A Grassland with Trees. California’s oak savannas are composed of broad expanses of open land blanketed by grasses and dotted with widely-spaced trees. Wildflowers brighten this habitat with color for a few weeks in spring, then the grasses turn golden brown in the dry summer. When autumn rains begin to fall, a new batch of grasses sprouts. Valley oaks, the most common trees in oak savannas, are the largest broad-leaved trees in California. They can grow more than 100 feet tall with trunks as large as six or seven feet in diameter. Massive branches support their spreading canopies, leaving smaller branches to droop almost to the ground. Valley and blue oaks drop their leaves in winter to protect themselves from harmful frost, but a few evergreen live oaks keep some green in this grassland throughout the year.
Oak savanna forms in the lowlands and foothills of the Central Valley and the Coast Ranges, from the Pit River in Shasta County south to Baja California. Near rivers with intermittently flooded soil, oak savanna intergrades to gallery forest. The most common tree in valley lowlands is the valley oak, found south to Los Angeles County. Blue oak savannas develop on the drier slopes. In Southern California, most oak savannas are dominated by live oaks, but live oaks may be found intermittently farther north. The rare Engelmann oak forms scattered savannas from Los Angeles County south: the largest remaining populations are found on the Santa Rosa Plateau in Riverside County and Black Mountain in San Diego County.
Oak savanna develops only in areas with consistent levels of annual rainfall at the crossroads of valley grassland and foothill woodland. More tree-dominated habitats require greater amounts of water, but savannas, which support mostly grasses, can thrive on less moisture. Where more water is available, oak savanna transitions into foothill woodland.
Summers are hot and dry in oak savannas, with temperatures exceeding 100 degrees Fahrenheit. In winter, daytime temperatures hover in the 50s and 60s, but nighttime temperatures often drop below freezing. Engelmann oak savannas require cooler summer temperatures and only grow where winter frosts are rare.
Many plants, including oaks, have symbiotic relationships with fungi called “mycorrhizae,” which means “fungus root.” The hyphae of mycorrhizae penetrate the cells of trees’ root tips, allowing substances to be exhanged between the roots and fungi and effectively increasing the surface area of both organisms for absorption of soil nutrients. The fungi absorb essential minerals from the soil that they share with the tree, as well as sugars and water from the tree’s roots. Both the fungi and the plants benefit from this symbiotic relationship. The underground network of mycorrhizae connects many of the plants in this habitat to each other.
Oak Savanna Plants
Plants in this habitat:
- are mostly oaks or grasses
- have both shallow surface roots and deep tap roots
- support many animal species
purple owl’s clover
coast live oak
interior live oak
(Bromus laevipes rubens)
(Bromus [mollis] hordeaceus)
The types of oaks that grow in oak savannas vary with slope and latitude, but all species here prefer well-drained, alluvial soils. During fall and winter, valley and blue oaks in the savanna drop their leaves, while live oaks retain their leaves year-round. Abundant grasses carpet oak savanna, which is drier than most oakdominated habitats.
Plant Adaptation Case Studies
Valley Oaks Reach Down
A dense canopy of large green leaves covers the winter-deciduous valley oak in summer. The soft leaves absorb carbon dioxide faster and lose water at a greater rate than the thicker, tougher leaves of blue and live oaks, allowing the valley oaks to grow faster than the evergreen oaks. Valley oaks avoid dehydration and maintain a steady growth rate by extending deep roots into permanent underground water supplies. Their taproots can reach up to 60 feet underground.
Blue Oaks Keep Stiff Upper Leaves
Blue oaks are also deciduous, but their leaves are thicker and stiffer than those of valley oaks. A thick waxy substance covers the upper surface of blue oak leaves and helps them conserve moisture, an important feature since blue oaks don’t have the same long roots as valley oaks. However, blue oak seedlings develop a much greater root mass than leaf mass, so they maximize water uptake from the ground and minimize water loss through leaf surface evaporation. Even mature blue oaks form only sparse canopies.
Oak Savanna Animals
Animals in this habitat:
- use the shade of scattered trees
- are attracted to oaks
- are active all year
apple gall wasp
California oak moth
California ground squirrel
Grazers Eat Grass—and Oaks, Too
Herbivores in oak savanna live in, around, and on oaks. The most obvious are grazers who feed on grasses, but there are others that eat oak leaves, twigs, roots, and acorns. Many of these animals take shelter in the oak canopies.
Animal Adaptation Case Studies
Tent Caterpillars Weft and Warp
Tent caterpillars produce silk from glands in their mouths and use this substance to build shelters called tents. These caterpillars commonly feed on oak leaves, but few animals prey on them, despite their obvious colonies. With a thick, unpalatable layer of hair covering their bodies, these caterpillars only have to watch out for certain lizards and a few species of birds, including the Bullock’s oriole. The oriole punctures a hole in the caterpillar with its beak, pulls out the inside, and leaves the hairy skin behind.
Oh the Gall!
On many oaks, you’ll see tumor-like growths called galls. The galls range in size from a pinhead to a softball and can take many shapes, sizes, and colors. Many insects cause galls, mostly wasps in the family Cynipidae. Galls provide a protective environment and a source of food for developing wasp larvae. After the wasps deposit their eggs on an oak, the larvae emerge and secrete chemicals that stimulate the tree to produce the gall. It is not known whether gall formation hurts the oaks, but many healthy trees have them.
Federal and State Listed Species
The golden eagle is a California species of special concern. This bird’s highest population density is in the windy oak savanna near Livermore and the Altamont Pass in Contra Costa County, but it occupies other oak savannas throughout the state. The eagle builds its nests in oaks and hunts for rabbits and ground squirrels above the grasses. Many golden eagles have been killed in recent years along the Altamont Pass by outdated windmills that have not been retrofitted to protect low-flying birds.
Although the yellow-billed magpie is not a listed species, much of this bird’s habitat has been lost in areas such as the Salinas Valley and the Santa Clara Valley as a result of the clearing of oak savanna for agricultural and residential development. In addition, like jays and crows, magpies are particularly susceptible to West Nile virus. They are much more vulnerable to population decimation than crows and jays because their range is almost entirely within California.
Oak Savanna and Californians
Native Californians ate beardless wildrye seeds, which they made into mush or cooked whole and made into cakes. The Maidu and other tribes ate bluegrass, while others ate deergrass roots, although this latter plant was more valued for its flowering stalks, which could be used in coiled baskets. Native Californians also shredded deergrass leaves and used them to make skirts and cordage. Oak acorns provided the staple of most native peoples’ diets. As much as 75 percent of the food eaten by Native Californians came from acorns. Many thought coast live oak acorns were tastier than those of other evergreen oaks.
Several oak species, most notably valley oaks, blue oaks, and Engelmann oaks are no longer reproducing successfully in California. Many factors seem to be contributing to this lowered regeneration rate, including the presence of nonnative annual grasses, livestock grazing, deer browsing, acorn predation, and uprooting by pocket gophers. Early-growing introduced annual grasses deplete soil moisture earlier in the growing season than do later-growing native perennial grasses, reducing the water available to thirsty oak seedlings.
More than 75 percent of oak woodland and oak savanna ecosystems in California are grazed by cattle, which, along with sheep, eat leaves, acorns, seedlings, and saplings. Where land managers prevent or limit grazing, oaks will sometimes, although rarely, regenerate. Some plant ecologists believe that environmental changes brought on by grazing alter the land to such an extent that its effects endure for years after livestock are relocated.
Some studies show that native deer and rodent populations also hurt oak regeneration. An increase in the deer population results in more intense browsing of oaks and other plants, making it more difficult for oak seedlings to grow past sapling stage. Rodents, such as pocket gophers, mice, and ground squirrels, have also multiplied as the populations of their predators (gray foxes, bobcats, and badgers, among others) have declined, leading to a higher number of acorneating animals. When more rodents feed on acorns, fewer acorns can grow into new oaks.
Recent research suggests that regeneration only occurs when several factors coincide, including a good acorn crop, a series of wet winters and cool summers, and a decline in acorn and seedling predators, especially deer and pocket gophers. Scientists documented an example of regeneration under these ideal conditions at the Kaweah River Preserve in Tulare County. In 1983, one of the wettest winters in California history caused floods that reduced populations of ground-dwelling acorn predators, leading to a good acorn crop that year. Since then, many oak saplings have begun growing in the preserve.
Many of the state’s oak savannas have been cleared for ranching, farming, and residential development. Valley oaks have been especially affected because the same deep, rich soils that support these trees make prime farmland. Also, valley oaks typically grow in the flat bottoms of valleys or in low, rolling hills, making them targets of destruction for housing developments.
What People Are Doing
As early as 1909, botanist Willis Linn Jepson noticed that in areas where residential development and agricultural clearing occurred rapidly, almost no valley oaks remained. Not until 1986 did the state of California establish its Integrated Hardwood Range Management Program (IHRMP) to study the management, regeneration, and enhancement of oaks and other hardwood trees. The IHRMP sponsors oak research and communicates oak management information to citizens and agencies in California. In 1988 the California Oak Foundation was formed to conserve, restore, and manage California’s oaks. By 1991 more than 100 California municipalities had enacted oak tree preservation ordinances, but California’s oaks continue to disappear as a result of development and agricultural pressures. Since the 1940s, California has lost more than one million acres of oak habitat from rangeland clearing and agricultural conversion alone. It is estimated that another 250,000 acres of oaks will be destroyed by the year 2010.
Books and Periodicals
Griffin, James R. 1973. Valley oaks—the end of an era? Fremontia 1(1): 5-9.
Holland, Robert F. 1986. Preliminary descriptions of the terrestrial natural communities
of California. Sacramento: California Department of Fish and Game.
Keator, Glenn, and Susan Bazell. 1998. The life of an oak: An intimate portrait.
Berkeley: Heyday Books.
Palik, Bruce M., Pamela C. Muick, Sharon Johnson, and Marjorie Popper.
1991. Oaks of California. Cachuma Press and the California Oak Foundation.
Arroyo Seco Foundation
United States Geological Survey
Key terms: field guide, golden eagle
California Oak Foundation
Key term: tent caterpillar
Santa Clarita Organization for Planning the Environment
Key term: valley oak
Key term: yellow-billed magpie
Sites to Visit
Bidwell Park (Butte County)
Cheeseboro Canyon, Santa Monica Mountains National Recreation Area
(Los Angeles County)
Cosumnes River Preserve (Sacramento County)
Fort Tejon State Historic Park (Kern County)
Henry W. Coe State Park (Santa Clara County)
Indian Grinding Rock State Historic Park (Amador County)
Kaweah Oaks Preserve (Tulare County)
Liberty Canyon, Malibu Creek State Park (Los Angeles County)
Mission San Antonio (Monterey County)
Oak Grove Regional Park (San Joaquin County)
Santa Rosa Plateau Ecological Reserve (Riverside County)
Sunol Regional Park (Alameda County)
William B. Ide Adobe State Historic Park (Tehama County)