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

Challis milkvetch (Astragalus amblytropis Barneby) is endemic to the d ry, rain-shadow region created by the mountains of west-central Idaho. The objectives of the study were to determine significant autecologic al characteristics, identify significant threats, and provide manageme nt recommendations for this species. This perennial plant is restricte d to deep (> 1 m), relatively porous soils on south- and west-facing s lopes throughout the area mapped as the Challis volcanics. In a two-ye ar study, populations of Challis milkvetch decreased and showed low pe rcent survival of seedlings and of non-reproductive and reproductive i ndividuals. There were sufficient pollinators, and the plants set and dispersed viable seed. However, in the dry and unpredictable climate o f 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 t wo consecutive years of below average precipitation. Anthropogenic dis turbances, such as domestic livestock grazing, off-road vehicle use, m ining, and herbicide spraying appear to be only minor threats to Chall is milkvetch. Agencies should continue to monitor the species to deter mine population trends. Challis milkvetch habitat should be protected because it holds the seed bank for this and many other endemic species .