BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Quercus douglasii | Blue Oak 



GENERAL BOTANICAL CHARACTERISTICS : 
Blue oak is a native, deciduous, flood-tolerant, drought-resistant tree
[28,53].  It is generally short and straight, from 20 to 66 feet (6-20
m) in height and from 14 to 24 inches (36-60 cm) in d.b.h.  [16].  The
acorns are from 0.8 to 1.2 inches (2-3 cm) long [53].  The bark is thin
and flaky [19].  Roots are extensive, and may grow through fractured and
jointed rock to a depth of 80 feet (24 m) or more to tap groundwater
reserves [39].  Stands are typically from 90 to 100 years old [19].  The
oldest known blue oak is in Sequoia National Park and is about 400 years
old [16].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Sexual:  The age of sexual maturity is unreported for this species.
Catkins develop from flora primordia formed during the previous growing
season.  During drought, these flower buds may not develop [4].  Acorns
mature in 1 year [17].  Abundant crops are produced every 2 to 3 years,
with bumper crops every 5 to 8 years [57].  One 38-foot (11.6 m) blue
oak in Shasta County produced 3,750 acorns during a favorable season
[16].  Fresh acorns collected by Mirov and Kraebel [51] from various
locations around the state were 72 percent viable.  Acorns are
disseminated by various animals.  Magpies, scrub jays, and various
rodents bury them in caches, resulting in high rates of emergence [27].
The acorns are capable of immediate germination.  Germination may be
epigeal or hypogeal, with buried acorns showing greater recruitment [14].
At plots in various Sierra Nevada locations, germination was initiated
at the first rainfall and slowly continued through winter [42].
Germinants are susceptible to fungal infection during this time, and
many acorns in the various plots rotted over winter.  A study conducted
in Berkeley and Mendocino Counties found that emergence was greatest at
75 percent of normal rainfall [45].  Greater than normal rainfall
results in high rates of germinant death due to damping-off fungi.

Blue oaks show rapid, early root elongation prior to shoot development
[43].  Seedlings growing on the canopy edge or in open positions
establish more frequently than those beneath the parent tree.  Seedlings
do not compete well with annual grasses.  Radicles of unburied acorns
often fail to reach the soil surface before desiccation when growing
through the thatch of these grasses.  Additionally, annual grasses often
outcompete blue oak seedlings for water and light [15].  Nonnative
annuals now represent 50 to 90 percent of ground cover in blue oak
woodlands, and may have irreversably altered the seasonal availability
of soil moisture to blue oak seedlings [10].  

Because of a flush of blue oak establishment that occurred statewide
from 1850 to 1900, it has been suggested in the literature that
recruitment of this species occurs in episodic bursts [4,68,70].
Tree-ring age analysis of trees in Kern County, however, showed blue
oak recruitment to be fairly continuous from 1570 to 1850, when the
seedling flush occurred [50].  Successful establishment of this species
depends upon a favorable combination of many factors.  Major factors
include abundant acorn production, escape from acorn predation,
sufficient rainfall, protection from desiccation during germination,
limited competition for light and water, and escape from browsers and
burrowing gophers [44].  Seedling recruitment is successful in some
areas, but few blue oak survive to the sapling stage.  Lack of sapling
recruitment was once attributed mainly to livestock herbivory.  At the
San Joaquin Experimental Range, however, few blue oak have reached
sapling size despite lack of livestock grazing since 1934; lack of
sapling recruitment there is attributed to wildlife herbivory [21].
McClaran and Bartolome [47] suggest that seedlings must grow quickly
enough to surpass the browse line in 10 to 13 years for new stand
establishment, and that this may not be possible during periods of
prolonged drought.

In a statewide study, sapling stand establishment varied according to
geographical location [52].  In the northern Sierra Nevada, the steeper
slopes supported the greatest number of saplings.  Along the Delta and
in the central Coast Ranges, saplings were more frequent on mesic
slopes.  In the southern Sierra Nevada, sapling frequency was greatest
where shrub cover was low.

Vegetative:  Blue oak produces root crown and coppice sprouts after
cutting or burning but is generally regarded as a weak sprouter [16].
Sprouting ability varies with geographic location and may be poor or in
some areas nonexistent [19,72].  Frequent resprouting may result in
bushlike or stunted trees [19].  Sprouting is vigorous in some
locations, however.  At the University of California's Sierra Foothill
Range Field Station on the east side of the Sacramento Valley, coppice
and root crown sprouts grew rapidly from experimentally-cut trees
measuring 4 to 36 inches (10-91 cm) in diameter [36].  Sprouting ability
declines with age.  Mature trees produce a greater number of coppice
sprouts, which are less vigorous and show higher mortality rates than do
root crown sprouts [27].  Very old trees either do not sprout or produce
only coppice sprouts [16].  Winter cutting or burning results in more
vigorous sprouting than tree removal in other seasons [49].


SITE CHARACTERISTICS : 
Soil:  Blue oak grows in soils derived from a variety of parent
materials.  Soils are characteristically shallow, skeletal, infertile,
thermic, and moderately to excessively well drained.  Soil textures
range from gravelly loam to gravelly clay-loam [16].

Climate:  Blue oak occurs in a Mediterranean climate, with hot, dry
summers and cool, wet winters.  The mean maximum July temperature is 90
degrees Fahrenheit (32 deg C); the mean minimum January temperature
is 30 degrees Fahrenheit (-1 deg C).  The frost-free growing season
varies from 150 to 300 days.  Annual precipitation ranges from 20 to 40
inches (510-1,020 mm), with most occurring between November and April
[16].

Elevation:  Blue oak ranges in elevation from 165 feet (50 m) at the
northern Central Valley floor to 5,900 feet (1,800 m) in its
southernmost distributional limits [16].


SUCCESSIONAL STATUS : 
Blue oak is shade intolerant [22,30,52].


SEASONAL DEVELOPMENT : 
Acorns planted at the Hastings Reservation in Carmel Valley emerged from
late February to late March [26].  The following seasonal development
was reported for blue oak in Sequoia National Park [4]:

     leaf buds swell:  January to mid-May
     stem elongation:  February to mid-May
     new leaves appear:  mid-March to May
     catkins emerge:  March to mid-June
     leaves fall:  August to mid-November

Acrorns are disseminated from late summer to late fall [48].  Blue oak
undergoes premature leaf abscission during drought [16].

Related categories for Species: Quercus douglasii | Blue Oak