Photo © Robert K. Moseley CLICK PHOTO FOR A LARGER IMAGE

Astragalus amnis-amissi Barneby

Lost River milkvetch

Fabaceae (Pea; Legume family)

General Description: A slender, low, weakly ascending perennial forb with a taproot and shortly forking above-ground caudex. The thinly pubescent herbage has short, sub-appressed, basafixed hairs pointing in the same direction. Leaves are compound with 7-13 elliptic to broadly egg-shaped, thin-textured leaflets, each 3-15 mm long. Leaflets are medium to dark semi-glossy green in color and generally tipped with an apical notch. The inflorescence is a loose raceme of spreading, small white flowers faintly marked with purple. The calyx is 4-6 mm long and has black or partly white appressed hairs. Fruit pods are sessile, moderately inflated, about 15-17 mm long by 7-8 mm in diameter, green or purplish, and with closely appressed short hairs.

Illustration.

Field Identification Tips: Lost River milkvetch's slender, weakly ascending habit, semi-glossy, relatively dark green, apically notched leaflets, small whitish flowers faintly marked with purple, somewhat inflated pods, and steep limestone cliff and associated talus habitat are good field characteristics.

Phenology: Flowering begins in early to mid-June and continues into July.

Similar Species: Lost River milkvetch is closely related to Astragalus amblytropis (Challis milkvetch), another species endemic to east-central Idaho. Challis milkvetch differs in several characteristics, including its buried root crown; a more divaricately branched habit; thicker-textured, silvery leaflets; dull straw-yellow flowers; and larger (20-35 mm long), more inflated fruit pods. Astragalus alpinus (alpine milkvetch) superficially resembles Lost River milkvetch, but has a slender, subterranean, adventitious root system, and narrower, pendulous, stipitate fruit pods.

Photo © Robert K. Moseley CLICK PHOTO FOR A LARGER IMAGE

Habitat: Ledges, crevices, and other outcrops on steep limestone cliffs, and talus along cliff bases; often in partial shade. Commonly associated with Cercocarpus ledifolius, Petrophytum caespitosum, Draba oreibata, Erigeron caespitosus, Leymus cinereus, and Pseudotsuga menziesii.

Global Distribution: Endemic to east-central Idaho.

Idaho Distribution: Eastern and western slopes of the southern half of the Lost River Range, and the southern end of the Lemhi Range, in Custer and Butte counties.

References:

Moseley, R. K. 1989. Field investigation of four astragali, all Region 4 sensitive species on the Salmon National Forest, with notes on two others. Idaho Department of Fish and Game, Conservation Data Center, Boise, ID. 23 pp. plus appendices.

Barneby, R. C. 1964. Atlas of North American Astragalus. Memoirs of the New York Botanical Garden Vol. 13. New York Botanical Garden, Bronx, NY. 1188 pp.

Spahr, R., L. Armstrong, D. Atwood, and M. Rath. 1991. Threatened, endangered, and sensitive species of the Intermountain Region. U.S. Forest Service, Intermountain Region, Ogden, UT.