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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.
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.