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

oak An oak is a hardwood tree or shrub in the genus ''Quercus'' of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisp ...

endemic Endemism is the state of a species being found only 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 foun ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

, common in the Coast Ranges and the

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

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 primari ...

. 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 Leaf, 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, e.g., including only woody plants with secondary growth, only ...

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 county located in the U.S. state of California. As of the 2020 census, the population was 1,682,353, making it the 7th-most populous county in the state and 21st most populous nationally. The county seat is Oakland. A ...

, at . The trees grow slowly, about per year. Individual trees over 500 years old have been recorded. The

bark Bark may refer to: Common meanings * 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) Arts and entertainment * ''Bark'' (Jefferson Airplane album), ...

is light gray with many medium-sized dark cracks. The blue-green

leaves A leaf (: leaves) is a principal appendage of the stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, stem, ...

are tough and leathery, deciduous, long, and entire or shallowly lobed. The

acorn The acorn is the nut (fruit), nut of the oaks and their close relatives (genera ''Quercus'', ''Notholithocarpus'' and ''Lithocarpus'', in the family Fagaceae). It usually contains a seedling surrounded by two cotyledons (seedling leaves), en ...

s are long, with a moderately sweet kernel, and mature in 6–7 months from

pollination Pollination is the transfer of pollen from an anther of a plant to the stigma (botany), stigma of a plant, later enabling fertilisation and the production of seeds. Pollinating agents can be animals such as insects, for example bees, beetles or bu ...

. ''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 comparable with gynomonoecy, andromonoecy and trimonoecy, and contras ...

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''). It contains many, usually unisexual flowers, arra ...

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 ''Quercus'' subgenus ''Quercus'' is one of the two subgenera into which the genus ''Quercus'' was divided in a 2017 classification (the other being subgenus ''Cerris''). It contains about 190 species divided among five sections. It may be calle ...

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 (''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 An old-growth forest or primary forest is a forest that has developed over a long period of time without disturbance. Due to this, old-growth forests exhibit unique ecological features. The Food and Agriculture Organization of the United Natio ...

ecosystems remaining in the state after European colonization. The acorns are eaten by wildlife and livestock.

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 traditionally 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 gene ...

s, at more than 50. The wasps trigger the formation of oak galls 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 The Oomycetes (), or Oomycota, form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms within the Stramenopiles. They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction o ...

''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

The acorns can be eaten but, if bitter, may need to have the

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and Precipitation (chemistry), precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' is widel ...

s leached. Native Californians commonly gathered the 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.

Notes

References

External links

* {{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 Taxa named by George Arnott Walker Arnott Taxa named by Robert Wight