The Essential Tree
Delving Deeper into Oak Woodlands
A blue oak is the pot of gold at the end of this rainbow in the Mount Diablo foothills, Easter Sunday, 2002.
Photo by Brad Perks, Lightscape Photo Gallery.
Acorns may be California’s single greatest natural resource. An oak tree can bear more than 400 pounds of acorns a year. There are an estimated 1 billion oak trees in California. That’s hundreds of millions of pounds of nutrient that serves as the staple for more kinds of creatures than any other food source in the state. But the bulk of nutrients oaks churn out is only the beginning of their contribution. Oak trees form the organizational backbone of numerous habitats from coastal valley bottoms to highland meadows, providing food, shelter, and stability for whole communities of organisms. According to a 1997 University of California study, California’s oak woodlands harbor more biodiversity than any other major habitat type in the state: At least 4,000 kinds of insects inhabit them, along with 2,000 kinds of plants, thousands of fungi and lichens, 170 different birds, 60 amphibians and reptiles, and 100 different mammals.
These most wonderful and generous habitats—California’s quintessential landscapes, many would say—are under siege on numerous fronts. As development and other threats surround and engulf private oak lands throughout the state, our parks and protected areas serve as refugia for oaks and the plants and animals that rely on them, as laboratories for scientists studying these habitats, and as points of access for potential future oak advocates. Many of them are also sublimely beautiful and a hell of a lot of fun to walk through.
The Sunol Regional Wilderness, east of Milpitas, may offer one of the best introductions to the Bay Area’s oaks and oak woodland habitats. Within a mile radius of the oak-shaded visitor center you can see at least five species of the Quercus genus that makes up the true oaks. This mixed oak forest is nourished by the wide and lively Alameda Creek, whose wetness also draws yellow-billed magpies, wild turkeys, black phoebes, titmice, and scores of other birds. Splendid examples of valley oak (Q. lobata), the most impressive and archetypal of the oaks (and one of the most beleaguered statewide), stretch their muscular limbs out and up toward the sky. The valley oaks’ dense foliage sheds welcome shade in the hot summer months. But when this deciduous tree drops its leaves in the winter, its skeleton reveals an unmatched genius for filling three-dimensional space in the most beautiful ways. If there is a drop of tree-climber in you, these trees will draw it out.
Sunol also hosts beautiful examples of canyon live oaks (Q. chrysolepis), coast live oaks (Q. agrifolia), and interior live oaks (Q. wislizenii). And the wilderness is home to the biggest blue oak (Q. douglasii) on record (as measured by trunk diameter), according to Ron Russo, chief of Interpretation and Recreation Services for the East Bay Regional Park District (EBRPD), which manages Sunol. The tree is somewhere near Indian Creek on the Ohlone Wilderness Trail. A kestrel is supposedly nesting in its upper branches. I haven’t seen it yet, as far as I know, but I can’t walk through Sunol without thinking of it and the hopeful continuity it represents.
Sunol’s 7,000 acres are an easy introduction to Bay Area oaks, but Sunol is also a link in a four-piece chain of wild areas that offer the advanced degree. Mission Peak Regional Preserve, to Sunol’s west, and Ohlone Regional Wilderness, to Sunol’s east, connect to Del Valle Regional Park to the north of Ohlone. Together they make up one of the largest continuous stretches of wilderness remaining in the Diablo Range.
Take the 28-mile Ohlone Trail hike through that wilderness, and you will see examples of dense oak riparian forest, where canopies overlap to create damp, primeval habitat where little sun penetrates. You would also see the more sparsely treed oak woodland, where the treetops barely reach one another, creating an open forest community filled in with grasses. The sparser-still savanna woodlands could also be described as grasslands punctuated by oaks. Along the trail you’ll find each type: forest, woodland, and savanna, as well as grassland, chaparral, and many intermediate grades and combinations of all of these. In the spring, when the grass is crazy green, and the oaks have been rejuvenated by the rains, and the wildflowers are in resplendent bloom, this is one of the most beautiful places on earth.
Not all trees in an oak forest or woodland are necessarily oaks—bay laurel, California buckeye, madrone, and big leaf maple are frequent companions—but to earn the classification, oaks must dominate.
What determines which kind of oak woodland appears where on these hills and valleys? The availability of water, soil nutrients, and light as well as slope and aspect (i.e., direction of exposure to the sun) all weigh in heavily. But chance plays its part as well. Fires, floods, and blights can wipe out a forest, and the availability of viable acorns of one kind or another will greatly influence the type of oak habitat that replaces it. Different conditions catalyze bumper crops for different kinds of oaks. One year may see abundant blue oak acorns; the next year may produce few blues but tons of live oak acorns. Which big crop is followed by a catastrophic fire, say, may greatly influence what kind of forest gets the jump on the next generation in that spot.
A great number of conditions need to be met for seedlings to make it to the sapling stage, let alone for saplings to survive to maturity, says Mike Moran, a naturalist at Black Diamond Regional Park (near Antioch), where there is an excellent example of blue oak forest. In the first place, there must be enough acorns. Fire must make an appearance at the right time, to clear out the brush, but must not reoccur when young saplings are still vulnerable. There can’t be too many foragers such as rodents or deer or, today, cattle, which devour saplings. Conditions must be wet enough (without flooding) and temperatures warm enough (but not too hot). The chances are slight in any year that all these requirements will be met for any species of oak and slighter still that a would-be forest will survive a decade to adolescence, after which it becomes less susceptible to browsing and weather fluctuations. But for trees that live many centuries, and in unbridled terrain like California used to be, the conditions wouldn’t have to have been satisfied very often to allow new woodlands to establish themselves from time to time.
Work by UC Davis ethnoecologist Kat Anderson suggests that native Californians, who relied heavily on acorns as a dietary staple, probably manipulated ecological variables to cultivate productive oak crops. Their main tool would have been fire, used both to initially clear land and then to periodically cleanse it of underbrush (which could cause more catastrophic fires later) and competition from other shrubs and trees. Anderson’s work also suggests that the original Californians pruned dead wood from trees and pulled up unwelcome plants from around seedling oaks. If Anderson is correct, this might help explain the frequency of single-age stands, such as the extensive blue oak forest at Round Valley Regional Preserve.
Round Valley, eight miles east of Mount Diablo along Marsh Creek Road, was the proposed site for a Contra Costa County dump before the Murphy family decided to sell it to the EBRPD instead. Round Valley Regional Park contains several historic Native American sites and, in its southwest corner, hosts a mixed oak woodland with coast live, interior live, black, blue, and valley oaks mixed in with California buckeye and bay laurel. Such a mix of trees increases the chance that whatever the weather in a given year, there will be a bumper crop for some kind of acorn. It is such mixed oak woodlands that support maximum diversity.
On the micro level there are the thousands of insect species that live on, in, or around oaks. The bugs that exploit oak habitats first became a special interest of the EBRPD’s Ron Russo 35 years ago when a park visitor brought him a blue oak branch with some odd sea-urchin-shaped forms on it. “I had no idea what they were, but I knew they were gorgeous,” Russo says. “It was an awakening.” He has spent the subsequent decades studying and classifying these oak galls from all over California, becoming a respected authority on the subject.
It turned out that the magnificent ornaments on that first oak branch were produced by a kind of Cynipid wasp that drills a hole into an oak leaf and deposits its eggs there. As with all gall wasps, a salivary secretion from the newly hatched larva stimulates the production of plant tissue surrounding the larva, which provides it with food and shelter. “What’s remarkable,” says Russo, “is that the tissue formed by the tree has very specific, and often very beautiful, characteristics that appear to be determined by the wasp. Research shows that larval RNA is involved, though still no one knows just how it controls specific growth patterns.”
Galls provide a glimpse into the recursive creativity of oak ecosystems. Russo found a blue oak in Briones Regional Park that had 31 distinct species of Cynipid wasp galls on it. Just one of those galls had 90 other insect species associated with it. “The number of different organisms living off that one gall staggers the mind,” says Russo.
The herpetological fauna of oak trees is also impressive. At least 80 species of amphibians and reptiles live in California oak communities, according to The Oaks of California (Cachuma Press & California Oak Foundation, 1991). Perhaps weirdest is the arboreal salamander, which climbs high into the canopy of coast live oaks to hunt insects during the wetter months. Arboreal salamanders, like all salamanders, absorb oxygen directly through their skin, which must remain moist for transpiration to occur. To help keep themselves wet the salamanders will often take up residence in oak tree cavities. In spring the females find moist places underground or under rotting logs to lay their eggs, returning above ground to forage until the dry season arrives, after which both sexes retreat to burrows or logs to settle down for a period of summer torpor (estivation). When it begins to rain again, the dried-out salamanders emerge and station themselves on the trunks of trees to rehydrate.
More open woodlands support “herps” that can tolerate, or require, more sun exposure, such as rattlesnakes, gopher snakes, and, in the East Bay, endangered Alameda whipsnakes. And before there were fences, fence lizards would have been tree lizards, hunting in the branches of oaks and foraging in the leaf litter beneath them.
The birds of oak woodlands are magnificent and varied as well. The acorn woodpecker is probably the most closely associated with oaks. Its practice of storing acorns in granary trees, its communal habits (an extended family group collectively shares both acorn caches and the raising of its chicks), and its preposterous getup (a red beret and whiteface) make it irresistible to watch. According to Oaks of California at least 20 other bird species nest in the cavities of oak trees, and 150 or so more rely on oaks in one way or another.
At Round Valley you can watch a pair of nesting golden eagles hunting for ground squirrels. The empty burrows of the preyed-upon squirrels are sometimes taken over by resident burrowing owls, which, like the eagles, are a rare and protected species.
Biologist David Herlocker, interpretive naturalist for the Marin County Open Space District, says that oak woodlands and forests are preferred habitats for black bears, which gorge on acorns as a primary protein source before entering winter torpor. The same goes for grizzlies. Though bears have not yet returned to Round Valley, there are enough coyotes, bobcats, and mountain lions (all preying on acorn-loving rodents) to keep a hiker’s senses alert.
If you took the gold out of the Sierra Nevada, it would still be the Range of Light. If you took the Nobel laureates out of Berkeley, you’d still have one stellar university. But take the oaks out of California and what have you got? Unfortunately, this is not as far-fetched a scenario as it sounds.
Indeed, there are few places where oaks are more threatened, or more precious, than here in the Bay Area. The most obvious cause is the runaway development gobbling up chunks of oak country around the Bay. Because oak woodlands have historically been so ubiquitous and hence taken for granted, they don’t enjoy the same legal protections as California’s other forests. The state doesn’t require special permits or mitigations to clear-cut even old-growth oak. Hence we have housing developments like Blackhawk and Rossmoor and Canyon Oaks where oak woodlands stood a few decades ago. And in addition to sprawl, the booming 1990s saw a sharp rise in the conversion of oak woodland and savanna into vineyards.
Climate change also poses a threat to oaks across the state. As average temperatures rise and precipitation patterns change, some areas may become inhospitable to the woodlands they now host. In past epochs woodlands could accommodate climate changes by migrating north or south or up- or down-slope. In those times, change occurred slowly and potential habitat was continuous, so forests could adapt or follow the optimal climate as it went. Today’s human-caused climate change is orders-of-magnitude quicker and the potential migration routes are fragmented by civilization.
The development of new generations, also called recruitment, is another serious problem. In many oak forests today it is rare to see a single sapling. Interrupted natural (or set) fire regimes, competition from exotic plants, and the browsing of seedlings by cattle all probably contribute. For all we know, shifts in climate over the past century may already have deprived today’s oak forests of the conditions required for reproduction and/or made them more vulnerable to diseases such as Sudden Oak Death (SOD), the great wild card in California oak conservation today.
Sudden Oak Death was first identified in the mid-1990s in Marin County, where tan oaks had mysteriously begun to “bleed” and die. (Tan oaks, or Lithocarpus densiflora, also known as tanbark oaks, are not in the Quercus genus but are close relatives with much in common.) The disease-causing organism (or pathogen) has since been identified as a previously unknown member of the Phytophthora genus, and it is infecting, and killing, tanbark, coast live, canyon live, black, and Shreve oaks in coastal counties from Monterey to Humboldt.
In some counties, SOD has killed thousands of trees, nearly wiping out the tan oaks and coast live oaks in places such as Marin’s China Camp State Park. It was not until two years ago that the SOD pathogen was found in the East Bay; so far it has only been confirmed in several locations. Perhaps the drier climatic conditions in parts of the East Bay will limit the pathogen, which requires moisture to spread. However, researchers caution that it is much too early to say with any confidence how Phytophthora ramorum will respond to a range of possible conditions and too soon to predict how it will affect oaks in the East Bay (and other inland counties). But while the scientific jury is still out on how devastating Sudden Oak Death will prove to be in the long run, there is no doubt that exposure to new pathogens, shifting climate, altered water patterns, and fragmentation of habitats make the future of our oak woodlands much more precarious.
Before we can marshal the political and scientific resolve necessary to keep our oak habitats with us and well, we must stop taking them for granted. When it comes to oaks and the natural communities they create, there is no better path to appreciation than experience. Get out there and learn your oaks; see how many oak species you can identify on a walk through Briones, Mount Burdell, or Henry Coe. See how many other organisms you can find relying on a single tree in an hour, an afternoon, a week. The longer you spend, the more likely you’ll be to count yourself among them.