Astragalus cremnophylax var. cremnoplylax
|Listed||December 5, 1990|
|Description||Low, mat-forming perennial with short, compound leaves and small, white or pale purple flowers.|
|Threats||Trampling by humans.|
Sentry milk-vetch, or Astragalus cremnophylax var. cremnoplylax, is a small, mat-forming perennial in the pea family. Plants grow from a thick tap root to a height of about 1 in (2.5 cm) and form mats up to 10 in (25 cm) across. The short stems have compound leaves, less than 0.5 in (1.3 cm) long, composed of from five to nine tiny leaflets. Minute white or pale purple flowers (0.2 in-0.5 cm) bloom in late April or early May. The fruit is egg-shaped and densely hairy; seeds set in late May and June.
This species is restricted to a single site on the south rim of Grand Canyon National Park. It grows in shallow soils in crevices and depressions on Kaibab limestone, preferring sunny, well-drained soils or limestone surfaces. Sentry milk-vetch and rock-mat (Petrophytum caespitosum ) are the dominant species in the dwarf plant community there.
Sentry milk-vetch was discovered m 1903 but misidentified at the time. It was rediscovered in 1947, west of E1 Tovar, Grand Canyon National Park, and described as a new species the following year. In 1979 a closely related form was described from specimens collected on Buckskin Mountain in Arizona. It was given the name A. cremnophylax var. myriorraphis. Sentry milk-vetch then became known as A. cremnophylax var. cremnophylax. The species has not been found at any other location.
Sentry milk-vetch is known only from the single population on the south rim of the Grand Canyon. Extensive searches of nearby areas have failed to find other populations. An inventory of plants in 1988 counted 489 plants; in 1989 the number had declined to about 440.
The main threat to sentry milk-vetch is the trampling of plants by visitors to Grand Canyon National Park. The site is near the rim and accessible by car or on foot. In the past sentry milk-vetch mats have endured considerable foot traffic. The 1988 survey determined that 65% of all plants had suffered trampling and that more than half had been severely damaged. Between May 1989 and May 1990, plants in the most heavily visited area suffered a 63% decline. Trampling not only destroys seedlings, but reduces the vigor of mature plants, leading to decreased flower and seed production.
Conservation and Recovery
Monitoring of the site will continue in order to establish whether the sole remaining population is self-sustaining. The National Park Service has rerouted pedestrian traffic, which has lessened, but not altogether eliminated, damage to the plants.
Ecological Services Field Office
3616 W. Thomas Rd., Suite 6
Phoenix, Arizona 85019
McDougall, W. B. 1964. Grand Canyon Wildflowers. The Museum of Northern Arizona, Flagstaff.
O'Brien, S. 1984. "Status of Astragalus cremnophylax and Recommendations to Protect It." Unpublished report to Grand Canyon National Park.
Phillips, A. M., III, et al. "Status Report, Astragalus cremnophylax. Barneby." U. S. Fish and Wildlife Service, Albuquerque.