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Blue Oak

Quercus douglasii

Summary 4

Quercus douglasii, the Blue Oak, is an oak in the white oak section of the genus, Quercus sect. Quercus. It is endemic to California and is found in foothills surrounding the Central Valley of California and certain locations in the California Coast Ranges, USA. It is also sometimes known as the Mountain Oak, and occasionally the Iron Oak.

Description 5

General: Oak Family (Fagaceae). Blue oak is a deciduous tree that is endemic to California. It has a rounded crown and grows from 6-20 m. high. The gray bark is shallowly checked into small thin scales. The leaves are shallowly lobed and blue-green above. The acorns mature in one year, are oval, and have slightly tubercled scales.

Distribution 6

More info for the term: natural

Blue oak is native and endemic to California [98,114,116,188,196]. It is very common within its narrow range, dominating almost half of California's oak woodlands [196]. It occurs in valleys and on low slopes of the Coast Ranges and on low foothills of the Sierra Nevada. The core area of blue oak distribution almost completely encircles the Central Valley, from Shasta County in the north to Los Angeles County in the south [98,114,116,188]. Some blue oak populations extend into the Central Valley. Blue oak reaches its northernmost distribution in Del Norte County and its southernmost distribution in Riverside County. Isolated populations occur in the Siskiyou, Klamath, and Trinity mountains, east of the Cascade Range, and on Santa Cruz and Santa Catalina islands [98]. Populations in coastal southern California and on the Channel Islands consist of small stands or solitary individuals, and some doubt exists as to whether some or all of those populations are natural stands or are historical introductions near Native American settlements [72]. The Jepson Flora Project provides a distributional map of blue oak.

Quercus × alvordiana is the most common of the blue oak hybrids and frequently forms hybrid swarms. The Q. × alvordiana complex is a variable group of semideciduous oaks that are a "conspicuous part" of the vegetation on the inner Coast Ranges from Carmel Valley in Monterey County south to the Tehachapi Mountains. Q. × alvordiana displaces blue oak as the dominant foothills oak in parts of that range. Although Griffin and Critchfield [98] describe Q. × alvordiana as an "unsatisfactory" taxonomic unit, they concede that these "problem oaks should be considered if the southern distribution of blue oak is to be fully understood." The Jepson Flora Project provides a distributional map of Q. × alvordiana.

Habitat characteristics 7

Blue oak grows on low-elevation slopes and foothills [114]. It is usually restricted to dry sites [93,114], although it occasionally grows on spring-fed and other moist soils [13]. It is reported as flood intolerant [129] to intermediate in flood tolerance [188,249]. Flood tolerance may depend on depth of the root system and/or soil depth. When the water level of Black Butte Reservoir was raised for 50 to 98 days, flooding both alluvial and shallow soils, blue oaks on alluvial soils suffered little mortality. Blue oaks on shallow soils suffered 50% mortality [109].

Soils: 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 clay [80,173]. Blue oak can grow over hardpans [126]. A study in Sequoia National Park found blue oak woodland soils were lower in nitrogen, phosphorus, and organic matter content compared to soils of an adjacent mixed-evergreen woodland [18]. In a San Luis Obispo County study comparing soils on sites dominated by blue oak and sites dominated by coast live oak, blue oak occupied erosional soils that were relatively more acidic and had finer textures than soils with coast live oak. Subsoil pH on blue oak sites ranged from 3.9 to 7.9 [67].

Climate: Blue oak occurs in a mediterranean climate, with hot, dry summers and cool, wet winters. In summer, midday temperatures in blue oak woodlands can exceed 100 °F (38 °C) for weeks at a time [196]. The mean maximum July temperature is 90 °F (32 °C); the mean minimum January temperature is 30 °F (-1 °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 [173]. Using blue oak tree-ring chronologies from the 18th, 19th, and 20th centuries, Gervais [81] found blue oaks in the Tehachapi Mountains experienced "disproportionately" long periods of both extreme drought and heavy precipitation, with "normal" or mean precipitation poorly representing the extreme ranges. For example, there was 10-year drought in the 1770s, while the 1790s was an extremely wet decade [81].

Elevation: Blue oak typically occurs below 3,900 feet (1,200 m) elevation [114]. Its elevational range is from sea level on the Central Valley floor to 5,900 feet (1,800 m) in its southernmost distributional limits [72,173]. In Sequoia National Park, blue oak occurs from 2,000 to 3,000 feet (600-800 m) on south-facing slopes and below 1,600 feet (500 m) on north-facing slopes [18].

Establishment 8

Adaptation: Blue oak woodlands cover 3 million acres in California, one-half of all oak covered lands. It is the most abundant hardwood forest type in the state. Blue oak may occur in near pure stands in dense woodland or savanna. It may also occur as a dominant in mixed stands that include foothill pine (Pinus sabiniana), interior live oak (Quercus wislizenii), valley oak (Quercus lobata), and/or coast live oak (Quercus agrifolia), or as a minor component in mixed stands of oaks and other hardwoods.

If possible, gather acorns from many different trees locally, to maintain genetic diversity of blue oak and to ensure that the plants are adapted to the site. The seeds of most oaks are short-lived and must be sown or refrigerated quickly, otherwise they lose their ability to germinate. They are best gathered directly from the tree or from the ground within a short time of their dropping, usually within several days. To test their ripeness, select an acorn that is still in its cap and twist it lightly. If it pops out of the cap, the acorn is ripe. Store the seeds without their caps in a grocery sack until ready to plant. The seeds can be sown in autumn outdoors (see under direct seeding), or if the seeds are started in pots in the greenhouse in the early spring, they should be stratified for one and one-half months between 33 and 40 degrees in a refrigerator in a mix.

Fill a gallon Ziploc™ bag about half full with acorns, and then add about a cup of dry perlite and a little bit of vermiculite (3:1 mixture). Shake the bag to distribute the perlite around the acorns, label the bag, and place it in the refrigerator. Check the bag weekly and discard moldy acorns. Any acorns that have germinated need to be taken out of the bag and sown. Plant the seeds on their sides directly in long, deep pots (2 in. in diameter by 10 inches long) with potting soil and a slow release fertilizer. Water the tubes and place them in diffused light in the nursery or outside, making sure to keep each tube suspended off the ground or bench so the large air holes at the bottom of the tube are exposed. When the tree roots hit the air, they'll stop growing. Water the pots when the surface of the soil is dry to the touch. Do not over-water.

Plant the seedlings the following winter in a sunny location and water the transplants to ensure sufficient moisture and eliminate air pockets. Be careful to keep the soil from falling off of the roots. Place the seedlings in the ground such that the top of the soil from the container is even with the ground line. Use a pick mattock for planting, which has a pick on one surface and a triangular wedge on the other surface. Break up the soil, which will foster root growth. Protect the seedlings from weeds, drying winds, grasshoppers, and small and large mammals that might feed on the roots, leaves, or trunks. Also, water the seedlings deeply the first summer. A good seedling protector is essential and can be a simple wire window screen mesh and wooden stake. Bend the mesh into a cylinder that is about 6-8 inches in diameter and 3 feet tall. Attach the mesh cylinder onto the wooden stake with staples or carpet tacks. Place the tube over the seedling and pound the stake into the ground. Seedlings should be kept free of vegetation for 2 to 3 feet surrounding each seedling.

Direct Seeding: Pick acorns without insect exit holes or diseases. Store acorns for one and one half months in a sealed plastic bag in the refrigerator. Dig a hole with the trowel, shovel, or hoe and be sure to break up the soil much deeper (one to two feet) than the acorn is planted and backfill with loosened soil to accommodate the growing of roots. Plant the acorns on their sides, one to two inches deep in the fall of the year. Plant several acorns in each hole and thin multiple seedlings down to a single most vigorous plant. Water the acorns deeply the first summer. Weed several feet around each seedling for several years. Protect the seedlings from animals until well established.

Fire ecology 9

More info for the terms: association, cover, density, fire exclusion, fire frequency, fire regime, fire severity, fire suppression, fire-return interval, forbs, frequency, fuel, hardwood, low-severity fire, mean fire-return interval, natural, root crown, severity, shrub, surface fire, top-kill, tree, wildfire

Fire adaptations: Blue oak sprouts from the root crown and/or bole after top-kill by low- to moderate-severity surface fire [28,103,154,165,173,176,176,235,236,253]. Young blue oaks are best adapted to sprout after top-kill [235,236]. Blue oak probably establishes from acorns after fire, likely from several sources including animal-dispersed acorns and acorns dropping from surviving parent trees.

Ability to sprout decreases with blue oak age [130,235]. Bark of mature blue oak bark is thin compared to bark of most mature, associated oaks, and it tends to flake off as trees age [235], so blue oaks are less insulated against fire than associated oaks. Longhurst [159] noted blue oaks on the Hopland Field Station sprouted less "vigorously" as they aged, with seedlings showing the most vigorous sprouting after top-kill. In a 13-county study, blue oak sapling recruitment was positively associated with fire (P≥0.01) [226].

Blue oak's ability to withstand extreme drought by dropping leaves under water stress and producing a flush of new leaves when wet weather returns probably also aids in blue oak's postfire recovery. In wet years, crown-scorched blue oaks may produce a flush of new leaves soon after fire [100].

FIRE REGIMES:
Ignition sourcesIn contrast to higher-elevation ecosystems, lightning ignitions are relatively rare in California's oak woodlands [95,204]. For example, a mean of 23 lightning strikes/million acres occurred over 10 years in a mixed-oak woodland spanning Amador and El Dorado counties. Strike rate in higher-elevation conifer sites on the El Dorado National Forest was 148 lightning strikes/million acres [137]. People may have historically been, and continue to be, the primary cause of ignitions in blue oak woodlands [221]. Lightning must have ignited some fires in prehistoric blue oak woodlands, though. Fire spread from more fire-prone adjacent ecosystems, such as chaparral and low-elevation ponderosa pine woodlands, was likely before fire exclusion [35]. Even given the low number of lightning strikes in blue oak ecosystems, lightning fires probably burned considerable acreage. A history of lightning-ignited fires in the lower foothill region found that in 1936, 11 lightning-ignited wildfires burned about 10 square miles before the fires were suppressed. It is likely that total acreage burned would have been much larger had the fires been allowed to spread [204]. The low incidence of lightning in blue oak and low low-elevation woodlands, however, may have increased the relative impact of Native American-set fires in blue oak woodlands [256].

Historic FIRE REGIMES Blue oak woodlands historically had a regime of frequent summer and fall surface fires, fueled by groundlayer perennial bunchgrasses and forbs and downed woody debris [8,101,164,165,176,212]. In 1902, Leiberg [154] noted that wildfires were "extensive" in blue oak-gray pine communities in the foothills of the Sierra Nevada, and that blue oak sprouted from the root crown or stump after wildfire or cutting. Blue oak ecosystems have experienced 3 periods with differing FIRE REGIMES: the presettlement, settlement (approximately 1850-1920), and postsettlement periods (after 1920). Presettlement and settlement FIRE REGIMES were most favorable to blue oak populations.

Presettlement period: Surface fires occurred about every 8 to 10 years in presettlement blue oak ecosystems. In the foothills of the northern Sierra Nevada, median fire-return interval in the presettlement era was 8 years, with minimum and maximum intervals of 2 and 49 years, respectively [165]. In a fire history study on 2 blue oak woodland sites on the Sierra Foothill Range and Field Station, McClaran [164] found blue oak woodlands historically experienced frequent surface fires. Percentage of blue oaks with fire scars ranged from 10% to 65% across sites. Fire frequency increased from presettlement intervals after the Gold Rush (1852), then dropped again in the late 1940s. Mean fire-return intervals on the 2 sites were 8.3 and 7.7 years from 1890 to 1948. No fires were detected from 1948 to 1958. There was a strong positive relationship between fire and subsequent successful blue oak establishment on both sites (P<0.025) [164].

A fire history study of isolated redwood groves in Annadel State Park found fire-return intervals ranged from 6.2 to 23.0 years before the early 1800s, with 67% of the intervals between 2 and 10 years. The redwood groves were small and surrounded by oak woodlands where blue oak was common to dominant, and by mixed-evergreen forests where coast Douglas-fir was common to dominant. Finney and Martin [71] concluded that the fire history recorded in the redwood groves probably reflected the fire regime of surrounding oak woodlands and mixed-evergreen forests. As of 1990, the Park had experienced 2 fires since fire exclusion began in the 1900s. In the absence of frequent surface fires, coast Douglas-fir was invading the oak woodlands but not the redwood groves [71].

Native American use of fire: There is high probability that Native American use of fire had important effects on foothills vegetation [256], although historical accounts of Native American use of fire in blue oak woodlands are inconclusive [156]. Based on sparse historical records, Sampson [212] concluded in a 1944 report that Native American use of fire in blue oak woodlands was negligible, with "the most extensive and destructive fires occurring since the coming of the white man." Lewis [156] proposed that Native American use of fire may have been important, but acknowledged a dearth of conclusive information. Early pioneers' accounts of Native American use of fire rarely distinguished between fires in the very low-elevation California prairie and the slightly higher-elevation blue oak savannas and woodlands [156]. It is likely, however, that Native Americans set frequent, low-severity fires in blue oak woodlands. Although blue oak acorns were not preferred for making meal, the abundance of blue oaks in the lower foothills made blue oak acorns an important food source for Native Americans. Native Americans used surface fire in blue oak woodlands to kill acorn weevils, which damage acorn crops [15]. A Mono tribeswoman specified blue oak as one of the species intentionally burned to produce sprouts for basketry (Turner, personal communication in [14]). Jepson [131] stated it was likely that Native American burning helped keep blue oak woodlands adjacent to chaparral or ponderosa pine woodlands from shrub and ponderosa pine invasion. Greenlee and Moldenko [91] suggested Native Americans burned low-elevation oak woodlands every 1 to 2 years, so fire severity would have been very low. Agee and Biswell [8] surmised that Native Americans set low-severity surface fires in spring or late fall in the blue oak woodlands of what is now Pinnacles National Monument.

Settlement FIRE REGIMES: Fire frequency increased during the settlement period due to rangeland burning by ranchers and wildfires in the gold fields [165]. A fire history study on 3 blue oak woodland sites in the Tehachapi Mountains found that prior to European settlement around 1856, blue oak recruitment occurred at a relatively steady rate, and the woodland had open structure. Mean fire-return interval in the presettlement period was 10 years. A burst of blue oak recruitment occurred in the 1850s and 1860s, when fire frequency increased during the settlement period (x=4.5-year return interval). Since the 1860s, the blue oak woodland had been used as livestock rangeland. Fires were suppressed, with only a single fire that occurred in the 1920s. The blue oak woodland had increased in density compared to presettlement times, and there was almost no blue oak regeneration with cattle grazing and fire exclusion [176]. Repeat photography studies near Sequoia-Kings Canyon National Park showed a "large increase" in blue oak cover and density beginning in the late 1800s, when Native American fires ceased and livestock grazing began. In 1981, most blue oaks were 60 to 100 years old, with few young trees [99].

The policy of fire exclusion began in higher-elevation forests before it was practiced in blue oak woodlands. Fire exclusion was officially adopted as policy in California 1905, but active fire suppression in blue oak woodlands only began in the 1930s [54]. Prior to the 1930s and 1940s, many ranchers used frequent prescribed surface fire to increase forage production in blue oak woodlands [154,164,212]. A fire history study of a mixed oak-foothills pine-ponderosa pine community was conducted in El Dorado County, using stumps of logged ponderosa pine. The site was logged in 1952, and the fire history spanned the settlement and postsettlement periods from 1850 to 1952. Blue oak was not a dominant oak but was a component of the vegetation. The study found fire-return intervals ranging from 2 to 18 years, with a mean of 7.7 years. Stephens [221] suggested that ranchers set most of the fires in the early settlement period, and that the fires were of low severity. Ranchers continued to burn blue oak rangelands in 8- to 15-year intervals until fire exclusion began in the 1940s [154,212].

In a fire history study on 2 Sierra Foothill Range and Field Station sites, McClaran and Bartolome [165] found that fire frequency increased from 1848 to 1940 compared to earlier and later times, with a peak of fire activity around 1848. Gold mining and ranching began in the area in 1848, and fire exclusion began in 1940. Cattle had grazed the site for over a hundred years at the time of study (1982-1983). For one of the study sites, the researchers selected a site that was relatively inaccessible to cattle and had no free-standing water, so it was only lightly grazed. The other site has heavily grazed. Probably due to tree harvest, there were few trees on the lightly grazed site older than 150 years. Blue oak recruitment had been sparse since fire exclusion was implemented. McClaran and Bartolome found a positive association between blue oak ages and fire dates, while cattle grazing was negatively associated with blue oak recruitment (P>0.01). Most blue oak recruitment occurred during the period of high fire frequency in the mid-1800s. McClaran and Bartolome suggested blue oak recruitment at that time was due to rapid growth of blue oak sprouts after fire. Comparing recent blue oak recruitment on heavily and lightly grazed plots, they found blue oaks sprouts were able to grow above the browse line only on lightly grazed plots. They suggested that blue oak sprouts required about 10 to 13 years to surpass the cattle browse line, while true seedlings may require 18 to 20 years [165].

Fire exclusion in the postsettlement period: It is difficult to assess the impact of fire exclusion on blue oak ecosystems. Besides fire exclusion, so many other human-caused changes have occurred in blue oak ecosystems that it is impossible to isolate the effects of any 1 change. Type conversion to a nonnative annual grassland understory, decline of rodent predators such as foxes and bobcats, loss of the top carnivore (the California grizzly bear), moderate to heavy livestock grazing in an ecosystem that evolved with only light grazing, a rapidly lowering water table, and urban development have all probably influenced the response of blue oak populations to fire [214].

Fuels: Surface fuels in blue oak woodlands are mostly comprised of nonnative annual grasses and downed woody debris. Without grazing, herbaceous fuels are continuous and can carry surface fires [71].

The type change from blue oak/perennial bunchgrass to blue oak/annual grass has probably altered fuels and fire behavior in blue oak ecosystems. Since there are few descriptions of pristine California oak woodland vegetation, it is difficult to compare groundlayer fuel loads in presettlement and contemporary blue oak woodlands. The perennial bunchgrass groundlayer was thought to be a southern extension of northern palouse prairie vegetation, which consists of spaced bunchgrass clumps with some forbs, soil crust organisms, and/or bare ground between grass clumps. Groundlayer vegetation was probably even sparser in California oak woodland understories than in palouse prairie due to reduced precipitation around the Central Valley compared to farther north [22]. In contrast to perennial bunchgrasses, annual grasses are usually closely spaced, creating a more continuous horizontal fuelbed [221]. Annual grass fuels are usually drier than bunchgrass fuels. California's perennial bunchgrasses generally stop growing, go dormant, and start drying after early June rains [128], while the annual grasses are generally dead and dry by early May [22,112]. In ungrazed blue oak ecosystems, changes in fuel loads caused by annual grass invasion have probably increased fire spread rate and altered fire seasonality.

Most blue oak/annual grass types are on private rangelands, and livestock grazing often reduces annual grass fuels. This reduction can be enough to stop fire spread, depending upon livestock utilization; however, fuel loads may be heavy where livestock are excluded 214.

Standiford [215] provides models to predict blue oak crown cover and height. Tietje and others [232,236] provide inventories of coarse woody debris size and volume in blue oak and other hardwood woodlands.

The following table provides fire regime information that may be relevant to blue oak. Fire regime information on vegetation communities in which blue oak may occur. For each community, fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models [150]. These vegetation models were developed by local experts using available literature, local data, and/or expert opinion as documented in the PDF file linked from the name of each Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.

Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics Percent of fires Mean interval
(years) Minimum interval
(years) Maximum interval
(years) California GrasslandCalifornia grassland Replacement 100% 2 1 3 California ShrublandCoastal sage scrub Replacement 100% 50 20 150 Coastal sage scrub-coastal prairie Replacement 8% 40 8 900 Mixed 31% 10 1 900 Surface or low 62% 5 1 6 Chaparral Replacement 100% 50 30 125 California WoodlandCalifornia oak woodlands Replacement 8% 120     Mixed 2% 500     Surface or low 91% 10     Ponderosa pine Replacement 5% 200     Mixed 17% 60     Surface or low 78% 13     *Fire Severities:
Replacement=Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Surface or low=Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area.
Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects [108,149].

Sources and Credits

  1. (c) Jill Matsuyama, some rights reserved (CC BY-NC-SA), http://www.flickr.com/photos/48496790@N00/3894772913
  2. (c) dloarie, some rights reserved (CC BY), uploaded by dloarie, http://www.flickr.com/photos/57556735@N08/6182307809
  3. (c) charlie dubbe, some rights reserved (CC BY-NC), uploaded by charlie dubbe
  4. Adapted by dlimandri from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Quercus_douglasii
  5. (c) USDA NRCS National Plant Data Center, some rights reserved (CC BY-SA), http://eol.org/data_objects/1392345
  6. Public Domain, http://eol.org/data_objects/24643071
  7. Public Domain, http://eol.org/data_objects/24643076
  8. (c) USDA NRCS National Plant Data Center, some rights reserved (CC BY-SA), http://eol.org/data_objects/1392347
  9. Public Domain, http://eol.org/data_objects/24643079

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