When I approach Sibley Volcanic Regional Preserve from the Skyline Blvd. entrance in Oakland, I pass the volcanic exhibit area near the parking lot on my way to see the park’s exposed geologic features. The Volcanic Trail leads to signposts for Miocene Epoch volcanic outcrops. Years of quarrying have exposed basalt, rhyolite, and other volcanic remnants from about 10 million years ago.
More recently, park visitors assembled smaller rocks into labyrinths. Labyrinth visitors have trampled bare dirt paths between the stones, which are surrounded by yellow annual grasses. Tasmanian blue gum trees (Eucalyptus globulus) tower over some coast live oaks (Quercus agrifolia) and bay trees (Umbellularia californica) on the hillside. Park rangers have cut eucalypts near the backpacking camp and replaced them with new coast redwoods (Sequoia sempervirens).
The East Bay Regional Park District publicizes Sibley Preserve for—surprise!—numerous exposures of volcanic rocks. If you wish to learn more about the park’s geology, I recommend two Bay Nature articles, by Horst Rademacher and Gordy Slack.
Plants recolonized Sibley Preserve in the epochs after eruptions formed Round Top, the highest hill in the park. Twentieth-century demand for crushed rock and paved roads led to much more recent plant disturbance from quarrying volcanic rocks. Fewer types of plants have regrown in the quarried area, in part because Sibley did not receive any soil replacement or restoration before it was turned over to the East Bay Regional Park District.
Rather than remain in the volcanic area to dwell on missed opportunities, I usually head north on the East Bay Skyline National Recreation Trail (Skyline Trail) towards the north end of the park above the Caldecott Tunnel. The Sibley-to-Tilden portion of the Skyline Trail starts among large eucalypts near the Sibley parking lot. However, the eucalypts and poison hemlock (Conium maculatum) quickly give way to more native vegetation as you descend on the trail. Start by looking at the tall arroyo willows (Salix lasiolepis) in wet areas at the bottom of the first slope. Some may debate whether to call arroyo willow a shrub or a tree. Either way, insects flock to arroyo willows and birds follow their insect meals into willow branches.
Further along the trail, coast live oaks and California bay trees replace arroyo willows, while the path draws nearer to the seasonally dry Round Top Creek. California buckeyes (Aesculus californica) that sometimes grow through the foundation of broadleaf evergreens are noticeable by their yellowing leaves and developing fruit as they enter summer dormancy.
More deciduous plants grow along the streambed and oak understory, including western hazelnut (Corylus cornuta ssp. californica) and creek dogwood (Cornus sericea). Bracken fern (Pteridium aquilinum) has already covered at least one creekside patch where CNPS volunteers started pulling French broom (Genista monspessulana) a decade ago. Goldback ferns (Pentagramma triangularis) and wood ferns (Dryopteris arguta) grow in shady patches under oaks and bay trees along the trail.
After traversing much of Sibley Preserve northward, I cross the footbridge over Round Top Creek. In this part of the East Bay, the Bay Area Ridge Trail runs in tandem with the Skyline Trail. And in this part of Sibley Preserve, the Bay Area Ridge Trail follows the creek’s canyon instead of the higher ridgeline on Grizzly Peak Blvd. Although sturdy coast live oaks grow up most of the slope to the road, the Sibley-to-Tilden portion of the Skyline Trail follows the more varied and dense vegetation in the canyon.
Unfortunately, French broom spread from its original foothold to the rest of the park. CNPS volunteers have been pulling it in Sibley Preserve for the past decade with the cooperation of park staff and other park volunteers. While many Genista thickets still bloom, volunteer work has taken out much of the broom near the trail and cleared it out of some native needlegrass areas uphill.
Plants in areas cleared of French broom have responded well. Along the stream, ninebark (Physocarpus capitatus) and wild roses (Rosa sp.) bloom in June, and rosilla (Helenium puberulum) blooms now, with its tiny fringes of ray flowers encircling globes of yellow disk flowers. Last summer, many candelabras of pink Muhlenberg’s centaury (Zeltnera muehlenbergii) flowered amid the needlegrasses, but the dryer winter of 2020 may have caused their absence this July.
Although rosilla blooms abundantly now, I found no exotic dandelion flowers where they usually sprout by the metal gate at the bottom of the trail. While I pondered the flowerless rosettes of green dandelions, I saw a tiny mouse-brown body dash into the poison oak (Toxicodendron diversilobum) clutching non-native dandelion leaves in its jaws. This California vole definitely had a taste for dandelion greens because it braved my presence to dart out repeatedly and snag more leaves. I have no idea if only this vole eats dandelions, but I wish it bon appétit for the rest of its short life.
Beyond the gate lies the Old Tunnel Road Staging Area, very small but much less crowded than the Skyline Blvd. parking lot. If you wish to continue your hike to visit and compare the less-disturbed Skyline Gardens to the north, follow trail signs to Tilden Regional Park out of Sibley Preserve. The Skyline Trail passes by easement through EBMUD lands, so you do not need an EBMUD permit to use this trail.
If you go: The main parking lot and entrance to Sibley Volcanic Regional Preserve is located in the Oakland hills on Skyline Blvd. just east of Grizzly Peak Blvd. You can find more information, including a wildflower list, on the East Bay Regional Park District’s page for the park.
— Janet Gawthrop, Field Trips Chair, CNPS East Bay Chapter