Blue oak (Quercus douglasii): what the agencies record

This page reports what US federal publications say about one tree. It does not identify trees from descriptions or photos and does not say what should be done with any tree. The family overview, tree species in the agency records, explains the sources and holds the comparison table. A general page on oak galls covers the gall wasps the handbook counts on this tree.

At a glance

Item	What the source records
Scientific name	Quercus douglasii Hook. & Arn., beech family (Silvics and PLANTS, symbol QUDO)
Other names, Silvics	Iron oak, mountain white oak and mountain oak; named for its blue-green foliage
Native status, USDA PLANTS	Native in the lower 48 states
Size, Silvics	Seldom over 125 cm (49 in) in breast-height diameter or 25 m (82 ft) tall; a champion in Alameda County, California, measured 196 cm (77 in) across, 28.7 m (94 ft) tall, with a crown spread of 14.6 m (48 ft)
Weight table	Not published for blue oak: NRS-38 lists it in Table 4 only, with specific gravities assigned from the genus average in Table 1A (footnote a), 0.59 on a green volume basis and 0.66 at 12 percent moisture content, and gives no green weight per cubic foot. The Wood Handbook strength table has no row for it

Native range and climate

The Silvics of North America handbook (Agriculture Handbook 654, Volume 2, 1990) calls blue oak a California endemic with a north-south range of 740 km (460 mi), in general surrounding the Central Valley. The northern limit is Montgomery Creek in Shasta County, the southern limits are the Liebre Mountains of Los Angeles County and the Santa Ynez Valley of Santa Barbara County, and trees also grow on Santa Cruz and Santa Catalina Islands. Elevations run from 50 to 1,800 m (165 to 5,900 ft). The handbook describes hot dry summers and cool wet winters, with a mean maximum July temperature of 32 degrees C (90 degrees F), a mean January minimum of minus 1 degree C (30 degrees F), a frost-free season of 150 to 300 days, and annual precipitation of 510 to 1,020 mm (20 to 40 in) in the main range, 60 to 90 percent of it between November 1 and April 30.

Soils and forest cover

Silvics says blue oak soils are characteristically shallow, skeletal, infertile and moderately to excessively well drained, often with extensive rock fragments, with as much as 50 percent of the surface covered by stones or rock outcrops. More than 40 California soil series are named as supporting it. The one characteristic found most often is high base saturation. Soils within the range that do not carry blue oak are generally poorly drained or heavy clay, often with a hardpan, and deep fertile soils are seldom occupied because blue oak cannot compete with taller conifers, interior live oak and California black oak. It is the principal part of the Blue Oak-Digger Pine cover type (Society of American Foresters type 250), with Digger pine its most common tree associate, and the handbook says it should be regarded as a component of a mosaic that includes savanna, chaparral, other oaks and at least one common conifer. Blue oak sheds leaves when moisture stress becomes prohibitive, and one dry-year reading showed predawn stress of 27 atmospheres on an upland blue oak against 2 to 5 on nearby valley and coast live oaks.

Acorns and wildlife

The handbook describes a monoecious tree that flowers from late March to mid-May. Acorns mature in one growing season, are 2.5 to 4.0 cm (1.0 to 1.6 in) long and 12 to 21 mm (0.5 to 0.8 in) across, and fall mostly between mid-September and the end of October. Abundant crops come every 2 to 3 years and bumper crops every 5 to 8. One average-sized tree produced 73 kg (160 lb) of acorns in a good year. Silvics names the filbert weevil and filbertworm as the two insects whose larvae destroy many acorns before maturity. Principal consumers include acorn woodpeckers, scrub jays, band-tailed pigeons, California quail, western gray squirrels and California ground squirrels, and the acorns are called critical to migrating deer moving from the Sierra Nevada to winter range. Silvics also notes the acorns were a favored food of California Indians.

Reproduction and sprouting

Silvics says blue oak has the potential to reproduce well from seed but appears to have reproduced poorly for the past 50 to 80 years before 1990: in Tulare County only 7 percent of 405 trees were under 60 years old, and a 0.8 km (0.5 mi) stretch of fuelbreak in southern Shasta County held only eight seedlings. The handbook lists heavy acorn and seedling losses to deer, cattle, sheep, hogs, insects, rodents, ground squirrels and pocket gophers, and possible chemical inhibition of germination by introduced annual grasses, and says no single factor seems enough to explain the shortage. Germination is hypogeal, acorns do not need stratification, and the seed must be covered. The tree is regarded as a weak sprouter after cutting or fire: stumps over 40 cm (16 in) across showed no sprouting in a Shasta County study.

Growth, age and roots

The handbook says about 90 percent of trees in natural stands are single stems, height growth is slow and after 65 cm (26 in) in diameter nearly stops. On good sites in Nevada and Shasta Counties, trees 20 cm (8 in) across were 40 years old and those 60 cm (24 in) across averaged 125 years; on poorer sites trees 36 to 51 cm (14 to 20 in) across were 175 to 280 years old, and a large tree in Sequoia National Park was 390 years old. Epicormic branching is common at all ages. Silvics reports a deep taproot system, with about 73 percent of dry weight below ground in the first growing season and roots of an 18 cm (7 in) tree reaching groundwater 24 m (80 ft) down. It classes blue oak as intolerant of shade and says it is rarely found in an understory.

Damaging agents the handbook names

Fire. The bark is thin relative to other oaks, catches fire easily and gives little protection, but leaves killed on part of the crown by ground fire are replaced in the following growing season with no apparent ill effect.
Animals. Damage is mostly loss of foliage to deer, cattle and other browsers and root injury by pocket gophers, with seedlings the most vulnerable.
Heartwood and root diseases. Silvics calls heartwood rots of trunk and large limbs probably the most severe: Inonotus dryophilus (white pocket rot), sulphur conk (brown cubical rot), hedgehog fungus and artist's fungus. Armillaria mellea is named as a root rot that gradually weakens trees at the base until they fall.
Leaf diseases and mistletoe. Powdery mildews are common. An unknown disease of blue oaks in well-watered lawns kills nearly every leaf in midsummer, with normal leaf development in the spring that follows. True mistletoe often infects older open-grown trees and its effect on them is undetermined.
Insects and galls. One study recorded 38 insect species in 21 families on blue oak. A local epidemic of the fruit-tree leafroller defoliated blue oaks in Contra Costa County in the early 1970s, with a second crop of leaves by mid-July. More than 40 species of cynipid wasps form galls on every part of the tree, including the spined turban gall wasp and the jumping oak gall wasp.

For who takes reports of regulated pests, see reporting a pest in the United States, and pests and diseases by country sets out which ones agencies regulate.

Wood, range use and hybrids

Silvics calls the wood strong, hard and heavy but of little or no commercial use because of the tree's short stature and poor form, with products limited to fenceposts and fuelwood. It records that oaks have been removed on rangeland to raise forage, that grass under low or moderate blue oak density was taller and stayed green longer, and that living oak roots hold soil on steep slopes. It says blue oak hybridizes with valley oak, Oregon white oak, California scrub oak and turbinella oak, and that the hybrid with turbinella oak, Alvord oak, is the dominant oak in some foothill woodlands.