rainfall and are subject to high light intensities; thus, soil water evaporation is 

 high and is probably a limiting factor in plant establishment and survival. This 

 species often occurs on barren, eroded microsites and may depend on the 

 climate that maintains these sites. At some locations, particularly steep slopes, 

 the soils erode during intense rain storms. Because of its cushion habit and 

 dense foliage, soil is protected from water erosion beneath A. barrii plants; 

 therefore, these plants frequently remain perched atop small pedestals of soil 

 (Appendix E-4). 



3. SOIL RELATIONSHIPS: A. barrii sites on the Custer National Forest are 



on the Midway soil series, and the study area distribution may be partially 

 explained by soil properties. These soils are residuum 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 of these soils, at 

 8.0, is quite high (Schassberger pers. measurement 1988; U.S. Department of 

 Agriculture 1971), and the texture is silty to sandy. 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 limits the availability of iron and manganese (Hausenbuiller 

 1972). One or more of these soil characteristics may influence the distribution 

 of this species directly. Ahematively, such characteristics could indirectly 

 affect /i. barrii by restricting the establishment of other plant species, thereby 

 limiting interspecific competition. 



Like A. barrii, the similarly tufted species, Astragalus hyalinus occupies 

 Badlands slopes, but tends to be on coarse sandstone parent material in the 

 study area, rather than on shale or siltstone (Schmoller pers. comm.). In the 

 Pryor Mountains, A. hyalinus is on a wide range of sandy to clay soils (Lesica 

 and Achuff 1992). A. barrii occurs on sandstone in Montana and elsewhere, so 

 that soil texture does not differentiate the habitats of these two species 

 rangewide. This potential distinction in soil relationships has not been 

 examined with rigor in the study area. 



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: theRockyford Ash Member of the Sharps Formation. The Rockyford 

 Ash Member is comprised of a zeolitic volcanic ash; zeolite minerals are 

 ftinctionally similar to vermiculite and montmorillonite (David Ode pers. 

 comm. to Schassberger). 



POPULATION BIOLOGY AND SPECIES BIOLOGY 



1. PHENOLOGY: An early-blooming species. Astragalus barrii flowers from 



24 



