13 



4. Physiographic and topographic characteristics: 



In Montana, populations of A. barrii are most 

 often found on eroding knolls, buttes, and 

 barren hilltops of a particular calcareous, 

 soft shale and siltstone complex. These sites 

 range from level areas to steep slopes (0- 

 35%) . Most sites occur on slopes of 

 intermediate steepness (3-8%) . All slope 

 aspects are represented; however, many of the 

 sites are on north- or northeast-facing slopes. 

 Populations occur from 3,140 - 4,000 feet in 

 elevation. 



Many of the populations are located along 

 rivers or streams, where downcutting has 

 exposed shale and siltstone outcrops. 



5. Edaphic factors: In Montana, A. barrii sites 

 on the Custer National Forest are on the Midway 

 soil series. These soils are calcareous 

 residumm from calcareous, platy, soft shale 

 parent material, in which the clay is mostly a 

 montmorillonitic mineral type (U.S. Department 

 of Agriculture 1971). The pH (8.0) of these 

 soils is high (Schassberger , pers. measurement 

 1988, U.S. Department of Agriculture 1971), and 

 the texture is silty to sandy. 



In South Dakota, within the White River 

 drainage, A. barrii appears to be restricted to 

 barren outcrops or erosional remnants of one 

 specific geologic unit - the Rockyford Ash 

 Member of the Sharps Formation. The Rockyford 

 Ash Member is comprised of a zeolitic volcanic 

 ash; zeolite minerals are functionally similar 

 to vermiculite and montmorillonite (David Ode, 

 S. Dakota Natural Heritage Program, pers. 

 comm. ) . 



The distribution of A. barrii may partially be 

 determined by soil properties. Soils that have 

 a high content of expandable clay minerals 

 (zeolite, montmorillonite) have a high cation 

 exchange capacity, and hold water longer. 

 However, the high pH of the soils in Montana 

 would limit the availability of iron and 

 manganese (Hausenbuiller 1972) . One or more of 

 these aspects of the soil may influence the 

 distribution of this species directly. 

 Alternatively, these soil properties might 

 indirectly affect A. barrii by restricting the 



