unusual floras in the state (Dorn 1978, Lesica et al. 1984). The arid climate 

 and unusual and varied soils provide desert-like habitats that are otherwise unknown in 

 Montana. Furthermore, this region lies at the north end of the Bighorn Basin, a broad 

 trough that extends south to the Red Desert of southern Wyoming. The Bighorn Basin 

 has undoubtedly been a migrational pathway for desert plants (Dorn 1977). For these 

 reasons, numerous species of desert plants reach the northern limit of their range in the 

 Pryor Mountain Desert (Lichvar et al. 1985). Many of these species occur nowhere else 

 in the state. In addition, at least four species are endemic to limestone-derived 

 substrates in this region. 



The Montana Natural Heritage Program lists 24 species of special concern 

 occurring in the Pryor Mountain Desert. In addition, there are 11 species of limited 

 distribution found in the area (Lesica et al. 1984, Lesica and Shelly 1991). Much of the 

 desert below 1980 m (6500 ft) is public land administered by the Bureau of Land 

 Management. Many populations of rare plants occur on these public lands. Although 

 the presence of these plant species in the area has been documented, the size and 

 number of populations has never been assessed. It is necessary to determine which 

 species are truly rare and which are locally common in order to afford protection to 

 those species that are most sensitive. The purpose of this study is to survey BLM lands 

 in the Pryor Mountain Desert and document the presence and size of rare plant 

 populations. 



THE STUDY AREA 



The Pryor Mountain Desert lies at the north end of the Bighorn Basin in south- 

 central Montana and adjacent Wyoming (Figure 1). It is bordered on the north by the 

 Pryor Mountains, a northwest extension of the Bighorn Mountain Range. The Prj'ors are 

 a large dome-shaped range rising to nearly 2740 m (9000 ft), composed of Paleozoic 

 sedimentary formations, principally Madison limestone. Numerous canyons incised in the 

 limestone occur on the south slope of the range. The east boundary of the Pryor 

 Mountain Desert is formed by the Bighorn River which has formed a spectacular canyon 

 in the sedimentary formations of the Bighorn-Pryor uplift. The slopes of the Bighorn 

 Mountains rise abruptly on the east side of the River. This portion of the river has been 

 impounded, and it and a strip of land 2-6 miles wide on the west side comprise Bighorn 

 Canyon National Recreation Area administered by the U. S. Park Service. The valley of 

 the Clarks Fork of the Yellowstone River forms the west boundary of the Pryor 

 Mountain Desert. Just to the west of the Clarks Fork is the east slope of the Bearlooth 

 Mountains which rise to over 3800 m (12,500 ft). The Pryor Mountain Desert lies in the 

 rain shadow produced by this massive uplift. The southern border of the region is 

 formed by the valley of the Shoshone River in Wyoming. 



