Raven’s manzanita (Arctostaphylos hookeri ssp. ravenii)

No discussion of our chapter area’s rare and endangered plants would be complete without mention of Raven’s manzanita. Also known as the Presidio manzanita, it is perhaps the single most endangered taxon in San Francisco County, with only one naturally occurring individual remaining in the wild. Historically, Raven’s manzanita occurred on three sites, but these populations were all lost to urbanization during the 1930’s and 1940’s. These historic locations include the Laurel Hill Cemetery, situated between Parker and Presidio avenues, Geary Blvd. and California Street; the Masonic Cemetery, bounded by Fulton and Turk streets and Parker and Masonic avenues; and Mt. Davidson. The only place the species still occurs in the wild is the Presidio.

Raven’s manzanita, prostrate, evergreen member of the heath family (Ericaceae), is a serpentine endemic, which means that it evolved on and is entirely restricted to serpentine-derived soils. Arctostaphylos hookeri ssp. ravenii is one of the manzanitas that do not produce basal burls (enlarged root crowns). It has finely hairy stems that root on contact with the soil. It has light green round to round-elliptic leaves less than one inch across, and produces small pink urn-shaped flowers from February through March.

Raven’s manzanita has probably always been rare. Even before colonization of the San Francisco Peninsula, its distribution is thought to have been highly restricted. Because only a single naturally occurring specimen is known to occur in the wild, Raven’s manzanita has received a great deal of attention. The California Department of Fish and Game (CDFG) listed it as an Endangered species in 1978 and the U.S. Fish and Wildlife Service (USFWS) followed suit in 1979. The California Native Plant Society (CNPS) has always placed it at the top of its list of threatened species, where it remains on the List 1B (rare, threatened, or endangered in California and elsewhere), with a 3-3-3 rarity, endangerment, and distribution (R-E-D) code (restricted to one occurrence, endangered throughout its range, and endemic to California). As required by the Endangered Species Act, a recovery plan was prepared for Raven’s manzanita in 1984. Since assuming control of the Presidio, the National Park Service (NPS) is actively protecting the remaining specimen and undertaking efforts to expand its last stand. In 1987, the CDFG, USFWS, NPS and the Army signed a Memorandum of Understanding to protect the Presidio’s rare plants.

Perhaps no other native California genus has been the subject of so much controversy among taxonomists as has Arctostaphylos. Raven’s manzanita is no exception. It has been collected, classified, and reclassified numerous times throughout botanical history. In 1892, Kathleen Brandegee identified our single Presidio specimen as A. pungens. In 1952, Peter Raven noted that this plant exhibited strong similarities with A. franciscana, which is extinct in the wild, and to A. montana from Mt. Tamalpais. Philip Wells was the first to treat Raven’s manzanita as a separate taxon, treating it as a subspecies of A. hookeri. James Roof argued for a closer kinship instead with A. pungens, but his proposed name change never gained acceptance.

In addition to accidental catastrophes, the last specimen of Raven’s manzanita is threatened by competition for light by such species as Monterey pine, Monterey cypress, and ice plant, as well as the native Ceanothus thyrsiflorus. Vandalism and foot traffic are also a perpetual threat. In 1985, road widening destroyed some of the plant’s habitat. Despite all these dangers, the plant has been successfully propagated. Cuttings that were transplanted in 1988 flowered for the first time in 1991. Plants are currently being grown at the Presidio nursery. Reintroduction efforts will be restricted to the Presidio as it supports the only remaining historical habitat for the subspecies.

What can you can do? The NPS is undertaking the monumental task of controlling invasive plant species that threatened to overgrow some of the last remaining examples of San Francisco’s native plant communities and volunteers are always needed. If you would like to learn more about these efforts, check out our Habitat Restoration page for regularly scheduled restoration work parties.