Quercus durata, commonly known as leather oak, is a species of oak in the white oak group (subgenus Quercus, section Quercus). The common name "leather oak" is derived from the leathery texture on the lop of its leaves. It is endemic to California, common in the Coast Ranges and the foothills of the Sierra Nevada. It is found nearly exclusively on serpentine soil.
Description Leather oak is a dicot. As described by English horticulturist and botanist Theodore Payne, it is a "rather low spreading shrub with rigid branches, foliage rich deep green. Desirable for hillside planting." Quercus durata is a short species of oak generally growing to 1–2 metres (3+1⁄2–6+1⁄2 feet) in diameter with a height of about 0.5–1.5 m (1+1⁄2–5 ft). In more extreme cases they have been known to grow to be 3–4 m across. The gray or yellowish twigs have scaly bark and are about 1–3 centimetres (1⁄2–1+1⁄4 inches) in diameter with the trunk diameter reaching 4–5 cm; the branches can be densely or sparsely distributed. The buds are smooth brown or reddish brown ovals. The leaf blades are cupped or convex, rarely somewhat planar; their dimensions are 1.5–3 cm long and 1–1.5 cm broad densely or sparsely distributed along the branches while the margins of the leaves can be entirely or irregularly toothed. The adaxial (upper) surface of the leaves are greenish or yellowish with short semi-erect hairs; the secondary veins are obscure, dense or scattered. The abaxial (lower) surface of the leaves are covered with erect rayed hairs 1–4 millimetres (1⁄16–3⁄16 in) in length that are felty to the touch with prominent secondary veins. It flowers in the spring, typically April or May, with male and female flowers on the same plant. The staminate flowers are in catkins, the pistillate ones singly or in pairs in leaf axils. The acorns of the leather oak can be found solitary or paired at the end of a small stalk; the cup which encloses up to half of the nut is reddish or yellowish with a scaly texture. The nut itself is cylindrical, measuring 1.5–2.5 cm in diameter and 1–2.5 cm long, the apex can be rounded or obtuse. Because leather oak is a species of white oak, its acorns mature in about 6 months, are hairless inside the acorn shell, and are sweet or slightly bitter tasting.
Var. gabrielensis The gabrielensis variant, found in the San Gabriel Mountains, has leaves that are not as densely crowded and still commonly have regular teeth along the edges. The upper surfaces are colored a darker green and are glossy, glabrous (without hair) and usually moderately cupped or sub-planar. The lower surface of the leaves have persistently wooly surfaces with hairs 2–4 mm in length.
Distribution, habitat and ecology The range of Quercus durata stretches from Shasta County in Northern California to the bottom of the South Coast Range in Santa Barbara County. The most common variant is found in foothill woodlands and chaparral ecosystems at elevations between 30 and 1,570 m (100 to 5,150 ft). It is especially abundant in Sargent cypress (Hesperocyparis sargentii) woodlands which can be found in portions of Mendocino, Sonoma, San Mateo, Santa Clara, Monterey, San Benito, San Luis Obispo and Santa Barbara counties as well as the San Francisco Bay region. In more sheltered, inland areas like MacNab cypress (Hesperocyparis macnabiana) woodlands this shrub can extend into riparian zones where summer fogs persist. Q. durata is well suited for ultramafic soils which are reddish and typically nutrient poor, having abundant nickel, magnesium, and chromium content while lacking calcium. Ultramafic chaparral communities are found below 500 m (1,500 ft) from Santa Barbara County north through the North Coast Ranges and also within the foothills of the Sierra Nevada and portions of the California Central Valley. Here, Q. durata can be found with shrubs including chamise (Adenostoma fasciculatum), coffeeberry (Frangula californica), buckthorn (Rhamnus crocea) and toyon (Heteromeles arbutifolia). Scattered occurrences of canyon live oak and coast live oak are also found here. In cases outside of serpentine environments, leather oaks typically occur as isolated individuals within a backdrop of plant associations dominated by California scrub oak (Q. berberidifolia) and interior live oak (Q. wislizeni). The climate in Q. durata habitat is characterized by 41 to 170 cm (16 to 67 in) of annual precipitation during a wet season of 3 to 8 months, temperatures range from a December low of 30 °F to a July high of 96 °F. It can sustain life in soil pH between 5.5 and 8.3. It is found nearly exclusively on serpentine soil it is a strong indicator species for this soil type. Based on controlled studies done at Jasper Ridge Biological Preserve the harsh environment of Q. durata limits species re-establishment after forest clearing or in colonization of grassland at chaparral-prairie boundaries. Low summer precipitation, high solar insolation, and herbivory are the basis for this difficulty rather than germination rates.
Var. gabrielensis Occurs at elevation 370–2,290 m (1,215–7,515 ft) as a limited distribution taxon in the San Gabriel Mountains within the San Bernardino and Los Angeles counties. The dry, exposed slopes of the San Gabriels stretching from La Cañada to Pomona are covered in a chaparral with non-serpentine soils suitable for Quercus durata var. gabrielensis. The precipitation, temperature, and soil pH ranges for the gabrielensis variant are much narrower than the common variant sitting at approximately 24-47 in of water annually, 44-51 °F, and 6.4 to 6.9 pH respectively. The December low in this region is 36 °F while the July high can reach 90 °F.
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