East Bay Regional Park District, move over. In 1974, long after the establishment of the park district, voters in Walnut Creek decided to save Shell Ridge (and other lands in and around the growing city) as public open space rather than proceed with residential development. By the mid-1980s, the land purchases were completed. Now, homes fill the valleys on either side of this northwest spur of Mount Diablo and then stop at its base.
The land protected by Walnut Creek voters as Shell Ridge Open Space interlocks in a complex jigsaw puzzle with other local pieces protected by the East Bay Regional Park District in Diablo Foothills Regional Park and by the State of California and Save Mount Diablo in Mount Diablo State Park. Shell Ridge is a significant piece of the contiguous and near-contiguous protected open space on and around Mount Diablo that forms important habitat corridors and protects biodiversity in the region. Shell Ridge itself is a 1,400-acre series of parallel ridges that rise to 829 feet at their highest point. Footpaths lead up from trailheads into Diablo Foothills Regional Park on the north side of Shell Ridge, while several trailheads in Walnut Creek city parks and on residential streets provide access to Shell Ridge Open Space on the south side.
When I moved to the East Bay from the Chicago area, I had to replace my expectations of summer grasses from green to yellow, as well as think of summer as a time of partial dormancy. However, I have since learned to rearrange my expectations of fall color for native trees. Coast live oaks (Quercus agrifolia) remain evergreen, but the diversity of deciduous oaks in and around Mount Diablo shows equally diverse but muted fall colors. In my post-Thanksgiving travels, I have left the coast live oaks of the Oakland–Berkeley hills and paid several visits to five oak species found on Shell Ridge.
Deciduous colors on Shell Ridge arrive earlier for the summer-deciduous plants, such as the flame-red leaves of poison oak (Toxicodendron diversilobum) and the variable green-yellow-brown of California buckeyes (Aesculus californica). Valley oaks (Quercus lobata) and blue oaks (Quercus douglasii) change color around Thanksgiving, shifting from mostly green to mostly brown. On just one section of slope, blue oaks vary from the mostly blue-green of summer to brown to mostly leafless.
Deciduous tree colors and plant communities differ on these slopes leading to Mount Diablo, not just because of colder night temperatures, but also with variations in soils and available moisture. A few coast live oaks grow in scattered, shadier locations, but deciduous blue and valley oaks take center stage on the clay soils in grasslands. Some of the evergreen oaks among the blue and valley oaks on the southern trails of Shell Ridge Open Space, including the trails I accessed from the end of Sutherland Drive, may turn out to be interior live oaks (Quercus wislizeni), distinguished by their often-flat leaves with hairless undersides from the usually concave leaves and “hairy armpits” (leaf vein junctions) of coast live oaks.
I also found what I believe to be an Oregon white oak (Quercus garryana), holding brown leaves with shallower lobes than those of valley oaks, plus blunt acorns with a flatter end instead of the narrow “bullet” acorns of valley or coast live oaks. Squirrels, acorn woodpeckers, and scrub jays make short work of acorns, so I usually figure out oak trees by leaves. Acorns figure prominently in many plant keys for the oak genus (Quercus), which can stymie identification of some oaks for much of the year. I recommend using The Flowering Plants and Ferns of Mount Diablo California by Barbara Ertter and Mary L. Bowerman, not only for its user-friendly oak key with simple language, but also for the first 100 pages that describe Mount Diablo in the context of its plant diversity.
The preservation of Shell Ridge as open space saved much of the land’s native tree diversity, but some areas needed a helping hand to restore understory. After Borges Ranch, which straddles the border between Shell Ridge Open Space and Diablo Foothills Regional Park, was opened for public use, volunteers working to improve habitat for California quail weeded out many of the exotic invasive forbs and grasses. They replanted weeded areas with native vegetation, including quail bush (Atriplex lentiformis), coyote bush (Baccharis pilularis), blue elderberry (Sambucus nigra ssp. caerulea), and a few coast live oaks. The quail population increased tenfold. The habitat also attracted other birds, brush rabbits, snakes, and coyotes. The project’s leader declared the work complete when he spotted a Cooper’s hawk in a nearby tree: the project now had a predator. Since then, this restoration has been stable with no maintenance other than a couple of years of removing stinkwort (Dittrichia graveolens) from the immediate area. Increasing the quail population by creating a new area of appropriate habitat was the volunteers’ goal, but they created a foothold for native plants as well.
More plant communities grow on Shell Ridge than oak woodlands, notably chaparral communities on harder sandstone areas. In the part of Shell Ridge that’s in Diablo Foothills Regional Park, I climbed towards the ridgeline from the trailhead beyond the end of Castle Rock Road and came up out of a switchback into a sudden transition to black sage (Salvia mellifera) and chamise (Adenostoma fasciculatum), leaving the oak woodlands in the background. In this year’s dry, warm early winter, much of the black sage foliage had died back, leaving evergreen leaves to the chamise thickets.
My ambitious eyes outpace my feet, and I still have not hiked across from Shell Ridge Open Space to Diablo Foothills Regional Park. However, I can return when travel restrictions ease and try looking for new growth after the latest rains.
— Janet Gawthrop, Field Trips Committee Chair, CNPS East Bay Chapter
Learn more about identifying our area’s oaks in Kate Marianchild’s online presentation “Who’s Who Among the Oaks” on January 27, 2021.