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Davis Mountain Oak
Quercus depressipes
Blue Oak
Quercus douglasii
  Hook. & Arn.
Oregon White Oak
Quercus garryana
  Owner:   C. Michael Hogan    

Blue Oak, Annadel State Park, California
Blue Oak
Quercus douglasii

C. Michael Hogan PhD
September 24, 2008

Blue Oak, a California endemic, is a dominant tree of the Coast Ranges and western Sierra Nevada foothills, occurring over approximately two million hectares of mixed oak forest and savanna. This species is associated with considerable ecological biodiversity within the California Floristic Province, and within its range it is viable in arid, hot climate zones.

Q. douglasii is common within its range, and dominant in almost fifty percent of the California oak woodlands. The species is found in valleys and on lower slopes of the Coast Ranges and in lower elevations of the Sierra Nevada, with the distribution forming an almost complete two-dimensional ovate torus with almost null distribution in the core of the Central Valley. Blue Oak occurs in 39 of the California counties ranging from Riverside County north to Del Norte County. Isolated populations are found in the Central Valley, mainly in the form of savannas. Disjunctive stands are found within the Trinity, Siskiyou and Klamath Mountains, east of the Cascade Range. Disjunctive populations occur on Santa Cruz and Santa Catalina islands as limited stands, and are likely the result of prehistoric transport by Chumash and other coastal Native American peoples using plank canoes crossing the narrower ocean channel that existed in the early Holocene.

Q. X alvordiana is the most common hybrid of the Blue Oak, occurring in the Inner Coast Ranges from the Tehachapi Mountains to Carmel Valley; this semi-deciduous hybrid may be dominant within that range.
Close-up of Blue Oak fall foliage

Most often occurring as a single trunked form, Blue Oak does not uncommonly exhibit a multi-furcate version. In any case the crown presents a generally rounded somewhat open canopy with crooked branches and sparsely distributed leaves. Bark is a light ashy gray in thin, scaly, checkered form. This tree has little commercial value beyond firewood, and it has rich brown heartwood, whereas sap wood is light brown. (Pavlik). The bluish green leaves are tough and leathery; serving as a drought protection mechanism; moreover, they manifest smooth wavy margins and extend two to seven cm in length. While normally deciduous, the tree may exhibit a semi-deciduous form in moister habitats.

Q. douglasii may attain a height of seven to 22 meters, with depth of roots commensurate. In shallow soils, this tree may take on a more shallow rooted laterally spreading form, whereas in deeper soils or fractured-rock soil roots will attain their full depth equal to above-ground height. Acorns are light brown at maturity and have dimpled shallow caps, rarely covering the acorn length by more than nine mm. The total acorn length is two to four cm, with a shape of a tapering elongated oval. Mass of acorns is typically three to ten grams.

Acorn of the Blue Oak
Blue oaks typically reach reproductive capability at a height of five meters, but often such growth takes a considerable number of years (approximately 20 to 35); however, trees can also regenerate from stumps following fire, even though this is not a robust method of growth. Acorns typically germinate within 30 days of falling, and this germination is facilitated by a forest floor of moderate leaf litter. Very low early winter temperatures impede germination and seedling survival. The wind pollinated flowers are unisexual, and little biotic energy is allocated to reproduction until after pollination (Koenig) Flowering occurs from March to May, with earlier blooming transpiring in more southerly or warmer regimes. Almost three fourths of the first year seedling growth is invested in root, rather than aboveground growth, as measured by dry-weight. In many cases sapling development is very slow.

Blue Oaks are adapted to considerable aridity and summer heat, with the ability to tolerate weeks of daily maxium temperature of 35 degrees. In years of extreme summer heat, Q. douglasii aestivates and slips into true dormancy, after which the tree continues to produce acorns using its food stores. Following such aestivation, no growth will occur until the ensuing spring Within the core range, typical average annual precipitation spans the range of 50 to 100 cm, although a small part of the southern range is less than half that minimum, and the far northern range sees up to 150 cm per annum.

The typical density of Blue Oak is 10 to 50 trees per hectare, with higher densities on moderate slopes of well drained soils. Q. douglasii will not thrive in heavy clay and hardpan soils (Whitney); moreover, Blue Oak is out-competed by Q. kelloggii (Coast Ranges) and Q. wislizeni (Sierra foothills) in deep rich soils. A shade intolerant species, Blue Oak is rarely an understory plant, (McDonald) but is found in more open canopy environments, including savanna, oak woodlands with a paucity of taller species and rocky arid hillsides of thin soils. A common associate on higher and drier regimes is the Digger Pine (e.g. Inner Coast Ranges and Sierra foothills), which is tall but exhibits a more open canopy, that does not deter Blue Oak sapling growth.

In the North Coast Ranges Blue Oak occurs frequently in association with California Black Oak. (Hogan) Q. douglasii also occurs with Pinus ponderosa and P. Attenuata in the Sierra foothills; and Heterondes arbutifolia and Aesculus californica in the North Coast Ranges. Common understory forbs in sunny patches of the forest floor include Calochortus luteus, Mimulus aurantiacus and Wyethia helenioides. Frequently Blue Oak is associated with a tall grass understory, prehistorically perennial bunch grasses such as Nassella pulchra or other species within the genus Stipa, but since European contact increasingly with introduced grasses such as Avena fatua.

Blue oaks evolved from angiosperms that emerged in the Early Cretaceous period. Q. douglasii apparently developed in response to drier, warmer climates during the previous ten million years The Paleobotanic record illustrates that Q. douglasii comes from a Miocene progenitor taxon, the Quercus douglasoides which had a very broad distribution (McDonald). Subsequent to formation of the Sierra Nevada mountains, Blue Oaks became isolated and thus evolved into a differentiated species.

* Muick Pavlik, Pamela C. Johnson, Sharon G. and Popper Marjorie (1991) Oaks of California.
Cachuma Press, Los Olivos, California, 184 pages
* W. Koenig, R. Mumme,. W.Carmen, and M. Stanback (1994). Acorn production by oaks in Central Coastal California: variation within and among years Ecology. 75: 99-109.
* Stephen Whitney (1994) Western Forests. National Audubon Society, Knopf, New York, N.Y.
* C. Michael Hogan (2008) California Black Oak: Quercus kelloggii, Globaltwitcher, ed. N. Stromberg

(The foregoing was prepared for use by GlobalTwitcher by C.Michael Hogan, who retains copyright to this work. Facts derived from the above may be used by full citation.)