|Listed||February 3, 1995|
|Description||Herbaceous annual with slender, threadlike stems, 4-16 in (10.1-40.6 cm) long; bears rose to whitish flowers.|
|Habitat||Serpentine chaparral or serpentine bunchgrass.|
|Threats||Residential and recreational development, foot traffic, and competition with normative species.|
Marin dwarf-flax, Hesperolinon congestum, is an herbaceous annual of the flax family (Linaceae) with slender, threadlike stems, 4-16 in (10.1-40.6 cm) tall. The leaves are linear. The flowers are borne in congested clusters; the pedicels are 0.04-3.2 in (0.1-8.9 cm) long. The sepals are hairy, and the five petals are rose to whitish. The anthers are deep pink to purple; this character helps distinguish H. congestum from H. californicum, found in the same geographic area, which has white to rose anthers as well as hairless sepals. Two other species that are found in the same region are H. micranthum, smallflower dwarf-flax, and H. spergulinum, slender dwarf-flax. They differ from H. congestum in having hairless sepals and a long, open inflorescence, with pedicels 0.08-1 in (0.2-2.5 cm) long.
Marin dwarf-flax is an annual herb flowering from May to June or July. The species is pollinated by native insects including bee flies and pollen beetles. Populations range in size from one plant to thousands of plants. Population sizes can fluctuate greatly from year to year. No further data on the reproductive biology or demography of the species are available.
Marin dwarf-flax is endemic to serpentine soils. Populations are found in serpentine chaparral or serpentine bunchgrass habitat. Known populations occur between approximately 100 and 1,200 ft (30.5 and 365.7 m). Marin dwarf-flax grows with or in the vicinity of other federally listed plants: Tiburon jewelflower, Tiburon mariposa lily, and Tiburon paint-brush, in Marin County; Presidio clarkia and Presidio manzanita in San Francisco County; and fountain thistle, San Mateo thornmint, and whiterayed pentachaeta in San Mateo County. The federally listed bay checkerspot butterfly also occurs in the vicinity of Marin dwarf-flax.
Marin dwarf-flax is found on serpentine soils from Marin County south to San Mateo County, a range of 50 mi (80 km). Marin County locations include the Tiburon Peninsula (five populations), Carson Ridge (three populations), Mt. Burdell Open Space (two populations), Big Rock, and Golden Gate National Recreation Area (one population discovered in 1995). Two extant populations are known from the Presidio in San Francisco County. San Mateo County contains four specific locations near Crystal Springs Reservoir, two in Edgewood County Park, and one near Woodside Glens. Previously identified populations, now extirpated, include two from San Mateo County and two from San Francisco County. One additional population on the Tiburon Peninsula in Marin County is possibly extirpated.
Marin dwarf-flax is threatened by residential and recreational development, foot traffic, and competition with normative species. There are eleven documented populations which exist in Marin County. On the Tiburon Peninsula, one population is on Ring Mountain Preserve, formerly managed by The Nature Conservancy and currently managed by Marin County. The preserve is fenced to reduce the incidence of four-wheel drive vehicle and motorcycle use, but is still accessible to bicycles, equestrians, and hikers. Soil slumping may also threaten Marin dwarf-flax populations on the preserve. Another population on the Tiburon Peninsula occurs partially on a small preserve at St. Hilary's Church and partially on private land which has been proposed for development. Some botanists report that the portion of the population near St. Hilary's may be threatened by invasive non-natives; others disagree. The population may also be threatened by trampling when people and dogs walk off of established trails. A second proposed development in the same area was denied by the Town of Tiburon. The area that would have been involved in this second development is proposed to be set aside as open space. A few scattered groups of plants occur in the Middle Ridge area of the Tiburon Peninsula. Some of these plants grow on land designated as open space by the Town of Tiburon. The remainder of the plants in the Middle Ridge area occur on private land and are threatened by ongoing or proposed residential development.
Off of the Tiburon Peninsula, the Carson Ridge populations of Marin County are on Marin Municipal Water District land. These populations may be threatened by trampling from hikers. A new threat to Marin dwarf-flax on Marin Municipal Water District land is invasion of non-native barbed goatgrass in the Azalea Hill area.
In San Mateo County, populations of Marin dwarf-flax are known to occur on private property. These plants are threatened by proposed development and by the consequences of completed development, such as trampling trash pumping, and changes in hydrology caused by irrigation runoff. Three populations in the vicinity of Crystal Springs Reservoir, including Puglas Ridge, are on land managed by the San Francisco Water Department. Their habitat is threatened by the trails in the watershed as well as by invasion of non-native plants and road construction. The construction of future trails and accompanying fences may damage Marin dwarf-flax habitat in this area as well. A portion of the Marin dwarf-flax population located in Edgewood Park is suffering from foot traffic and inadequate trail maintenance. The population at Woodside Glens is in an area set aside as mitigation, but water runoff from upslope homes threatens the population.
Conservation and Recovery
Two populations of the species occur in Edgewood Park which San Mateo County manages as a natural preserve. The County has recently adopted a master plan to guide future activities in the park. The National Park Service has fenced one population of Marin dwarf-flax and removed some non-natives at the Presidio in an attempt to restore serpentine habitat.
Between 1982 and 1995, Ring Mountain was protected from development because the land on which it occurs was owned and managed by The Nature Conservancy. The Ring Mountain property was transferred to Marin County Parks and Open Space in 1995. Marin Open Space District has developed a management plan for Mt. Burdell Open Space that includes rotational grazing but no rare plant monitoring.
U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232
U.S. Fish and Wildlife Service. 1998. "Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area." Portland, Oregon. 330+ pp.