Quercus douglasii
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Quercus douglasii, known as blue oak, is a species of oak endemic to California, common in the Coast Ranges and the foothills of the Sierra Nevada.<ref name="hogan">Template:Cite web</ref> It is California's most drought-tolerant deciduous oak,<ref name="fs">Template:FEIS</ref> and is a dominant species in the blue oak woodland ecosystem. It is occasionally known as mountain oak and iron oak.<ref name="usda">Template:Silvics</ref><ref>Template:Calflora</ref>
Description
Quercus douglasii is a medium-sized tree with sparse foliage, generally Template:Convert tall, with a trunk Template:Convert in diameter at breast height.<ref name="fs"/> Trunks are typically solitary, but some trees have multiple trunks.<ref name="fs"/><ref name="flora">Template:EFloras</ref> The tallest recorded specimen was found in Alameda County, at Template:Convert.<ref name="usda"/><ref>Template:Cite web</ref> The trees grow slowly, about Template:Convert per year.<ref name="Cal Poly">Template:Cite web</ref> Individual trees over 500 years old have been recorded.<ref name="Stahle1">Template:Cite web</ref>
The bark is light gray with many medium-sized dark cracks. The blue-green leaves are tough and leathery,<ref name="hogan"/> deciduous, Template:Convert long, and entire or shallowly lobed. The acorns are Template:Convert long,<ref name="flora"/> with a moderately sweet kernel, and mature in 6–7 months from pollination.
Q. douglasii is monoecious and wind-pollinated. Flower buds take a growing season to develop into catkins.<ref name="fs"/> Blue oak pollen is severely allergenic.<ref>Template:Cite web</ref>
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.<ref name="IOS" />
Taxonomically it is placed in the white oak group<ref name="sudden">Template:Cite web</ref> (subgenus Quercus, section Quercus).
Ecology
Q. douglasii prefers dry to moist soil and plenty of sunlight.<ref name="Cal Poly" /> Its sparse foliage allows more light to reach the ground, and young trees may grow for decades below their parents' canopies.<ref name="fs" /> The species often co-habitates with gray pine (Pinus sabiniana),<ref name="fs" /><ref name="Peeters">Template:Cite book</ref> and is also found with interior live oak (Q. wislizeni), coast live oak (Q. agrifolia), valley oak (Q. lobata), Oregon white oak (Q. garryana),<ref name="fs" /> and canyon live oak (Q. chrysolepis).<ref name="Canyon">Template:FEIS</ref> Natural hybrids between Q. douglasii and the related Q. lobata, Q. garryana,<ref name="fs" /> 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<ref name="Alvord">Template:Cite web</ref> or Q. john-tuckeri.<ref name="fs" />
Old-growth blue oak woodland may be one of the most widespread old-growth forest ecosystems remaining in the state after European colonization.<ref name="Stahle2">Template:Cite journal</ref>
The acorns are eaten by wildlife and livestock.<ref>Template:Cite book</ref>
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.<ref name="fs"/> The leathery blue-green leaves contribute to its drought resistance;<ref name="hogan"/> 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.<ref name="fs"/>
Gall wasps
Author Ron Russo writes that Q. douglasii hosts the "largest number of known species" of gall wasps,<ref name="Russo">Template:Cite web</ref> at more than 50.<ref name="Wirka">Template:Cite web</ref> The wasps trigger the formation of oak galls in a wide variety of shapes, colors, and sizes.<ref name="Russo"/><ref name="Wirka"/>
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.<ref name="sudden"/>
Uses
The acorns can be eaten but, if bitter, may need to have the tannins leached.<ref>Template:Cite book</ref> 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.<ref name="fs"/><ref name="week">Template:Cite web</ref> Commercially, the blue oak is mainly limited to use as firewood.<ref name="hogan"/>