Triple-Ribbed Milk-vetch

Astragalus tricarinatus

Description

Triple-ribbed milk-vetch, Astragalus tricarinatus, is a short-lived erect perennial, reaching 2-10 in (5-25 cm) in height. Leaves are 1.3-2.7 in (3.3-6.8 cm) long, with 17 to 20 leaflets that are silvery strigose on the upper surface. The flowers are white or pale cream-colored, arranged in loose 6-to 17-flowered racemes. The fruit is narrow, 0.8-1.6 in (2-4 cm) long, glabrous and distinctly three-ribbed.

Habitat

Triple-ribbed milk-vetch grows in sandy and gravelly soils in dry washes, at the base of canyon slopes, and on steep scree slopes of decomposed granite. Although A. tricarinatus is a short-lived perennial, its numbers fluctuate significantly from year to year and the species may not be present above-ground in drought years.

Distribution

Historically, the range of triple-ribbed milk-vetch extends from Morongo and Whitewater Pass, located at the north end of the Coachella Valley, south to the Orocopia Mountains. During the last 2 decades, triple-ribbed milk-vetch has been located in four areas—in the north at Big Morongo Canyon and its tributary canyons, at two nearby locations at Whitewater Canyon and Mission Creek, and at a disjunct location about 40 mi (64 km) to the south in Agua Alta Canyon.

The occurrence of triple-ribbed milk-vetch in Agua Alta Canyon was discovered in 1985 and consisted of only one plant. The taxon had not been seen during previous explorations of this canyon wash nor has it been seen since, although the site was searched the following two years. In the north, Whitewater Canyon is the type locality for triple-ribbed milk-vetch and specimens were collected there in the 1940s, 1960s and mid-1980s. A search of the east ridge of Whitewater Canyon over several days in 1995 failed to locate a population there, although a single immature plant was discovered in alluvial sands from the wash. The Mission Creek occurrence is also known from only one plant, discovered during 1995 surveys for this taxon. Although triple-ribbed milk-vetch has the potential to occur in other canyons within its range, populations of greater than one plant are currently known only from Big Morongo Canyon and may occur at Whitewater Canyon.

Triple-ribbed milk-vetch at Big Morongo Canyon is within the Big Morongo Preserve, managed by the Bureau of Land Management. In 1984, one site in Big Morongo Canyon that supported fewer than 10 plants was bulldozed during maintenance for a gas pipeline. No plants have been found at that site since 1984, although searches were conducted in 1987, 1992, and 1994. Triple-ribbed milk-vetch also occurs 2 mi (3 km) farther down Big Morongo Canyon and within the mouths of two tributary canyons. In 1992, botanists surveyed this region and counted 70 plants in 5 groupings scattered along a 1-2 mi (1.5-3 km) stretch of canyon floor. In 1993, 33 plants were counted along this same stretch and in 1994, a total of 20 plants in five patches were found there.

Threats

Triple-ribbed milk-vetch is threatened by maintenance activities for the crude oil pipeline which runs through its habitat at Big Morongo Canyon and by vehicle use in the canyons. Its limited number of individuals make it especially vulnerable to unanticipated events, such as pipeline leaks, breaks, or emergency repairs.

Triple-ribbed milk-vetch is known to be extant only in Big Morongo Canyon. This canyon bottom has been disturbed by pipeline maintenance activities several times in the last decade and these activities are likely to continue. One occurrence of fewer than 10 triple-ribbed milk-vetch plants at the north end of the canyon was graded during maintenance of a gas pipeline access road in 1985 and has not been seen since, despite searches. In 1995, the Four Corner's Pipeline Company excavated and realigned three segments of a crude oil pipeline that extended through habitat for triple-ribbed milk-vetch in Big Morongo Canyon and had been exposed by streambed scouring. One section of the realignment extended through a site that had supported 20 triple-ribbed milk-vetch plants in 1992. Plants present at the time of construction were shielded from the construction zone by protective fencing, and the topsoil scraped from the site was stockpiled and later replaced. However, the project, originally scheduled for October 1994, was not carried out until April 1995, the period when plants are flowering but before fruits have matured. Any damage to plants during this period would have resulted in diminished seed production by the population that year. Triple-ribbed milk-vetch population sizes fluctuate widely from year to year and may depend on the persistence of a soil seedbank during years when weather limitations are unfavorable for growth or reproduction. Due to poor growing conditions for this taxon throughout the Canyon in 1996, the effect of this pipeline realignment on triple-ribbed milk-vetch in Big Morongo Canyon has not yet been determined.

Conservation and Recovery

Existing regulatory mechanisms that may provide some protection for this taxon include: (1) the California Endangered Species Act, (2) the California Environmental Quality Act, (3) the Federal Endangered Species Act, in those cases where these species occur in habitat occupied by other listed species, (4) the Clean Water Act, (5) the Federal Land Policy and Management Act, and (6) regional planning efforts.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Reference

U. S. Fish and Wildlife Service. 6 October 1998. "Determination of Endangered or Threatened Status for Five Desert Milk-vetch Taxa From California." Federal Register 63 (193): 53596-53615.