Marin Dwarf Flax & serpentine (Hesperolinon congestum)

In the December issue of Yerba Buena News, we introduced Clarkia franciscana (which has just been federally listed as Endangered),a species occurring only on soils formed on serpentine parent material.Serpentinite, California’s state rock, is the greenish rock occurring in a lineof outcrops visible from Hunter’s Point, Potrero Hill, under the U.S. mint onMarket Street, and underlying Laurel Hill and Baker Beach. Serpentine isabundant above Fort Point, and the south tower of the Golden Gate Bridge wasconstructed on what the newspapers called “pudding stone.” Elsewherein the Bay Area, serpentine can be seen near Crystal Springs in San MateoCounty, in the Oakland-Berkeley hills, on Angel Island, and from Tiburon tonear the summit of Mt. Tamalpais. Serpentine occurs on every continent of theworld. In California serpentineoutcrops cover over 1,100 square miles, primarily in the northern half of thestate.

Serpentine soils are ofspecial interest to ecologists because of the unique plant assemblages usuallyassociated with them. Botanists are especially interested in serpentine derivedsoils because such soils have given rise to a remarkable degree of speciation.The so-called “serpentine syndrome” consists of a high level ofendemism, the occurrence of indicator species, and sparse vegetative cover.
Of the 1,742 taxa listed inthe California Native Plant Society’s Inventory of Rare and EndangeredVascular Plants of California, 285(16 percent) occur on serpentine soils. Of the 33 sensitive taxa occurring inSan Francisco County, 11 (33 percent) occur on serpentine. Presidio clarkia andMarin dwarf flax are two of these. The others include Arctostaphylos hookeri ssp. franciscana (extinct in the wild), Arctostapylos hookeri ssp. ravenii (known from a single location), Cirsium andrewsii, Grindelia hirsutula var. maritima, and Triphysaria floribunda.

Fritillaria lileacea,Sanicula maritima, and Sidalceahickmanii ssp. viridis, occur elsewhere on serpentine, but the San Franciscopopulations have been extirpated. Erysimum franciscanum sometimes grows on serpentine, but in San Francisco itis found only on ocean-blown sand.

Hesperolinon congestum is a member of the flax family (Linaceae). It is alow annual herb with dichotomously branching stems two to six inches high. Thestems are sharply angled and frequently pubescent above the nodes. Leaves arelinear, alternate, glabrous to occasionally pubescent, and one-half to one inchlong. The species has well-developed stipule glands with a pronounced reddishexudate. Inflorescences are congested near the tips of the stems. Sepals arelance-ovate, pubescent on back, acuminate, less than one-quarter of an inchlong, and have distinctively gland-dotted toothed margins. Petals are pink torose and slightly larger than the sepals. Anthers are pink to purple, while thethree styles are whitish. The ovary has six chambers and develops into a minute,short ovoid capsule. The flowering season is from May through July.

Marin dwarf flax isrestricted to serpentine grassland and chaparral. It is known from only roughlyfifteen locations in Marin, San Francisco, and San Mateo counties. The type localityof the species is on the Tiburon Peninsula in Marin County, and the NatureConservancy’s Ring Mountain Preserve is the only population being activelymanaged for the species. Only one other stand is known in Marin County and thesite is planned for development. In San Mateo County, the largest and mostsignificant populations occur within the Crystal Springs Watershed. Recently,the County has published plans for an extensive trail system that could damageat least two populations of Marin dwarf flax. The species also occurs atEdgewood County Park; with the rejection of a proposed golf course on the site,hopefully these populations will be preserved.

Hesperolinon congestum is currently listed as threatened by the CaliforniaDepartment of Fish and Game, has just been listed as threatened by the U.S.Fish and Wildlife Service, and is on the CNPS List 1B: 3-3-3 (restricted inoccurrence, endangered throughout its range, and endemic to California).Twenty-five percent of the historic populations of the species have been lostto residential development and road construction. Extensive searches ofsuitable habitat have been conducted with very limited results. No managementplan for this species has been developed.

In San Francisco, Marin dwarfflax has been collected historically at Laurel Hill Cemetery (1912), on Mt.Davidson, Inspiration Point, and above Baker Beach. The populations at LaurelHills, Mt. Davidson, and possibly Inspiration Point have been extirpated. Onlythe Baker’s Beach population persists. The National Park Service has initiatedefforts to preserve this population and is in the process of developing amanagement plan for the species through consultation with the U.S. Fish andWildlife Service. We hope that this last known San Francisco occurrence can beadequately preserved.