THE AUTECOLOGY OF CHALLIS-MILKVETCH, AN ENDEMIC OF EAST-CENTRAL IDAHO

Challis milkvetch (Astragalus amblytropis Barneby) is endemic to the dry, rain-shadow region created by the mountains of west-central Idaho. The objectives of the study were to determine significant autecological characteristics, identify significant threats, and provide management recommendations for this species. This perennial plant is restricted to deep (> 1 m), relatively porous soils on south- and west-facing slopes throughout the area mapped as the Challis volcanics. In a two-year study, populations of Challis milkvetch decreased and showed low percent survival of seedlings and of non-reproductive and reproductive individuals. There were sufficient pollinators, and the plants set and dispersed viable seed. However, in the dry and unpredictable climate of east-central Idaho, establishment of Challis milkvetch seedlings is dependent on rapid growth of the taproot. Establishment of new Challis milkvetch plants and population size both changed dramatically over two consecutive years of below average precipitation. Anthropogenic disturbances, such as domestic livestock grazing, off-road vehicle use, mining, and herbicide spraying appear to be only minor threats to Challis milkvetch. Agencies should continue to monitor the species to determine population trends. Challis milkvetch habitat should be protected because it holds the seed bank for this and many other endemic species.