Saturday, April 22, 2017

A Land That Time Forgot: San Bruno Mountain

Or, rather, a land that "progress" has mercifully not destroyed, an unlikely island of refuge in a sprawling sea of development. 

My relationship with (and, I admit, my knowledge of) San Bruno Mountain began one foggy March day a couple of years ago, when Eric and I went to visit a friend living in South San Francisco.  Our friend suggested that we go hiking at a place nearby, a mountain, he called it.  I was skeptical, and understandably so, situated as we were in a maze of suburbia on the San Francisco Peninsula.  My knowledge of the immediate area didn't extend much beyond the San Francisco airport, and I had heretofore failed to notice the 1,300 foot mountain rising above the city (probably because it is so often shrouded in fog). 

Little did I know a glorious and unexpected wilderness awaited me atop that mountain.

Our ascent of San Bruno Mountain brought us through a thick grove of eucalyptus trees: non-native, rather weedy, entirely to be expected (unfortunately).  Before long, we broke through the trees, climbing steeply through a lovely grassland.  Were those native grasses I spied, tucked into the typical exotic mix?  There certainly was a beautiful patch of Blue-eyed Grass (Sisyrinchium bellum) along the trail! 

Blue-eyed Grass (Sisyrinchium bellum)

As we climbed, I became more intrigued, and a sort of slow dawning came over me: this was a special place, a remnant of what used to be.  Being the lone naturalist in our hiking party, the significance of the mountain was mostly lost to the others at the time.  As we reached the top of the mountain, the wind blew and the fog surrounded us, enveloping us in our own private world.  We could see neither sea nor city; not another human being, not a sign of buildings or roads (except for the trail we were on), not a blight on the natural landscape.  For all we could tell, we were on a high mountain peak in a remote wilderness, forgotten by humanity and left to progress as nature intended.

Once upon a time, all of the San Francisco Peninsula looked like San Bruno Mountain.  That is, the Peninsula once supported the vast assemblage of plant communities and wildlife that are now holding out as remnant populations on 3,600 acres of the mountain's slopes.  The mountain is home to 13 rare and endangered plant species, and three federally listed endangered butterflies: Mission Blue (Icaricia icarioides missionensis), Callippe Silverspot (Speyeria callippe callippe), and San Bruno Elfin (Callophrys mossii bayensis).

A Northern Checkerspot (Chlosyne palla): not one of the endangered butterflies, but the only one I photographed!

In 1999, famed ecologist E.O. Wilson called San Bruno Mountain one of the most important and threatened biodiversity sites in the world.  Located in San Mateo County, San Bruno Mountain is now a state park, more or less protected from development, with miles of trails open to outdoor recreationists.

San Bruno Mountain Watch is the organization largely responsible for saving the mountain.  Born in 1970, the mission of the mountain watch group is to “preserve and protect the native ecosystems of San Bruno Mountain, in perpetuity.”  This group has taken on the responsibilities of environmental stewardship, in addition to creating outreach programs that aim to engage the public in protecting the mountain for its value as public open space as well as for its native ecosystems.  And the work is not finished.  Tracts of land adjacent to the mountain, including dune habitat and pockets of native grasslands, are on private property and at risk for development.  Even the 140 acre quarry on the east side of the mountain holds the potential for restoration, as stonecrop (Sedum spathulifolium), the host plant for the endangered San Bruno Elfin Butterfly, is found growing on the rocky outcroppings of the quarry walls.

Stonecrop (Sedum sp), a host plant for the endangered San Bruno Elfin Butterfly

Much of the mountain is composed of the sedimentary rock greywacke, part of the Franciscan complex, which geologists estimate formed about 130 millions years ago during the Mesozoic era.  At that time, the present coastline was underwater, and the sedimentary rock formed as layers of sediment were deposited in oceanic trenches.  The mountain itself is a fault block that began to rise about one million years ago.  The Franciscan greywacke has formed relatively thin, rocky soils on the steep mountain slopes, with pockets of clay soils found where the slopes are more gradual.  According to the website of San Bruno Mountain Watch, there is an inland sand dune on the western slope of the mountain, a remnant from a time long ago when the coastline was farther inland.  (I haven't visited the dune area myself.)

Coast Iris (Iris longipetala) is a rare, California endemic with limited distribution; it is associated with the diminishing
coastal prairie plant community.

Microclimates on the mountain, created by varying combinations of fog, high winds and sun exposure, create the opportunity for a diverse assemblage of plant communities to share a relatively small area.  In all, ten distinct plant communities can be found on the mountain.

The most common plant community is Northern (or Franciscan) Coastal Scrub, comprised largely of coyote brush (Baccharis sp.), sagebrush (Artemisia sp.), lizard tail (Eriophyllum staechadifolium), bush monkey flower (Mimulus aurantiacus), and coffee berry (Frangula californica, formerly Rhamnus californica).  San Bruno Mountain boasts the largest remaining example of this plant community, which once covered the peninsula along with coastal grassland.

Coast Rock Cress (Arabis blepharophylla), a rare, endemic plant found on San Bruno Mountain, in association with
coastal prairie and northern coastal scrub communities.

Two types of grasslands or coastal prairies can be found on San Bruno Mountain; these plant communities contain the greatest diversity of plants, have the highest productivity, and provide habitat for all three endangered butterflies.  San Bruno Mountain preserves possibly the largest, most diverse and intact area of coastal grassland prairie left in California.

Valley Needlegrass Grassland graces the southern slopes of the mountain, a mix of purple needle grass, California melic, June grass, blue wild rye and San Francisco blue grass.  Wildflowers also abound in season, on these warmer, drier slopes.

Coastal Terrace Prairie is found on northern exposures, able to withstand a greater amount of fog and wind.  Grasses found in the Coastal Terrace Prairie community include Pacific reed grass, California oat and hair grass, and three types of fescue: California, Idaho and red.

Ceanothus, or Blue Blossom, in full bloom on the mountain - an important butterfly host plant!

Central Coast Riparian Scrub, comprised of willows, elderberry and ferns, can be found at sites of seeps and springs.  Less conspicuous communities include Blue Blossom (Ceanothus) Chaparral, the rare Valley Wild Rye Grassland, Freshwater Marsh (near Colma Creek) and Freshwater Seep, as well as Coast Live Oak Woodland in a few sheltered canyons.  A small remnant of Central Dune Scrub, a community that is rapidly disappearing in California, is tucked away on the west side of the mountain.

A variety of lupine species on the mountain are host plants for
the endangered Mission Blue Butterfly.

The small creeks, seeps and wetlands of San Bruno Mountain support Pacific Tree Frogs (Pseudacris regilla) and the rare San Francisco Forktail Damselfly (Ischnura gemina).  Historically, these wetlands have been home to endangered San Francisco Garter Snakes (Thamnophis sirtalis tetrataenia) and threatened California Red-legged Frogs (Rana draytonii).  Hopefully, with continued conservation and restoration efforts, one day the mountain will again support populations of these unique species.

According to San Bruno Mountain Watch's website, the biodiversity counts for the mountain are:
662 plant species, 42 butterflies, 195 birds, 5 bumblebees, 30 ant species, 24 mammals, 13 reptiles, and 6 amphibians.  Quite impressive for one mountain island, isolated as it has become in a sea of civilization.

Mammal inhabitants include brush rabbits and jack rabbits, rodents such as gophers, ground squirrels, mice and voles, Gray Foxes, Long-tailed Weasels, raccoons, skunks, opossums, shrews, and of course, the ever-present feral cat.  Sightings of mountain lion, red fox, badger, bobcat and coyote have also been confirmed.  A coyote population on the mountain would be an exciting thing indeed, as the presence of coyotes leads to greater biodiversity, especially in bird species, as coyotes prey on feral cats, opossums and skunks, which are leading predators of birds, fledglings and bird eggs.  One interesting paper I read suggested that the abundance of cemeteries in the nearby town of Colma has inadvertently created enough of a wildlife corridor to allow coyotes and other wildlife to reach the mountain's isolated island of habitat.

Since my first visit to San Bruno Mountain, I have returned to this island a few more times, and each experience has been different.  I have witnessed the mountain in its many moods: damp, close fog one day, keeping the mountain's secrets; billowing, rolling fog another day, tumbling over the mountain on biting winds.  Refreshing summer breezes and wisps of cloud have met me on the mountain; dazzling sunshine and crystal skies have revealed views extending across the San Francisco Bay and out to the Pacific, the Farallon Islands on the horizon.

But the mountain I like the best is the brooding mountain: thick, moody fog cloaking its slopes, swirling on the wind and blotting out the surrounding cityscape; dense, sheltering fog protecting this secret, forgotten place from the busy world.  Because it is only after a climb up the mountain and into the fog that one feels as if it is truly possible to visit the California of days gone by, to return to a land that time forgot.

More information on this remarkable mountain island can be found at San Bruno Mountain Watch's very informative website, which I poured over eagerly after my initial, surprising visit to the mountain:


  1. Hi Siera,

    Nice article and lovely photos!
    Thank you for your thorough and engaging descriptions of the mountain and its moods.
    We're happy our website could be of help in providing information about the mountain!
    To you and your readers, please join us anytime on the mountain and at the Mission Blue Nursery, and feel free to reach out with questions or thoughts about the mountain's cultural and natural history!

    Ariel Cherbowsky Corkidi
    San Bruno Mountain Watch

    1. Hi Ariel!
      Thank you for reading, and for the excellent work that you do for San Bruno Mountain Watch! It's such a special place. I hope to visit again soon!