Clara Hunt's Milk-vetch

Astragalus clarianus

Description

Willis Jepson first described Astragalus clarianus (Clara Hunt's milk-vetch) in 1909 from specimens collected by Clara Hunt in the Conn Valley near St. Helena in Napa County, California. Axel Rydberg treated this taxon as Hamosa clariana in 1929, then Jepson treated it as A. rattani var. clarianus in 1936. Rupert Barneby reestablished A. clarianus as a full species in 1950, a treatment retained in 1993.

Clara Hunt's milk-vetch, A. clarianus, a low-growing annual herb in the pea family (Fabaceae), is a slender and sparsely leafed plant, sparingly covered with sharp, stiff, and appressed hairs. The simple single or few basally branching stems ascend 3-8 in (7.6-20.3 cm) in height. The leaves are alternate, 0.5-2.5 in (1.3-6.4 cm) long, with five to nine un-crowded leaflets 0.1-0.4 in (0.25-1.0 cm) long. The leaflets are oblong to obovate, narrow at the base, and notched at the tip. Small flowers appear from March through April. The petals are bicolored, with the wings whitish and the banner and keel purple in the upper third. The keel is longer and wider than the wings. The horizontal to declining seed pods are narrow, linear, slightly curved, pointed at both ends, and are borne on slender stalks that are 0.06-0.10 in (1.5-2.5 mm) long.

A. rattani var. jepsonianus resembles A. clarianus, but it grows 4-14 in (10.2-35.6 cm) in height, has larger flowers, and seed pods that are not elevated on a stalk.

Habitat

Clara Hunt's milk-vetch is found on thin, rocky clay soils derived from volcanic or serpentine substrates in grasslands and openings in whiteleaf manzanita (Arctostaphylos manzanita ) blue oak (Quercus douglasii ) woodlands over an elevation range of 240-840 ft (73-256 m).

Distribution

Six historical occurrences of Clara Hunt's milk-vetch were known from Napa and Sonoma Counties, two of which were extirpated by urbanization and viticulture. Of the remaining occurrences, three are found in northwestern Napa County and one occurs in adjacent Sonoma County. These four disjunct populations are restricted to about 70 acres (28 hectares) on private, municipal, and state land.

Threats

Clara Hunt's milk-vetch, whose four extant populations combined would occupy only a 1-acre area (0.4-hectare), is variously threatened by habitat destruction resulting from urbanization, a proposed water storage project, recreational activities, an approved subdivision, airport maintenance, elimination due to plant community succession, and competition from invasive weeds.

One population in Napa County was reduced in size during the 1950s when the creation of Lake Hennessey inundated much of the site. The City of Napa owns Lake Hennessey, uses it as a water source, and conducted a feasibility study in 1993 on increasing the height of the dam wall as part of a water storage enhancement. This proposed project would have raised the lake level and submerged the remnant population of Clara Hunt's milk-vetch but is currently considered too costly to implement. However, any future water storage project that would involve increasing the height of the dam and raising the level of Lake Hennessey would constitute a major threat to the population that lies along the lakeshore. This remnant population was nearly destroyed in December of 1990 when dredge spoils from the lake were placed on top of it. The City of Napa, in cooperation with California Department of Fish and Game, removed most of the dredge spoils, fenced the 2-acre (0.8-hectare) area, and placed a gate in the fence for fishing access to the lake. Ground disturbance caused by dredge spoil removal resulted in proliferation of invasive weeds that further threaten the site. The population has not recovered well. Eight plants were counted at this site in 1991, 325 plants in 1992, 156 plants in 1993, nine plants in 1994, and 15 plants in 1996. The area remains a favorite fishing access to the lake and receives significant use by the public. The City of Napa has repaired damage to the fence several times.

Another Clara Hunt's milk-vetch population occurs in Bothe-Napa Valley State Park; on a 2-acre (0.8-hectare) monitoring site here eight plants were reported in 1988, 220 plants in 1992, 101 plants in 1993, and 39 plants in 1996. The larger portion of this population occurs sparsely in an area of 15 acres (6 hectares) outside of the monitoring zone. This area has been partially protected by placing brush piles next to a foot trail to divert people away from the population. The general plan for the park indicates a campground to be placed over the larger portion of Clara Hunt's milk-vetch, but this is not considered an imminent threat because of lack of funding and possible revisions to the park plan. At present, no specific plans to develop a campground have been made. Although the campground development may be relocated away from the population of Clara Hunt's milk-vetch, the increased recreational use from an additional campground in this park constitutes a potential threat.

The third population occurs near the City of Santa Rosa in eastern Sonoma County. This population, estimated at 2,100 plants in 1996, is scattered over 15 acres (6 hectares) and appears stable at the present time. It is on private land under a voluntary protection agreement with The Nature Conservancy. Upslope and adjacent to this population is the approved Saddle Mountain subdivision of 1,350 acres (540 hectares). Soil erosion from proposed road and pad construction for house lots potentially threatens this population. Construction of this development had not yet begun as of 1996.

The fourth population consisted of 2,238 plants in 1993 scattered over less than 5 acres (2 hectares) of private land. Feral pigs uprooted a substantial number of plants during 1994; the number of plants at this site had declined in 1995-96, although this decline may be attributable to factors other than damage by pigs.

The population of Clara Hunt's milk-vetch located along the north shore of Lake Hennessey has an infestation of the invasive and dominating yellow starthistle (Centaurea solstitialis ). This infestation was a direct result of ground disturbance associated with the removal of dredge spoils that were placed on top of this population. Competition from this alien annual weed is also considered a threat to the population at the Bothe Napa Valley State Park. A proposed application to build two small agricultural water storage reservoirs along a creek in Napa County would avoid direct impacts to another population, but ground disturbance would most likely introduce this same alien invasive weed.

Plant succession may be excluding or reducing the population at one site where Clara Hunt's milk-vetch grows sparingly in the gaps between manzanita plants. As established plants continue to grow and new manzanita seedlings become established, less space is available for Clara Hunt's milk-vetch. Fire suppression has reduced fire frequency in the manzanita community. Periodic fire reduces manzanita cover and creates space for Clara Hunt's milk-vetch and other plants. This species is therefore vulnerable to habitat loss from plant succession. Another population of Clara Hunt's milk-vetch is threatened by competition from the invasive alien shrub French broom (Genista monospessulana ) and the rooting behavior of wild pigs.

Conservation and Recovery

The Nature Conservancy obtained a voluntary agreement with private landowners in 1990 to protect one population of Clara Hunt's milk-vetch.

Contact

Regional Office of Endangered Species
U. S. Fish and Wildlife Service
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232
http://pacific.fws.gov/

Reference

U. S. Fish and Wildlife Service. 22 October 1997. "Determination of Endangered Status for Nine Plants From the Grasslands or Mesic Areas of the Central Coast of California." Federal Register 62 (204): 54791-54808.