Blue Oak
Quercus douglasii
The blue oak is a medium-sized deciduous tree, growing 15-28 meters tall, with an irregular, crooked crown. The light gray bark is linear with many shallow fissures, creating a thick, flake-like effect. The dull blue-green leaves, which give the oak its name, are thick and waxy with shallow, wavy lobes. Each leaf is 4-10 cm long. The blue oak is monoecious, with yellow-green male catkins and inconspicuous female flowers. The thick acorns, about 2 cm long, have a warty, tan cap.
Basic Information
Member of the Fagaceae, or oak, family
Winter deciduous tree
Slow-growing, reaching a height of 25 meters
Typically only grow a few inches per year
Average lifespan of 300 years
Habitat
Native from central to southern California, though it primarily grows in the Central Valley
Endemic to the Central Valley of California, the North and South Coast Range, and San Francisco Bay Area
Found growing in California’s foothills as part of oak savannas and chaparral woodlands
Ecological Role
Oaks are a keystone species, supporting and homing a wide variety of wildlife
Acorns, leaves, and roots provide food for many birds, butterflies, amphibians, and insects
Large, dead, or damaged oaks are important for cavity-seeking mammals and birds
Hosts the larva for a number of butterflies including the California Sister, Propertius Duskywing, Mournful Duskywing, Golden Hairstreak, and Gold-Hunter's Hairstreak
Larval food plant for for the mournful duskywing butterfly (Erynnis tristis)
Supports many lichens, mosses, and parasitic plants, like mistletoe
The blue oak is the most drought tolerant of California’s deciduous oaks
The leaves are thick and waxy with underside hairs to help prevent water loss
Leaves can make adjustments to survive even a 30% water loss
Prevent wilting by increasing salt levels in photosynthetic cells
Build more cellulose and lignins to
Thinner leaf canopy
Extensive roots can grow through fractured and jointed rock up to 80 ft. or more deep to reach groundwater reserves
May go dormant during long drought or extreme heat, shedding its leaves, flower buds or developing acorns
To survive wildfires, the tree resprouts from the root crown
Reproduction
Blooms in from March to May
Monoecious, with separate male and female flowers on the same plant
Wind pollinated, like many oaks
Acorns mature quickly, about 6-7 months after pollination
Alternative Names
Iron Oak, Mountain Oak
Historical Uses
Native Americans in the Central Valley had a number of uses for the blue oak:
Acorns were an important food for many tribes, including the Miwok, Yokut, and Kawaiisu
The leaves were chewed for sore throats
A poultice of the ground oak galls and salt was applied to burns, sores, and cuts
Acorn leachate used for dying baskets
Wood was used to make bowls
Additional Information
Produces an especially abundant acorn crop (a mast) in irregular cycles, about every 5 years
Old-growth blue oak woodland may be one of the most widespread old-growth forest ecosystems remaining in California after European colonization
The Blue oak appears to be resistant to the Sudden Oak Death (SOD)
