''Quercus douglasii'', known as blue oak, is a species of oak

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...

to (and found only in)

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

, common in the

Coast Ranges The Pacific Coast Ranges (officially gazetted as the Pacific Mountain System in the United States) are the series of mountain ranges that stretch along the West Coast of North America from Alaska south to Northern and Central Mexico. Although ...

and the

foothills Foothills or piedmont are geographically defined as gradual increases in elevation at the base of a mountain range, higher hill range or an upland area. They are a transition zone between plains and low relief hills and the adjacent topogr ...

of the

Sierra Nevada The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primar ...

. It is California's most drought-tolerant

deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, ...

oak, and is a dominant species in the blue oak woodland ecosystem. It is occasionally known as mountain oak and iron oak.

Description

''Quercus douglasii'' is a medium-sized tree with sparse foliage, generally tall, with a trunk in diameter at breast height. Trunks are typically solitary, but some trees have multiple trunks. The tallest recorded specimen was found in Alameda County, at . The trees grow slowly, about per year. Individual trees over 500 years old have been recorded. The

bark Bark may refer to: * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, e ...

is light gray with many medium-sized dark cracks. The blue-green leaves are tough and leathery, deciduous, long, and

entire Entire may refer to: * Entire function, a function that is holomorphic on the whole complex plane * Entire (animal), an indication that an animal is not neutered * Entire (botany) This glossary of botanical terms is a list of definitions of ...

or shallowly lobed. The acorns are long, with a moderately sweet kernel, and mature in 6–7 months from

pollination Pollination is the transfer of pollen from an Stamen, anther of a plant to the stigma (botany), stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by Anemophily, wind. Pollinating agents can ...

. ''Q. douglasii'' is monoecious and wind-pollinated. Flower buds take a growing season to develop into catkins. Blue oak pollen is severely allergenic.

Taxonomy

The species is one of over 80 named after Scottish botanist David Douglas. The common name "blue oak" derives from the dark blue-green tint of its leaves. Taxonomically it is placed in the white oak group (subgenus ''Quercus'', section ''Quercus'').

Ecology

''Q. douglasii'' prefers dry to moist soil and plenty of sunlight. Its sparse foliage allows more light to reach the ground, and young trees may grow for decades below their parents' canopies. The species often co-habitates with gray pine (''Pinus sabiniana''), and is also found with interior live oak (''Q. wislizeni''), coast live oak (''Q. agrifolia''), valley oak (''Q. lobata''), Oregon white oak (''Q. garryana''), and

canyon live oak ''Quercus chrysolepis'', commonly termed canyon live oak, canyon oak, golden cup oak or maul oak, is a North American species of evergreen oak that is found in Mexico and in the western United States, notably in the California Coast Ranges. This ...

(''Q. chrysolepis''). Natural hybrids between ''Q. douglasii'' and the related ''Q. lobata'', ''Q. garryana'', and shrub live oak (''Q. turbinella'') often occur where the species grow together in the same area. Sources consider '' Quercus × alvordiana'' to be a hybrid of ''Q. douglasii'' and either ''Q. turbinella'' or '' Q. john-tuckeri''. Old-growth blue oak woodland may be one of the most widespread old-growth forest ecosystems remaining in the state after European colonization.

Drought tolerance

''Quercus douglasii'' is the most drought-tolerant of California's deciduous oaks. It has a smaller canopy than less drought-tolerant relatives, and invests proportionally more growth into roots rather than leaves throughout its life cycle. The leathery blue-green leaves contribute to its drought resistance; during drought, the leaf color is more pronounced. Trees can also drop their leaves in summer rather than fall in dry years, but usually continue to develop their acorns through the fall. Drought may cause trees not to flower in spring. Quercus douglasii-Galls

Gall wasps

Author Ron Russo writes that ''Q. douglasii'' hosts the "largest number of known species" of

gall wasp Gall wasps, also incorrectly called gallflies, are hymenopterans of the family Cynipidae in the wasp superfamily Cynipoidea. Their common name comes from the galls they induce on plants for larval development. About 1,300 species of this gener ...

s, at more than 50. The wasps trigger the formation of

oak gall Oak apple or oak gall is the common name for a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from in diameter and are caused by chemicals injected by the larva of certain kinds of gall w ...

s in a wide variety of shapes, colors, and sizes.

Sudden oak death resistance

As of 2002, the disease known as sudden oak death, caused by the oomycete ''Phytophthora ramorum'', had not been found in ''Quercus douglasii'' or any members of the white oak group. An experiment showed that ''Q. douglasii'' and ''Q. lobata'' (another white oak) appeared to be resistant to the pathogen.

Uses

Native Californians commonly gathered ''Quercus douglasii'' acorns, which they considered good-tasting, and processed them into

acorn flour The acorn, or oaknut, is the nut (fruit), nut of the oaks and their close relatives (genera ''Quercus'' and ''Lithocarpus'', in the family Fagaceae). It usually contains one seed (occasionally two seeds), enclosed in a tough, leathery Nutsh ...

. They made baskets out of blue oak seedlings, utensils such as bowls from the wood, and dye from the acorns. Commercially, the blue oak is mainly limited to use as firewood. The acorns are eaten by wildlife and livestock. They can be eaten but, if bitter, may need to have the tannins leached.

Notes

References

{{Taxonbar, from=Q666160 douglasii Endemic flora of California Natural history of the California Coast Ranges Natural history of the Central Valley (California) Plants described in 1840 Trees of Mediterranean climate Garden plants of North America Ornamental trees