Astragalus albens, known as the Cushenbury milkvetch and silvery-white milkvetch, is one of several plants endemic to San Bernardino County and the San Bernardino Mountains in particular. It is an endangered plant, of which there are a known total of about 7,000 plants growing in 30 to 50 distinct locations in the region, generally within the northeast portion of the mountains, with many on the northern slopes of the San Bernardino Mountains near the settlement of Cushenbury.
Located in the woodland and scrub of the slopes between Big Bear in the mountains and Lucerne Valley in the Mojave Desert at the foot of the range, Astragalus albens is described both as an annual and as a short-lived perennial herb or annual herb of the pea family producing a prostrate mat of delicate stems coated densely in silvery hairs. The leaves are generally a few centimeters long and made up of several gray-green oval-shaped leaflets less than a centimeter long each. The inflorescence arises upright from the low patch of foliage and bears up to 14 pealike flowers. Each flower is dark-veined pink to deep purple with a spot of white or light pink in the throat.
The fruit is a legume pod between one and two centimeters long. It is roughly hairy and crescent-shaped, drying to a thick papery texture. The fruits, which attach directly to the base have two locules, are about 0.4 to 0.7 inches long, crescent shaped, three-sided, with densely stiff hairs.
This fruit shape helps to distinguish the Cushenbury milkvetch from Bear Valley milk-vetch (A. leucolobus) which may also grow sympatrically on carbonate soils. It also resembles Mojave milk-vetch (A. mohavensis) from the northern Mojave Desert, but Mojave milk-vetch is not pubescent, as is the Cushenbury milk-vetch.
Cushenbury milk-vetch has been reported from Joshua tree woodland and blackbush scrub communities, but is most commonly found in pinon-juniper woodland. It has been reported growing with dominant species Utah juniper (Juniperus osteosperma), joint fir (Ephedra viridis), paper bag plant (Salazaria mexicana), mountain mahogany (Cercocarpus ledifolius), Mojave yucca (Yucca schidigera), manzanita (Arctostaphylos glauca), flannel bush (Fremontodendron californicum), Great Basin sagebrush (Artemisia tridentata), and needlegrass (Stipa coronata).
The main threat to this species is limestone mining, an industry in one of the areas of the San Bernardino Mountains where the plant proliferates. This form of mining alters the local habitat by physically removing plant life for quarries, road construction, and load dumping. It also produces major changes in the hydrology of the area and releases large amounts of carbonate dust into the air which combines with water and forms a very thin layer of what is essentially cement over the habitat. Most of the populations of this plant are located on sites of active mining or sites which are targeted for mining in the future.
Cushenbury milk-vetch is, with rare exceptions, restricted to carbonate and carbonate-related soils and outcrops from 4,000 feet to 6,600 feet. Its range extends from a ridgetop just east of Dry Canyon to the southeast through Lone Valley, east of Baldwin Lake, to upper Burns Canyon. An unverified population at Box ‘S’ Springs, two to three miles northwest of Cushenbury at 3,600 ft., is its northernmost and lowest reported location.
Cushenbury milk-vetch occurs on U.S. Forest Service lands as well as on Bureau of Land Management lands in Furnace, Bousic, Marble, and Cushenbury Canyons, below Monach Flat and Blackhawk Mountain, at Round Mountain, and at Terrace Springs.
Flowering occurs from late March to mid-June. Pods ripen at least as early as May, and become stiff and papery with long hairs as they mature.
Pollen vectors are most likely small bees, given the flower shape and color. Most Cushenbury milk-vetch reproduction presumably occurs by seed, and seeds have been found to have high viability. Vegetative reproduction has never been reported. Seeds require abrasion. It has long been known that seeds remain dormant in the soil during drought years, but the numbers of viable seeds present in the soil and the length of time they can remain viable is unknown. The extent of seed predation, the numbers and kinds of seed predators, and seed dispersal mechanisms are also unknown.