Braunton's Milk-vetch

Astragalus brauntonii

Description

Braunton's milk-vetch, Astragalus brauntonii, is a robust, short-lived perennial in the pea family (Fabaceae or Leguminosae). It is one of the tallest members of the genus, reaching a height of 60 in (1.5 m) and is covered with woolly hairs. A thick taproot and woody basal stem give forth several stems. The leaves are 1.5-6.5 in (3-16.5 cm) long and are pinnately compound with 25 to 33 oblong-ovate, abruptly pointed leaflets. The light purple flowers are clustered in 35-to 60-flowered racemes 1.5-5.5 in (3.8-14 cm) long. The beaked, slightly curved pods are oblong-ovoid and 2.5-3.5 in (6-8.8 cm) long. A. brauntonii is readily distinguished from the only other perennial species of Astragalus by having two-chambered rather than one-chambered pods.

Astragalus brauntonii was first collected in 1901 by Ernest Braunton near Sherman (now called West Hollywood) in Los Angeles County. Samuel B. Parish described it two years later as A. brauntonii. Per Axel Rydberg published the name Brachyphragma brauntonii in his 1929 revision of the genus; however, this name was not recognized by most botanists. Rupert Barneby recognized the name A. brauntonii in 1964 in his Atlas of North American Astragalus. A. brauntonii is also included in the current edition of The Jepson Manual, published in 1993.

Habitat

Braunton's milk-vetch is associated with the fire-dependent chaparral habitat dominated by chamise (Adenostoma fasciculatum ), yucca (Yucca whipplei ), and the rare Tecate cypress (Cupressus forbesii ).

Braunton's milk-vetch is considered a limestone endemic; the only populations not found on limestone are: on down-wash sites caused by seed drift following a fire event, an occurrence along the edge of a fire road in Monrovia, and at a location in Chino Hills where the substrate type is unknown. Surveys for Braunton's milk-vetch during post-fire floristic inventories within its known distribution on substrates other than limestone have not yet found it in non-limestone soils. Limestone outcrops are extremely rare within the limits of the known distribution of Braunton's milk-vetch.

Fire is a natural requirement for the survival of this species. The natural frequency of fire in the habitat of Braunton's milk-vetch is unknown, but estimates range between 20 to over 100 years with an average of 70-year intervals. Higher fire frequencies have resulted from increasing human populations in southern California, mostly in the form of arson-caused fires. This species has a life span of 2-3 years, and a given population is visible only once in 20 to 50 or more years, depending on the intervals between fires.

Distribution

Braunton's milk-vetch is currently known from four general areas in Ventura, Los Angeles, and Orange counties. One population is found along the south slope of the Simi Hills of eastern Ventura and western Los Angeles counties. Two occurrences in one population are known from Santa Ynez Canyon in the Santa Monica Mountains, Los Angeles County, which probably represents the original type locality. Two occurrences in one population are known from Coal and Gypsum Canyons in the Santa Ana Mountains, Orange County. Eight individuals were reported during the preparation of the Cloverleaf Canyon Specific Plan for the area in 1983, near where historical collections were made south of Clamshell Canyon, north of Monrovia in Los Angeles County.

Because reproduction of Braunton's milk-vetch is stimulated by fire events, the total number of individuals varies with current fire cycles. The largest known population ever recorded was an Orange County population of approximately 400 individuals three years after a 1982 fire. Recent researchers have not been able to find any plants here. Endangered plant specialists from the Rancho Santa Ana Botanic Garden surveyed nearby habitat for Braunton's milk-vetch in the Cleveland National Forest but found no plants. The remaining populations contain no more than approximately 20 to 30 individuals each, while the current total number of individuals is estimated to be fewer than 100. The seed bank for Braunton's milk-vetch may have the capability of generating approximately 1,000 individuals in four highly subdivided populations.

Most of the habitat of Braunton's milk-vetch is on private land in areas with expanding development. Four public agencies have small colonies that may not be viable within their jurisdictions: the California Department of Parks and Recreation (DPR), the Conejo Open Space Conservation Agency, the Rancho Simi Parks and Recreation District, and the National Park Service. All of the protected habitat occurs in the immediate vicinity of urban development.

Threats

Braunton's milk-vetch is threatened by direct habitat loss from urban development, the reduced ability of fragmented habitat to sustain normal ecological processes, multiple ownership of scattered individual populations that result in different landscape treatments, alteration in fire cycles, and extinction from naturally occurring events due to small population sizes and low numbers of individuals.

Steep terrain typifies the habitat of Braunton's milk vetch and, until the recent increase of urban sprawl, the species has remained relatively secure. Now that the majority of flat lands have been developed, several populations occupying rugged terrain have been destroyed by urban development. So extensive has this urbanization been that all of the population areas for this plant—Simi Hills, Topanga State Park, Monrovia, and the Santa Ana Mountains—have experienced habitat destruction, and what habitat remains is threatened by modification of natural ecological processes.

One Braunton's milk-vetch colony at Monrovia has been extirpated within the last 15 years, while colonies at Santa Ynez Canyon and Simi Hills have incurred significant losses through development. The Coal Canyon location has also been approved for development by the City of Anaheim; in fact, there are no known populations that are not facing primary or secondary threats to survival. Only a small portion of the Santa Ynez Canyon population, most of which was destroyed by previously approved development, occurs on public lands (DPR), and a portion of the population was bulldozed during fire suppression activities in the Old Topanga fire of 1993. The City of Anaheim has approved a development that will eliminate 50% of the population of Braunton's milk-vetch in the Santa Ana Mountains. The County of Ventura has approved a development with attendant mitigation efforts that will eliminate a portion of the habitat for Braunton's milk-vetch in the Simi Hills. These proposed mitigations, strictly experimental, involve establishing a rare-plant reserve for Braunton's milk-vetch on a highly altered water tank site that has little natural habitat and no limestone substrate. Limestone soils will be scraped from the destroyed site and placed on the reserve. Because the small reserve is bordered by development, it is unlikely that prescribed periodic fires will be used as a management tool. It is highly doubtful that these measures will eventually support viable populations of Braunton's milk-vetch; to compound the problem, there is no contingency plan in place should these measures fail.

The use of prescribed fire as a habitat management tool for Braunton's milk-vetch anywhere in its range will be difficult because approved development is situated extremely close to "protected" populations of the plant.

Overutilization is probably not applicable to Braunton's milk-vetch, although its large stature and striking appearance may make it vulnerable to casual collection, particularly along firebreaks adjacent to areas used for recreational activities.

Conservation and Recovery

Most of the critical habitat of the Braunton's milk-vetch is on private land in areas with expanding residential development. Several public agencies have small populations on lands within their jurisdiction: the California Department of Parks and Recreation, the Conejo Open Space Conservation Agency, the Rancho Simi Parks and Recreation District, the National Park Service, and the National Forest Service. All of these publicly owned habitats should be protected and managed to benefit the Braunton's milk-vetch. It is crucial, however, that the best critical habitats on private land are identified and protected. This could be done by acquiring the land and designating ecological reserves, or by negotiating conservation easements with the landowners. The habitat will have to be managed to maintain its suitability for the rare milk-vetch, primarily by extending the fire frequency. The populations of the rare plant must be monitored, and research undertaken into its basic biology and ecological requirements, particularly with regards to fire ecology.

Contacts

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/

U. S. Fish and Wildlife Service
Ventura Field Office
2493 Portola Road, Suite B
Ventura, California 93003-7726
Telephone: (805) 644-1766
Fax: (805) 644-3458

Reference

U. S. Fish and Wildlife Service. 29 January 1997. "Endangered and Threatened Wildlife and Plants: Determination of Endangered Status for Two Plants and Threatened Status for Four Plants From Southern California." Federal Register 62 (19):4172-4183.