Afoot and Afield: San Francisco Bay Area. David Weintraub

Kentfield, in Marin County, receives about 48 inches (that’s 4 feet!) of rain per year, whereas the town of Livermore, in eastern Alameda County, gets only about 15 inches. Yet the distance between these two towns is only about 55 miles.

      Climate is one thing, but daily weather is another, so it’s a good idea to check with a local source for current conditions and forecasts before heading out. You can use a weather radio, available at Radio Shack, outdoor stores, and other outlets, to receive broadcasts from the National Weather Service. You can also find up-to-the minute weather information on the Weather Channel or on the Internet at www.weather.com.

Geology

      The Bay Area lies within a geological province called the Coast Ranges, a complex system of ridges and valleys that stretches from Arcata to near Santa Barbara, and inland to the edge of the Central Valley. The Coast Ranges were formed millions of years ago, as the floor of the Pacific Ocean was dragged under the western edge of North American continent. This process scraped material from the ocean floor and piled it higher and higher on the continent’s edge, in what is now California.

      Within the Bay Area are sub-ranges such as the Sonoma, Mayacmas, and Vaca mountains in the North Bay; the Diablo Range in the East Bay and South Bay; and the Santa Cruz Mountains on the Peninsula and in the South Bay. The tallest peak in the North Bay is Mt. St. Helena (4343’), at the corner of Sonoma, Napa, and Lake counties. Other prominent Bay Area summits include Mt. Hamilton (4213’), Mt. Diablo (3849’), Loma Prieta (3806’), Mt. Tamalpais (2571’), and Sonoma Mountain (2295’).

      Olcott Lake in Jepson Prairie Preserve, a seasonal pool, hosts a variety of plants and animals, some quite rare.

      Most of the surface rock in the Bay Area is sedimentary, but volcanic activity has occurred here in the past. Young volcanic rock caps the Mayacmas and Vaca mountains bordering the Napa Valley, and there are four extinct volcanoes in the East Bay, including Round Top (1763’). California’s state rock is serpentine (more properly, serpentinite), gray-green in color and often seen as outcrops beside the trail. Serpentine forms a soil that is toxic to many plant species, but some have adapted to it. Among these are Sargent cypress, leather oak, and a variety of uncommon wildflowers.

      The San Andreas fault—which splits the Santa Cruz mountains and slices through Marin County—and a host of lesser faults crisscross the Bay Area. As the Pacific plate slides past the North American plate, tremendous bursts of energy are sometimes released in the form of earthquakes. Most of the time, however, the northward movement, which, over the last 25 million years has carried granite from southern California to Point Reyes, is imperceptible. Visit Point Reyes National Seashore in the North Bay, Sibley Volcanic Regional Preserve and Mt. Diablo State Park in the East Bay, and Los Trancos Open Space Preserve on the Peninsula to learn more about Bay Area geology.

      Little Yosemite is a rocky gorge on Alameda Creek, perfect for picknicking and nature study.

Plants

      California has a rich diversity of plant life. Some species, like coast redwoods, date back to the dinosaurs, whereas others have evolved within the past several thousand years. Roughly 30 percent of the state’s native plants grow nowhere else. These endemics, as they are called, include many types of manzanita (Arctostaphylos) and monkeyflower (Mimulus). Botanists divide the plant kingdom into several major groups: flowering plants, conifers, ferns and their allies, mosses, and algae. A plant community consists of species growing together in a distinct habitat. Here are the principal plant communities you will encounter along the trails.

      OAK WOODLAND

      Inland from San Francisco Bay, the fog-free hills between 300 and 3500 feet host a generally open woodland, sometimes called a savanna. Species here include various oaks, California buckeye, gray pine, California bay, buckbrush, toyon, coffeeberry, snowberry, and poison oak. Examples of this community can be found in Sugarloaf Ridge State Park, Black Diamond Mines Regional Preserve, and Henry W. Coe State Park.

      RIPARIAN WOODLAND

      Found beside creeks and rivers, these trees and shrubs provide the Bay Area’s best hope for an autumn display of color. Common riparian species include bigleaf maple, white alder, red alder, California bay, various willows, California rose, poison oak, California wild grape, elk clover, and giant chain fern. Point Reyes National Seashore and Monte Bello Open Space Preserve give you opportunities to enjoy this community.

      REDWOOD FOREST

      At one time, coast redwoods blanketed the Pacific coast from central California to southern Oregon. These giants are the world’s tallest trees and are among the fastest-growing. Commercially valuable, they were heavily logged, especially in the Santa Cruz Mountains. The remaining old-growth coast redwoods in the Bay Area are confined a few areas, most notably Muir Woods National Monument in Marin County and Armstrong Redwoods State Reserve in Sonoma County. Associated with redwoods are a number of plant species, including tanbark oak, California bay, hazelnut, evergreen huckleberry, wood rose, redwood sorrel, western sword fern, and evergreen violet. You can visit second-growth redwood forests and see a few old-growth giants at Muir Woods National Monument, Redwood Regional Park, and Purisima Creek Redwoods Open Space Preserve.

      DOUGLAS-FIR FOREST

      These majestic trees often occupy similar habitats as coast redwoods, but thrive where soil conditions do not favor redwood growth. In many parts of the Bay Area, Douglas-fir is the “default” evergreen, easily told by its distinctive cones, which have protruding, three-pointed bracts, sometimes called rats’ tails. Like redwood, Douglas-fir is prized for its lumber. Some of the common plants associated with Douglas-fir are the same as those associated with coast redwood, namely California bay, tanbark oak, and western sword fern. Others include blue blossom, coffeeberry, and poison oak. Point Reyes National Seashore, Mt. Tamalpais State Park, and El Corte de Madera Creek Open Space Preserve have beautiful Douglas-fir forests.

      Coast redwoods grow in association wtih other trees and shrubs, creating a plant community.

      MIXED EVERGREEN FOREST

      A mixture of evergreen trees, including California bay, canyon oak, coast live oak, and madrone, comprises this community. The understory often contains shrubs such as toyon, blue elderberry, hazelnut, buckbrush, snowberry, thimbleberry, oceanspray, and poison oak. Carpeting the forest floor may be an assortment of wildflowers, including milk maids, fairy bells, mission bells, hound’s tongue, and western heart’s-ease. Take a stroll through a mixed evergreen forest at China Camp State Park, Dry Creek Pioneer Regional Park, Sierra Azul Open Space Preserve, and Edgewood Park and Preserve.

      CHAPARRAL

      This community is made up of hardy plants that thrive in poor soils under hot, dry conditions. Chaparral is very susceptible to fire, but some of its members, such as various species of manzanita, survive devastating blazes by sprouting new growth from ground-level burls. Although chaparral foliage is mostly drab, the flowers of many species are beautiful, with some blooming as early as December. The word chaparral comes from a Spanish term for dwarf or scrub oak, but in the Bay Area it is chamise, various manzanitas, and various species of ceanothus that dominate the community. Other chaparral plants include mountain mahogany, yerba santa, toyon, chaparral pea, and poison oak. You can study this fascinating assembly of plants on Pine Mountain, Mt. Diablo, and at Sierra Azul and Rancho San Antonio open space preserves.

      GRASSLANDS

      Few if any grasslands in the Bay Area have retained their native character. Human intervention, in the form of fire suppression, farming, and livestock grazing, along with the invasion of nonnative plants, has significantly altered the landscape. Gone from most areas are the native bunchgrasses, perennial species that once dominated our area. Remaining, thankfully, are native wildflowers, which decorate the grasslands