The Coast Live Oak (Quercus agrifolia) is an evergreen oak, highly variable and often shrubby, found in the coastal regions of southwestern North America from Mendocino County, California south to northern Baja California in Mexico. The dark green leaves are usually small, thick and spiny-toothed. This native of coastal California is the dominant overstory plant of the Coast Live Oak woodland habitat, often joined by bay laurel and California buckeye north of Big Sur; moreover, Coast Live Oak has also become a common addition to landscaping. The stateliness of the Coast Live Oak has rendered it a subject of historical landscape painters throughout California modern history since the mid 1800s.
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Design characteristics
The Coast Live Oak exhibits a highly esthetic multifurcate trunk and reaches a mature height of 20 to 30 meters. Some specimens may attain an age exceeding 250 years. Leaves range from two to five centimeters in length arranged six to 16 irregularly along stems; the leaf appears to be serrate, but it actually owns sharp thistly fibers that extend from the lateral leaf points. The tree's growth habit displays a broad roundish dense helmet appearance, especially aged 20 to 70 years. In later life the trunk and branches are more well defined and the leaf density less severe.
Outer layers of leaves are designed for maximum solar absorption, since they contain two to three layers of photosynthetic cells and are convex in shape. These outer leaves are deemed to be small in size to more efficiently re-radiate the heat gained from solar capture. Inner leaves are generally broader and thinner, having only a single layer of photosynthetic cells. The convex leaf shape may be useful for interior leaves which depend on capturing reflected light scattered in random directions from the outer canopy. The trunk structure, particularly for older individuals, may be highly contorted, massive and gnarled yielding an eerie visage. This irregular shape let the tree escape widespread harvest for building timbers, and also led the early settlers to endow the Coast Live Oak with mystical qualities. Some of the largest trunk diameters may extend to three or four meters, such as the magnificent specimens at the Filoli estate in San Mateo County
Uses by early settlers
When migrating tribes descended from the ice bridge into coastal California between 10,000 and 3000 BC, at least twelve distinct cultures are known to have consumed Coast Live Oak acorns as a dietary staple. In the 18th century Spaniards in the San Fernando Valley used the wood for charcoal to fire kilns in making adobe. Later this form of charcoal would be utilized in the baking, gunpowder and electric power industries. In the 18th and 19th centuries shipbuilders sought out the odd angular branches to make special joints. Pioneers moving west would harvest small amounts of the tree for making farm implements and wagon wheels, but the greatest impact was the wholesale clearing of oak woodlands to erect sprawling cities such as San Diego and San Francisco.
Habitat and range
Coast live oak is the only California native oak that actually thrives in the coastal environment, although it eschews the immediate shore locale; it enjoys the mild winter and summer climate afforded by ocean proximity, and it is somewhat tolerant of aerosol-borne sea salt. The welcome coastal fog supplies further buffering from the rainless California summer heat. Coast Live Oak is often associated with the understory plants of toyon, manzanita and poison oak.
Normally the tree is found on well drained soils of coastal hills and plains, often near year round or perennial streams. It may be found in several natural communities including: Coast Live Oak woodland, Engelman Oak woodland, Valley oak woodland and both northern and southern mixed evergreen forests. While normally found within 100 kilometers of the Pacific Ocean at elevations less than 700 meters, in southern California it occasionally occurs at up to 1500 meters in altitude. The Coast live oak is sensitive to changes in grading and drainage; in particular, it is important to respect the root crown level and avoid adding soil near the trunk when construction or landscaping occurs. Also, if incorporating Coast live oak into a landscaping scheme with artificial irrigation, it is important not to water within the drip line of the Coast live oak.
Classification detail
The Coast live oak is classified in the red oak section (Quercus sect. Lobatae). Several hybrids between Coast live oak and other red oak species have been documented. Hybrids with Interior live oak (Q. wislizenii) are known in many areas in northern California. Coast live oak also hybridizes with Nuttall's scrub oak (Q. dumosa) and Shreve oak (Q. parvula var. shrevii). All these oak species show evidence of introgression with one another.
The name Quercus agrifolia literally means "field-leaved oak", and is generally thought to be an error by the describing botanist for "aquifolia", "holly-leaved". This species is sometimes known by the name "California Live Oak".
There are two varieties of Quercus agrifolia found in California. These are Q. agrifolia var. agrifolia and Q. agrifolia var. oxyadenia. Var. agrifolia has leaves that are glabrous to slightly hairy on the abaxial side, especially near the leaf vein axils. It hybridizes with Quercus kelloggii, Quercus parvula var. shevei, and Quercus wislizenii. Var. oxyadenia in contrast has leaves that are tomentose abaxially, with densely interwoven hairs. It prefers granitic soils and hybridizes with Quercus kelloggi.
Bibliography
- Bruce M. Pavlik, Pamela Muick, Sharon Johnson and Marjorie Popper, Oaks of California, Cachuma Press (1992) ISBN 0-9628505-1-9
- E.K. Balls Early Uses of California Plants, University of California Press, Berkeley (1972)
- Hickman, James C. The Jepson Manual Higher Plants of California. University of California Press, Berkeley Los Angeles London. (1993).