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

oak An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...

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 elsew ...

to (and found only in)

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

, 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 the ...

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 topograp ...

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 primarily ...

. 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, aft ...

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 In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...

with sparse foliage, generally tall, with a trunk in

diameter at breast height Diameter at breast height, or DBH, is a standard method of expressing the diameter of the trunk or bole of a standing tree. DBH is one of the most common dendrometric measurements. Tree trunks are measured at the height of an adult's breast, ...

. Trunks are typically solitary, but some trees have multiple trunks. The tallest recorded specimen was found in

Alameda County Alameda County ( ) is a List of counties in California, county located in the U.S. state of California. As of the 2020 United States Census, 2020 census, the population was 1,682,353, making it the 7th-most populous county in the state and List ...

, at . The trees grow slowly, about per year. Individual trees over 500 years old have been recorded. The bark is light gray with many medium-sized dark cracks. The blue-green

leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

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

acorn The acorn, or oaknut, is the 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 shell, and borne ...

s are long, with a moderately sweet

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learnin ...

, and mature in 6–7 months from

pollination Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds ...

. ''Q. douglasii'' is

monoecious Monoecy (; adj. monoecious ) is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant. It is a monomorphic sexual system alongside gynomonoecy, andromonoecy and trimonoecy. Monoecy is conne ...

and wind-pollinated. Flower buds take a growing season to develop into

catkin A catkin or ament is a slim, cylindrical flower cluster (a spike), with inconspicuous or no petals, usually wind-pollinated (anemophilous) but sometimes insect-pollinated (as in ''Salix''). They contain many, usually unisexual flowers, arranged cl ...

s. 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 The genus ''Quercus'' contains about 500 species, some of which are listed here. The genus, as is the case with many large genera, is divided into subgenera and sections. Traditionally, the genus ''Quercus'' was divided into the two subgenera '' ...

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'' (sometimes spelled ''P. sabineana''), with vernacular names including towani pine, foothill pine, gray pine, bull pine, and digger pine, is a pine endemic to California in the United States. Some sources discourage using the n ...

(''Pinus sabiniana''), and is also found with

interior live oak ''Quercus wislizeni'', known by the common name interior live oak, is an evergreen oak, highly variable and often shrubby, found in many areas of California in the United States continuing south into northern Baja California in Mexico. It genera ...

(''Q. wislizeni''),

coast live oak ''Quercus agrifolia'', the California live oak, or coast live oak, is a highly variable, often evergreen oak tree, a type of live oak, native to the California Floristic Province. It may be shrubby, depending on age and growing location, but is ...

(''Q. agrifolia''), valley oak (''Q. lobata''),

Oregon white oak ''Quercus garryana'' is an oak tree species of the Pacific Northwest, with a range stretching from southern California to southwestern British Columbia. It is commonly known as the Oregon white oak or Oregon oak or, in Canada, the Garry oak. It ...

(''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 ''Quercus turbinella'' is a North American species of oak known by the common names shrub oak, turbinella oak, shrub live oak, and gray oak. It is native to Arizona, California, New Mexico, Utah, Colorado, and Nevada in the western United State ...

(''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 An old-growth forestalso termed primary forest, virgin forest, late seral forest, primeval forest, or first-growth forestis a forest that has attained great age without significant disturbance, and thereby exhibits unique ecological featur ...

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 Drought tolerance is the ability to which a plant maintains its biomass production during arid or drought conditions. Some plants are naturally adapted to dry conditions'','' surviving with protection mechanisms such as desiccation tolerance, detox ...

; 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 generall ...

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 ...

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

Sudden oak death resistance

As of 2002, the disease known as

sudden oak death James Green aka "Sudden" is a fictional character created by an English author Oliver Strange in the early 1930s as the hero of a series, originally published by George Newnes Books Ltd, set in the American Wild West era. Oliver Strange died i ...

, caused by the

oomycete Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...

''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. 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