Habitat and life history characteristics of Astragalus barrii (Fabaceae) in the South Dakota badlands

Dingman, Sandra

Astragalus barrii, Barr’s milkvetch, is a rare plant species endemic to the dynamic badlands of South Dakota, Wyoming, and Montana and it is one of a distinct group of Astragalus species, known as the Orophaca group, that are regionally endemic. A study was undertaken in Badlands National Park, located in southwestern South Dakota, to develop predictive habitat models, describe the physical habitat characteristics, and study the life history of A. barrii to aid local conservation efforts and to further understanding of the Orophaca Astragali. Deductive habitat similarity modeling, classification based on discriminant analysis, and probabilistic modeling based on logistic regression were used to predict the occurrence of A. barrii in Badlands National Park based on soils, vegetation, slope, and geology. Presence of A. barrii was associated with simple vegetation shapes, low shade levels, complex soil shapes, silty clay soil textures, simple slope shapes, and a range of slopes. Presence was also observed to coincide with the Chadron and Brule formations as well as the presence of coarse substrate materials, such as chalcedony cobble and limestone gravel. Plants were found in both the Cheyenne and White River watersheds but no plants were found above the Badlands Wall, thus indicating that topography is a major regional influence on seed dispersal with the Badlands Wall serving as an effective barrier to colonization. Populations were varied in their number of plants, mean plant size, mean density, and proportion flowering. Almost all plants flowered and all plants greater than 177 cm2 surface area flowered. The smallest plants showed the largest percent growth and small plants were much more common than large plants. Flowering resulted in viable seed that contributes to population growth. Recruitment was low at 4% while plant mortality was relatively high at 16%. Findings of this study indicate that A. barrii is an edaphic specialist specialized to grow in dynamic badlands habitats. Results contribute directly to the design of future studies and conservation of A. barrii and its habitat. Results also have implications for understanding speciation and adaptation in the Orophaca Astragali and meta-population dynamics.

Habitat and life history characteristics of Astragalus barrii (Fabaceae) in the South Dakota badlands

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