San Francisco’s Native Cherries : Bitter Cherry (Prunus emarginata), Hollyleaf Cherry (Prunus ilicifolia), Western Chokecherry (Prunus virginiana var. demissa)

When studying vegetation associations, we have a tendency to focus on anomalies. A high concentration of rare species is one aspect that immediately draws our attention. Rarity, of course, has a wide variety of definitions and possible explanations (see Yerba Buena News from 2000, vol.14, nos.3 and 4). But we can gain insight into the age, origin, and evolutionary relationships of the local flora when the rare species present are relictual or newly-evolved.

Finding a rare plant population here and there is always exciting. Its presence gives us a rationale for assigning a higher value to that location than another site lacking rare species. But then there are the “gold mines” of biological diversity, the mother lodes that host unprecedented numbers of rare species. Frequently, such sites are associated with geologic discontinuities, peculiar and isolated soil types derived from unusual geologic formations (see Yerba Buena News from 2001, vol.15, nos.3 and 4). Soils derived from serpentine and other ultramafic (ferromagnesian) rocks are famous for hosting unique (and highly evolved) plant species with restricted geographic distributions (see Yerba Buena News, 1995, vol.9, no.2). And with an estimated 1,100 square miles of such soils occurring in California (Kruckeberg 1985), such sites are hard to miss. A large band of ultramafic substrate runs diagonally across the entire city of San Francisco‹from the Golden Gate Bridge pilings, through the Presidio and Lone Mountain, under the US mint, through Potrero Hill and southeastern Bernal, to Hunters Point. An estimated 12.5% of species endemic to California are restricted to ultramafic substrates and 15% (some 285 taxa) of all listed threatened and endangered species in the state show some degree of association with these soils (Safford, et al. 2005).

Other unique plant assemblages associated with distinct geologies include the Ione chaparral (Amador County), the Pygmy Forest of Mendocino County’s hardpan marine terraces, the carbonate endemics of the San Bernardino Mountains, the pebble plains and limestone endemics of Big Bear Valley (which probably supports the highest concentration of endemic plants at a single location in the US), the chaparral of Pine Hill’s gabbro formation (El Dorado County), the basaltic lava grasslands of North Table Mountain (Butte County), and of course, the ultramafic soils of the Cook and Green Pass/Red Butte Wilderness area of the Siskiyous, to name a few. The beautiful book California’s Wild Gardens (CNPS 1997) provides a wonderful introduction to these locales and many more. For an excellent overview of California’s biodiversity, see the California Department of Fish and Game’s Atlas of Biodiversity in California (CDFG 2003).

One of the many factors contributing to the tremendous plant diversity found in California today has to do with the floras of past geological epochs that repeatedly advanced and retreated with global climate shifts. During the ice ages, the Arcto-Tertiary geoflora thrived and advanced southward. During periods of drought, elements of the Madro-Tertiary geoflora advanced westward from the Great Basin region. And during warm, moist periods, the Neo-Tropical geoflora left its mark on the California landscape with northerly invasions. With the advance and retreat of these floristic elements, overlaps resulted (e.g., at the Cook and Green Pass area you can see significant overlap of southern and northern floral elements). And where topography or substrate provided micro-environments favorable to the retreating flora, they created refugia capable of supporting plant species that were otherwise being extirpated. A simple but dramatic example of this is the giant sequoia (Sequoiadendron giganteum). These trees, once circumpolar in distribution, were literally scraped off the face of the earth by advancing glaciers. On the entire planet, they persisted only in small groves in the central Sierra, where they were protected from glacial ice movements.

That’s a ridiculously long introduction to the topic of this installment of “Focus on Rarities.” I hope you’ll forgive me this digression. Since becoming aware of such things (relatively late in life), I’ve been intrigued by localized assemblages of otherwise common related tree and/or shrub species not often all found in one location. The more species are packed into a small area, for whatever geologic or evolutionary reason, the more excited botanists become (and you know how excited they can get!). A spectacular example of this is Russian Peak in the Klamath Mountains, where you can see 16 species of conifers in a single square mile.

But assemblages don’t have to be so dramatic to provide us with insight into the evolutionary and geologic histories of the indigenous flora at a particular locale. There’s a unique example of this to be found right here at Bayview Hill in the heart of San Francisco. Near the summit of the hill, three native species of cherry can be found within a stone’s throw of each other. These are bitter cherry (Prunus emarginata), hollyleaf cherry (Prunus ilicifolia), and western chokecherry (Prunus virginiana var. demissa). None of these is rare. In fact, they are all quite widespread throughout the state. So what’s cool about that, you ask?

Bitter cherry and western chokecherry occur along the coast from the Oregon border to San Diego County, inland from Modoc to Riverside, and east of the Sierra Nevada; both extend into Oregon and Washington. Western chokecherry typically occurs in foothill woodland and yellow pine forest up to 8,200 feet in elevation. Bitter cherry occurs in yellow pine forest and red fir forest up to 9,000 feet. While these two species also occur in chaparral, they are restricted to moist settings.

Hollyleaf cherry has a somewhat narrower distribution, occurring in the Coast Ranges from Napa to San Diego counties, inland from Yolo to Imperial counties, and extending into Baja California. Hollyleaf cherry occurs in chaparral and foothill woodlands in more xeric environments; it does not occur above 5,000 feet.

While all three species also occur on nearby San Bruno Mountain, Bayview Hill is the only location where all three occur in San Francisco County. According to A Flora of San Francisco (Howell, et al. 1958), western chokecherry occurred at Lake Merced, Twin Peaks, and San Miguel Hills, in addition to Bayview Hill. Katherine Brandegee recorded hollyleaf cherry from throughout the “Mission Hills, Strawberry Hill, Hunters Point, and the bluffs near Visitación Bay” around 1890, but its range had already been reduced to Bayview Hill by 1958. Bitter cherry was not listed in the 1958 flora. Today, bitter cherry is found at Glen Canyon; western chokecherry is also present on Tank Hill; hollyleaf cherry is found only at Bayview Hill.

To me, Bayview Hill feels like one of those micro-habitat refugia. With a fractured chert bedrock at or near the surface, this site is fast-draining and typical for hollyleaf cherry. Hollyleaf cherry, which is most extensive in the southern half of the state, reaches its northernmost coastal distribution in San Francisco. Bitter cherry and western chokecherry, typically associated with moister, more forested habitats, overlap hollyleaf cherry here. It’s a subtle clue to the geologic history of the region, to the ebb and flow of floristic elements whose roots are farther to the north or to the south. This story might be a little vague to inspire much awe in most folks. But then, you know how excitable botanists can be! And it might not even be true. It’s just a story. Fun to contemplate.