Peirson's Milk-vetch

Astragalus magdalenae peirsonii

Description

Peirson's milk-vetch (Astragalus magdalenae peirsonii ) is a stout, short-lived perennial reaching 8-27 in (20.3-65.6 cm) in height. The stems and leaves are covered with fine silky hairs, and the leaves are 2-6 in (5-15.2 cm) long, with 3-13 small oblong leaflets. The dull-purple flowers are arranged in 10-to 17-flowered racemes; the resulting pods are 0.8-1.4 in (2-3.6 cm) long, inflated, and have a triangular beak. The variety peirsonii is separated from two other varieties of A. magdalenae based on the number of leaflets, the length of the peduncles, and the length and diameter of the fruits. With a length of 0.2 in (5.1 mm), Peirson's milk-vetch has the largest seeds of any Astragalus in North America.

Habitat

Peirson's milk-vetch grows in the Sonoran Desert on the slopes and hollows of windblown dunes.

Distribution

Peirson's milk-vetch is known from the Borrego Valley in San Diego County and the Algodones Dunes in Imperial County, which extend just south of the International Border into northeastern Baja California. Collections also occur in the Gran Desierto in Sonora, Mexico. The specimens from Sonora were all collected south and southeast of the Sierra Pinacate lava field in the southern Gran Desierto over a 15-year period. Although a 1980 sighting included San Felipe (in central Baja California) within the range of this taxon, no collections of variety peirsonii could be located from that region since then. A report of Peirson's milk-vetch occurring in the dunes west-southwest of the Salton Sea in Imperial County, California, remains unconfirmed.

Within San Diego County, Peirson's milk-vetch has not been seen for several decades. Surveys in 1978 failed to locate the variety in the Borrego Valley, where it was originally collected, and a portion of the dune habitat in Borrego Valley is currently used as a county landfill. A major landowner in the area, the California Department of Parks and Recreation, does not have any information or reports of this taxon occurring in Anza Borrego Desert State Park. The only location where Peirson's milk-vetch could be confirmed in the United States was on the Algodones Dunes, an active dune system located southeast of the Salton Sea and extending south about 1.5 mi (2.4 km) into Baja California. In 1977 a survey of the sensitive plant taxa of the Algodones Dunes showed that Peirson's milk-vetch was distributed in what can be considered one extensive population of scattered colonies spanning the length of the dune system, primarily along its western side.

The Algodones Dunes are a linear dune system, approximately 40 mi (64.4 km) long and 5 mi (8 km) wide, supporting several species of plants and animals that occur only in dune systems in the Sonoran Desert. Managed by the Bureau of Land Management, the Algodones Dunes—also known as the Imperial Sand Dunes Recreation Area—are among the most intensively used off-highway vehicle recreation areas in California's deserts, attracting several hundred thousand off-highway vehicle users each year.

Threats

The primary threat to Peirson's milk-vetch is destruction of individuals and dune habitat from off-highway vehicle use and the recreational development associated with it. Approximately 75% of the Algodones Dune system is open to motorized vehicle use, and between 75-80% of all known colonies of Peirson's milk-vetch in 1977 were within that area. The greatest concentration of colonies was located in the central dunes, within a 4-mi (6.4-km) radius of the southern end of Gecko Road, an area that has since been more fully developed for recreational use. Surveyors in 1977 reported that no seedlings of any of the sensitive plant species, including Peirson's milk-vetch, could be found in areas receiving heavy off-highway vehicle use, and large areas receiving intensive off-highway vehicle use showed virtually a complete loss of all plant cover. By 1990 colonies of mature Peirson's milk-vetch plants could not be located in areas of heavy off-highway vehicle use, and colonies located in areas receiving moderate off-highway vehicle use had lower reproductive success and poorer health than comparable populations located in areas closed to off-highway vehicles.

Conservation and Recovery

Approximately 23,000 acres (9,307.8 hectares; or 18%) of the Algodones Dunes has been closed to motorized vehicle use since 1972. In 1994 most of this closed area and an extension to the north, a total of 32,240 acres (13,047.1 hectares; or about 25%) of the dune system, was designated the North Algodones Dunes Wilderness. The wilderness, a linear section of the northern dunes, is bounded by an area designated for intensive off-highway vehicle use to the north and by Highway 78 and an intensively used off-highway vehicle area to the south. Roughly 20-25% of the known colonies of Peirson's milk-vetch occur in the wilderness area.

While loss of colonies and declines in reproductive success and health of Peirson's milk-vetch have been documented in areas receiving high and moderate levels of off-highway vehicle use, a large area in the central section of the dunes has been designated a "limited use" area under the California Desert Conservation Area Plan. According to this plan, the "limited use" designation is designed to protect sensitive resources, while allowing multiple use. However, Peirson's milk-vetch colonies in these areas may decline if present trends continue. Because the area is on a dune system, the "limited use" designation prohibits the construction of roads or campgrounds within its boundaries but does not include any restriction on off-highway vehicle use in the area.

In 1988 the Bureau of Land Management (BLM) constructed a campground at the south end of Gecko Road, just 0.75 mi (1.2 km) north of the boundary of the "limited use" zone and adjacent to the highest concentration of colonies of Peirson's milk-vetch in the dune system. This region of dunes was also a Wilderness Study Area in the 1970s and 1980s. When the BLM recommended against designating this Wilderness Study Area as wilderness in 1989, it cited four reasons for its recommendations: 1) a long tradition of motor vehicle use; 2) the projected continued demand for off-highway vehicle use; 3) the Wilderness Study Area's potential for energy and mineral development; and 4) the similarity of the area to a nearby Wilderness Study Area recommended for wilderness. While off-highway vehicle use is expected to increase throughout the recreation area, off-highway vehicle use in the former southern Wilderness Study Area is expected to increase faster than the overall rate, tripling from 1985-2000. In addition, these projections from BLM's 1987 Recreation Area Management Plan did not consider the increase in camping that is occurring along the railroad tracks and canal road that bound the central dunes on their east and west sides. Camping in these areas facilitates quick, easy access to the central "limited use" dunes for off-highway vehicle use. Construction of a bridge over the All American Canal in the southern portion of the Algodones Dunes will also increase ease of access to the central dunes and may thereby encourage additional off-highway vehicle use. The trend for habitat conditions of Peirson's milk-vetch in the central "limited use" zone of the dunes is one of continuing decline.

The BLM and the California Department of Fish and Game developed a habitat management plan for the Algodones Dunes in 1987. The plan included a monitoring program to track the effects of the 1988 construction of Roadrunner campground and the subsequent increase in off-highway vehicle use in the central dunes. In the habitat management plan, the BLM also agreed 1) to establish monitoring transects for sensitive plants, including Peirson's milk-vetch, in all land-use classes and 2) to monitor them every other year until trends were established. Little of the monitoring specific to sensitive plant species has been carried out.

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.