|A large blue oak in a pasture in Mariposa County, California|
|Subgenus:||Quercus subg. Quercus|
|Section:||Quercus sect. Quercus|
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. It is California's most drought-tolerant deciduous oak, and is a dominant species in the blue oak woodland ecosystem. It is occasionally known as mountain oak and iron oak.
Quercus douglasii is a medium-sized tree with sparse foliage, generally 6–20 meters (20–66 feet) tall, with a trunk 36–60 centimeters (1–2 ft) in diameter at breast height. Trunks are typically solitary, but some trees have multiple trunks. The tallest recorded specimen was found in Alameda County, at 28.7 m (94 ft). The trees grow slowly, about 30 cm (12 in) per year. Individual trees over 500 years old have been recorded.
The bark is light gray with many medium-sized dark cracks. The blue-green leaves are tough and leathery, deciduous, 4–10 cm (1+1⁄2–4 in) long, and entire or shallowly lobed. The acorns are 2–3 cm (3⁄4–1+1⁄4 in) long, 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. Blue oak pollen is severely allergenic.
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 group (subgenus Quercus, section Quercus).
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 ecosystems remaining in the state after European colonization.
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.
Author Ron Russo writes that Q. douglasii hosts the "largest number of known species" of gall wasps, 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 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.
Native Californians commonly gathered Quercus douglasii 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. The acorns are eaten by wildlife and livestock. They can be eaten but, if bitter, may need to have the tannins leached.
- ^ Not to be confused with Quercus gambelii Nutt., a separate species.
- ^ Beckman, E. (2016). "Quercus douglasii". IUCN Red List of Threatened Species. 2016: e.T78914533A78914539. doi:10.2305/IUCN.UK.2016-1.RLTS.T78914533A78914539.en. Retrieved November 12, 2021.
- ^ a b Wyly, Zarah (February 8, 2019). "Species Spotlight: Quercus douglasii Hook. & Arn". International Oak Society. Retrieved July 14, 2019.
- ^ "Quercus douglasii". World Checklist of Selected Plant Families. Royal Botanic Gardens, Kew – via The Plant List.
- ^ a b c d Hogan, C. Michael. "Blue Oak Quercus douglasii". GlobalTwitcher. Archived from the original on May 26, 2013. Retrieved July 14, 2019.
- ^ a b c d e f g h i j k l Fryer, Janet L. (2007). "Quercus douglasii". Fire Effects Information System (FEIS). US Department of Agriculture (USDA), Forest Service (USFS), Rocky Mountain Research Station, Fire Sciences Laboratory.
- ^ a b McDonald, Philip M. (1990). "Quercus douglasii". In Burns, Russell M.; Honkala, Barbara H. (eds.). Hardwoods. Silvics of North America. Washington, D.C.: United States Forest Service (USFS), United States Department of Agriculture (USDA). Vol. 2 – via Southern Research Station.
- ^ "Quercus douglasii". Calflora. Berkeley, California: The Calflora Database.
- ^ a b Nixon, Kevin C. (1997). "Quercus douglasii". In Flora of North America Editorial Committee (ed.). Flora of North America North of Mexico (FNA). Vol. 3. New York and Oxford – via eFloras.org, Missouri Botanical Garden, St. Louis, MO & Harvard University Herbaria, Cambridge, MA.
- ^ Ritter, Lyman V. "Blue Oak Woodland" (PDF). California Department of Fish and Game. Archived from the original (PDF) on August 10, 2009. Retrieved April 12, 2022.
- ^ a b "Quercus douglasii Tree Record". SelecTree. Cal Poly San Luis Obispo - Urban Forest Ecosystems Institute. Retrieved July 14, 2019.
- ^ Stahle, David. "Ancient Blue Oak Woodlands of California". University of Arkansas Tree-Ring Laboratory. Archived from the original on December 4, 2008. Retrieved July 14, 2019.
- ^ "Blue Oak (Quercus douglasii) Species Details and Allergy Info, Santa Clara County, California". PollenLibrary.com.
- ^ a b Rizzo, David M.; Garbelotto, Matteo; Davidson, Jennifer M.; Slaughter, Garey W.; Koike, Steven T. (2002). "Phytophthora ramorum and Sudden Oak Death in California: I. Host Relationships" (PDF). U.S. Forest Service. Retrieved July 14, 2019.
- ^ Peeters, Hans J.; Peeters, Pam (2005). Raptors of California. University of California Press. p. 147. ISBN 9780520242005.
- ^ Tollefson, Jennifer E. (2008). "Quercus chrysolepis". Fire Effects Information System (FEIS). US Department of Agriculture (USDA), Forest Service (USFS), Rocky Mountain Research Station, Fire Sciences Laboratory.
- ^ "Plants Profile for Quercus alvordiana (Alvord oak)". Plants Database. USDA Natural Resources Conservation Service. Retrieved July 15, 2019.
- ^ Stahle, D. W.; Griffin, R. D.; Meko, D. M.; Therrell, M. D.; Edmondson, J. R.; Cleaveland, M. K.; Stahle, L. N.; Burnette, D. J.; Abatzoglou, J. T.; Redmond, K. T.; Dettinger, M. D.; Cayan, D. R. (May 22, 2013). "The Ancient Blue Oak Woodlands of California: Longevity and Hydroclimatic History". Earth Interactions. 17 (12): 1–23. Bibcode:2013EaInt..17l...1S. doi:10.1175/2013EI000518.1. S2CID 130820896.
- ^ a b Russo, Ron (July 1, 2009). "Call of the Galls". Bay Nature Magazine. Bay Nature Institure. Retrieved July 15, 2019.
- ^ a b Wirka, Jeanne (September 22, 2015). "Nature: Sonoma County is bursting with galls". The Press Democrat. Retrieved July 15, 2019.
- ^ Gauna, Forest. "Plant of the Week: Blue Oak (Quercus douglasii Hook. & Arn.)". Celebrating Wildflowers. U.S. Forest Service. Retrieved July 15, 2019.
- ^ Little, Elbert L. (1994) . The Audubon Society Field Guide to North American Trees: Western Region (Chanticleer Press ed.). Knopf. p. 395. ISBN 0394507614.
- ^ Elias, Thomas S.; Dykeman, Peter A. (2009) . Edible Wild Plants: A North American Field Guide to Over 200 Natural Foods. New York: Sterling. pp. 228, 231. ISBN 978-1-4027-6715-9. OCLC 244766414.