14 



establishment of other plant species, which 

 would in turn limit interspecific competition. 



6. Dependence of this taxon on natural 

 disturbance: This species often occurs on 

 barren eroded microsites, and may depend on the 

 climate that maintains these sites in this 

 condition. 



7. Other unusual physical features: At some 

 locations, especially on steep slopes, the 

 soils erode during intense rain storms. 

 Because of its cushion habit and dense foliage, 

 soil is maintained beneath A. barrii plants, 

 with the result that the plants frequently 

 remain perched atop small pillars of soil. 



C. Biological characteristics. 



1. Vegetation physiognomy and community structure: 



In Montana, A. barrii occurs in silty to sandy, 

 calcareous, clay soils. When an upper 

 overstory is present, it usually consists of a 

 sparse cover of Pinus ponderosa (ponderosa 

 pine) and Juniperus scopulorum (Rocky Mountain 

 juniper) . Many of the sites only have a shrub 

 overstory, which is dominated by Artemisia 

 tridentata (big sagebrush) and/or Atriplex 

 confertifolia (shadscale) . Vegetative cover of 

 grasses and forbs at these sites is low, with 

 few dominant species. 



2. Regional vegetation types: All but one of the 

 sites in southeastern Montana occur in the 

 Clayey and Shallow ciay Range Site Association 

 according to Ross and Hunter (1976) . This 

 association is described as occurring in the 

 10-14 inch precipitation zone, and list as 

 dominants: western and thickspike wheatgrass, 

 green needlegrass, little bluestem, bluebunch 

 wheatgrass, prairie junegrass, native legumes, 

 big sagebrush, Nuttall saltbush, and winterfat. 



3. Frequently associated species: In Montana, 

 associated species include: 



Agropyron spicatum (bluebunch wheatgrass) 



Allium textile (textile onion) 



Andropoqon scoparius (little bluestem) 



Artemisia f rigida (fringed sagewort) 



Artemisia tridentata (big sagebrush) 



