protect ecological values and biological diversity. There are 

 six goals to our studies: 



(1) conduct a thorough floristic inventory, 



(2) map locations of putatively rare plant populations to 

 determine which are truly rare in the area and the habitat 

 requirements of truly rare species 



(3) develop a formal community classification system and provide 

 descriptions of these communities based on field samples 



(4) use the classification system to determine v,'hich communities 

 are endemic or rare in Montana or the region 



(5) develop a vegetation map for the study area 



(6) use maps of rare species and plant communities to delineate 

 areas critical for protecting the biological values of the area. 



We completed (1) and (2) in 1991 with a floristic survey of the 

 study area, mapping of target species locations, and habitat 

 descriptions (Lesica and Achuff 1992) . This paper presents a 

 classification system and descriptions for community types 

 (vegetation types) for public lands administered by the BLM in 

 the Pryor Mountains and the Bighorn Basin of Montana. In 

 addition, v/e evaluate the state and global distribution of these 

 communities. Goals (5) and (6) will be met in subsequent years 

 as funding becomes available. 



THE STUDY AREA 



The Pryor Mountain Desert lies at the north end of the 

 Bighorn Basin in south-central Montana and adjacent Wyoming. It 

 is bordered on the north by the Pryor Mountains, a northwest 

 extension of the Bighorn Mountain Range. The Pryors are a large 

 dome-shaped range rising to nearly 9,000 feet, 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 to 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 Beartooth Mountains which rise to over 

 12,500 feet The Pryor Mountain region 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. 



Our study area consists of all lands administered by the BLM 

 in the Pryor Mountains and that portion of the Pryor Mountain 

 Desert north of the Wyoming border. Elevations are ca. 4,000 



