Quercus douglasii



INTRODUCTORY


  Blue oak savanna. Mark W. Skinner @ USDA-NRCS PLANTS Database.
AUTHORSHIP AND CITATION:
Fryer, Janet L. 2007. Quercus douglasii. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
QUEDOU

NRCS PLANT CODE [242]:
QUDO

COMMON NAMES:
blue oak

TAXONOMY:
The scientific name of blue oak is Quercus douglasii Hook. & Arn. (Fagaceae). It is in the white oak (Lepidobalanus) subgenus [72,114,134,157,239].

Blue oak hybrids are:

Quercus × alvordiana Eastwood (Q. douglasii × Q. john-tuckeri Tucker) [72,98,114,157,239]
Quercus × kinselae (C. H. Muller) Nixon (Q. douglasii × Q. dumosa Nutt.) [72,98,239]
Quercus × eplingii C. H. Mull. (Q. douglasii × Q. garryana Dougl. ex Hook.) [98,114,157,239,240]
Quercus × jolonensis Sarg. (Q. douglasii × Q. lobata Nee) [61,98,114,157,239]

SYNONYMS:
None

LIFE FORM:
Tree

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
Over 100 cities and counties in California have ordinances providing some level of protection for oaks. At the state level, the Integrated Hardwood Range Management Program, a collaborative effort between the University of California and the California Division of Forestry, is monitoring blue oak populations to provide recommendations for future legal protection [70]. Information on state-level protection status of blue oak is available at Plants Database.

DISTRIBUTION AND OCCURRENCE

SPECIES: Quercus douglasii
GENERAL DISTRIBUTION:
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 TYPES AND PLANT COMMUNITIES:

Blue oak woodlands and savannas dominate many of California's lower foothills. Along low western slopes of the Cascade-Sierra Nevada ranges, blue oak types either 1) lie between chaparral or mixed-conifer forest above and annual grassland or valley oak (Quercus lobata) woodland below [95,102] or 2) form a mosaic with chaparral and annual grassland. Blue oak savannas generally occur near the Central Valley floor, on shallow soils, and/or low-elevation, south-facing foothills. Blue oak woodlands occur further upslope, sometimes closing to a nearly continuous overstory on moist sites [256]. At midelevation, oak woodland, annual grassland, and chaparral ecotones may be dynamic [58], with type shifts dependent on differences in soil, aspect, grazing patterns, and/or fire history [45]. On the Coast Ranges, blue oak woodlands and mixed-oak woodlands with a blue oak component typically lie within a mosaic that includes annual grassland, coastal sage scrub, chaparral, redwood (Sequoia sempervirens), and/or coast Douglas-fir (Pseudotsuga menziesii var. menziesii) communities. Blue oak woodlands finger into singleleaf pinyon-California juniper (Pinus monophylla-Juniperus californica) woodlands at the ecotones of the Great Basin and Mojave deserts [18,80,95,125].

Blue oak woodland. Mark W. Skinner @ USDA-NRCS PLANTS Database.

Blue oak-dominated communities are highly variable in composition. Blue oak frequently codominates with gray pine (Pinus sabiniana) [88,95]. It also occurs in monospecific stands or codominates with valley oak, Oregon white oak (Q. garryana), coast live oak (Q. agrifolia), and/or interior live oak (Q. wislizenii) [95,196]. In Annadel State Park, oak woodlands with various mixtures of blue oak, valley oak, California black oak (Q. kelloggii), and interior live oak form a mosaic with mixed-evergreen forest and redwood groves [71]. Blue oak is a component of some low-elevation riparian communities. A vegetation survey along watercourses of the Central Valley found blue oak grew in association with Fremont cottonwood (Populus fremontii), California sycamore (Platanus racemosa), northern California black walnut (Juglans californica var. hindsii), and valley oak [250].

The herbaceous ground layer in blue oak communities and annual grasslands is dominated by nonnative annual species. California grasslands were probably historically dominated by perennial bunchgrasses such as purple needlegrass (Nassella pulchra) and bottlebrush squirreltail (Elymus elymoides) [22,55,204,211,251]. The type shifts from blue oak/perennial bunchgrass to blue oak/annual grassland and from perennial bunchgrass to annual grassland are irreversible [139]. Groundlayer diversity is probably higher since invasion of nonnative annuals than when blue oak communities supported a ground layer of native perennial grasses [139]. Keeley [138] conducted an inventory of groundlayer vegetation in blue oak woodlands in and near Sequoia-Kings Canyon National Park. He found nonnative annuals comprised about three-fourths of the groundlayer species present at the smallest scale (1 m²) and about one-half the species at the largest scale (1,000 m²) [138].

The following vegetation typings describe blue-oak dominated communities. Typings are listed from north to south, with general, statewide typings below.

General typings:

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Quercus douglasii

 

@ USDA-NRCS PLANTS Database
GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (for example, [72,114]).

Blue oak is a medium-sized, fall- and drought-deciduous tree [109,168,188,196]. It may retain its leaves year-round on moist sites [196] or show a flush of new leaves after heavy rains [100]. It is generally short and straight, from 20 to 66 feet (6-20 m) in height and 14 to 24 inches (36-60 cm) in DBH [173]. Blue oak typically has a single trunk, although some trees have few to several trunks [72]. The trunk is seldom more than 2 feet (0.6 m) in diameter. The bark is thin and flaky [66,235]. The canopy is compact, round, and supported by many crooked branches [196]. Leaves are sparse [235], 1 to 3 inches (2.5-8 cm) long, and have wavy, spineless margins. They are bluish-green, waxy, and thick [1,72]. The bluish-green color becomes most pronounced with drought [196]. Blue oak's leaf canopy is proportionately smaller than canopies of other, less drought-tolerant oaks [196]. The fruit is a nut, commonly called an acorn, that is 5 to 10 mm long × 10 to 15 mm wide [72,188]. Pavlik and others [196] state that the mature root system is "not particularly deep or extensive"; however, shallow roots are probably only characteristic of blue oaks on shallow soils [42,47]. Blue oak roots are often extensive, growing through fractured and jointed rock to a depth of 80 feet (24 m) or more to tap groundwater reserves [155]. Blue oak tends to produce more fine roots on sites were the taproot does not reach the water table [48]. Milikin and others [179] present preliminary regression equations for estimating blue oak root biomass based on DBH.

Stand and age class structure: Blue oak types vary in physiognomy from widely spaced blue oak savannas with a grass and forb understory to partially-closed or closed-canopy woodlands [18,88]. Stands in late succession may have an understory of drought-hardy trees and/or chaparral shrubs [18]. There are usually blue oak seedlings in the understory but few sapling or pole-sized trees (see Regeneration Processes). On the Central Coast Ranges, blue oak stand density ranged from open stands with means of 25 trees/ha and 14 inches (36 cm) DBH to dense stands of 163 trees/ha and 7 inches (18 cm) DBH [252]. Trees in mature stands are typically 90 to 100 years old [18,66]. The oldest known blue oak, in Sequoia National Park, is about 400 years old [173]. McClaran and Bartolome [164,166] found blue oak tree height and DBH were poor indicators of age on sites in the Sierra Nevada, and stated that analysis of blue oak age class structure requires direct age measurements.

It is difficult to reconstruct recruitment dates of true blue oak seedlings as opposed to blue oak sprouts, and most blue oak recruitment studies either do not distinguish between true seedlings and sprouts or define "seedling" as a size class, not an age class (for example, [101,177]). Tree-ring data sometimes show sprout recruitment dating from the last stand-replacing fire [176,177]. In this review, "seedling" refers to a size class unless otherwise stated.

Karlik and McKay [133] provide leaf area index and leaf mass density measurements for a blue oak stand at California Hot Springs near Bakersfield.

Physiological adaptations: Blue oak is the most drought tolerant of California's deciduous oaks. Adaptations to drought include thick leaves and bluish-green color [1], high water-use efficiency [43,142,182], deciduous habit with summer drought, plasticity in leaf development, and plasticity in early root development. During leaf development, leaves on droughty sites may gain more leaf mass than trees on mesic sites in spring but lose more leaf mass and reduce their photosynthetic rates in summer. In early root development, root growth is directed toward either upper or lower soil-layer water sources, depending on water availability (see Seedling establishment/growth for further details) [46].

RAUNKIAER [203] LIFE FORM:
Phanerophyte

REGENERATION PROCESSES:
Blue oak regenerates from seed and vegetatively.

Pollination: Blue oak is wind pollinated [34].

Breeding system: Blue oak is monoecious and rarely self fertile. Since blue oak is mostly outcrossing [34] and its acorns are dispersed by animals [96], genetic diversity is probably greater among than within blue oak populations [151]. For example, studies have found more variation in water-use efficiency among than within blue oak populations [162,206], and a common garden study found high genetic variation in stem growth, phenology, and mineral accumulation among blue oak populations [163]. Reciprocal transplant and common garden studies show between-population differences and local adaptation in seedling emergence, survivorship, and growth traits, however [205,208]. For information on gene flow among blue oak populations, see Riggs and others [209].

Seed production: Blue oak is a masting species [144,147,164]. Catkins develop from flower buds formed in the previous growing season, although flower buds may not develop in drought years [19]. The acorns mature in 1 year [34,51,114]. A 10-year study on the Hastings Natural History Reservation in Carmel Valley found mast years occurred approximately every 3 years for blue oak [144]. Abundant crops are generally produced every 2 to 3 years, with bumper crops every 5 to 8 years [193]. Masting is apparently tied to climate cycles, not endogenous cycles. A mast year is often followed by a year of low acorn production (review by [90]). Warm April temperatures and hot summer temperatures result in the largest blue oak acorn crops [147]. Acorn production can vary widely among trees in a stand [89]. One 38-foot (11.6 m) blue oak in Shasta County produced 3,750 acorns during a favorable season [173].

Seed dispersal: Acorns are disseminated by various animals. Magpies, scrub jays, and various rodents bury blue oak acorns in caches, resulting in high rates of emergence compared to uncached acorns [96].

Seed banking: Given blue oak's lack of seed dormancy [34,196], the palatability of the acorns to wildlife (see Palatability/nutritional value), and the many diseases that infect acorns (see Germination), it is unlikely that blue oak forms a persistent seed bank.

Germination: Since they are not dormant, blue oak acorns germinate rapidly when cool October rains begin [34,196]. In various Sierra Nevada locations, germination was initiated at the first rainfall and slowly continued through winter [160]. Germination may be epigeal or hypogeal, with buried acorns showing more recruitment than acorns on the soil surface [38]. Some blue oak acorns begin germinating before they fall from parent trees [92]. Fresh acorns collected by Mirov and Kraebel [180] from various locations around the state averaged 72% viability.

Although fall moisture is required for germination, too much rainfall in winter and spring can reduce seedling establishment on woodland sites [92]. Blue oak germinants are highly susceptible to fungal infection in cool, moist weather, so many acorns and germinants rot over winter [160]. A study in Berkeley and Mendocino counties found that emergence was greatest at 75% of normal rainfall, with above-normal rainfall resulting in high rates of germinant death due to damping-off fungi [164]. However, above-average rainfall may increase blue oak establishment in annual grassland [92].

Aspect can influence blue oak germination and seedling survival. In a 3-year Carmel Valley study, acorns on mineral soil showed higher rates of emergence on north-facing woodland slopes than south-facing woodland slopes. Emergence rates were similar on north- and south-facing woodland slopes when acorns were buried; however, first-year seedlings on south-facing, open grassy slopes had high rates of mortality except in wet years [92].

Seedling establishment/growth: Blue oaks show rapid, early root elongation prior to shoot development [161,196]. Blue oak seedlings generally produce more root than shoot compared to associated oaks, and maintain this growth habit through sapling and mature stages of life [196]. Seedlings with access to deep soil layers tend to grow deep taproots [42]. When supplied with a deep water source in the greenhouse, blue oak seedlings rapidly grew a taproot but not an extensive lateral root system. When water was only available in the upper soil layer, however, the seedlings grew many lateral roots [48]. In field and greenhouse experiments, shaded seedlings elongated their taproots faster than seedlings in the open [43].

Blue oaks beneath their parents' canopies may show higher establishment and growth rates than seedlings in the open. Blue oak's deciduous habit allows nearly full-sunlight penetration to the ground in some seasons, and blue oak canopies are usually sparse and diffuse in all seasons [43], so light does not usually limit blue oak establishment beneath blue oak canopies. Surveys in north-central California suggest that blue oak seedlings may persist beneath their parents' canopies for decades before release by death of the parent trees [224,226].

Blue oak top-growth may be rapid when mesic conditions foster rapid, early root growth. On the Sierra Foothill Range and Field Station on the east side of the Sacramento Valley, planted blue oak seedlings were irrigated their first year in the field but not thereafter. The seedlings grew an average of 9.8 inches (25 cm) in their first field season. Annual growth rate for the next 3 years averaged 27 inches (68 cm) [170].

Seedlings that survive 10 or more years have the greatest chance of surviving in subsequent years, although growth rate of older seedlings may be very slow. In Kern County, blue oak seedlings >10 years old showed reduced mortality compared to younger seedlings. However, older seedlings tended to die back more during drought compared to younger seedlings, so relative growth rate was slower for older seedlings compared to seedlings <10 years old. Mean change in height over a 4-year period was a gain of 0.96 inch (2.4 cm) for young seedlings and a loss of 1 inch (2.5 cm) for seedlings 10 or more years old [198].

Barriers to regeneration: Blue oak is regenerating poorly in some areas of its distribution [69,85,165,186]. Causes for this failure include most environmental and managerial influences [25,164]. Ungulate herbivory, rodent herbivory, acorn predation, annual grass interference, and drought are barriers to successful establishment on many sites [4,5,65,104,106,164]. Cattle, mule deer, and/or northern pocket gopher browsing have all seriously reduced blue oak seedling and sapling recruitment [65,106,176].

Seedling recruitment: Fire or flood prior to acorn dispersal can reduce acorn predator populations. Fire kills the larvae of ground-dwelling beetle larvae that damage blue oak acorns [15]. On The Nature Conservancy's Kaweah River Preserve, a large blue oak acorn crop was followed by a wet winter that flooded the Preserve and killed many ground-dwelling, acorn predator insects. The Preserve now supports many saplings that date back to the flood year [196].

Seedlings do not compete well with annual grasses [3,65,86]. Radicles of unburied acorns often fail to reach the soil surface before desiccation when growing through annual grass thatch. Additionally, annual grasses often outcompete blue oak seedlings for space, water, and light [37,85,86,182]. In a study on the competitive effects of ripgut brome and cutleaf filaree (Erodium cicutarium) on blue oak seedlings, Gordon and others [85] concluded that "competition for soil water with introduced annual species contributes to the increased rate of blue oak seedling mortality currently observed in California woodland systems."

A 4-year study across blue oak's range found growth interference from annual grasses limited establishment of true blue oak seedlings more than herbivory. Blue oak emergence increased significantly when herbaceous species were controlled with herbicides and hoeing (50% increase from uncontrolled plots, P=0.01). Herbivory exclosures significantly increased first-year blue oak seedling survivorship another 18% over unprotected seedlings. Interactive effects of protection from annual grasses and herbivory were not significant [4,5]. However, grazing sometimes favors young blue oaks by reducing the fuel load in blue oak ecosystems, so fires are not as severe and are less likely to kill seedlings and saplings [153].

Nonnative annuals may have irreversibly altered the seasonal availability of soil moisture to blue oak seedlings [27]. An experiment using exclosures and herbicides on 6 sites across blue oak's distribution showed that when confounding effects of ungulate herbivory were removed, growth interference from annual grasses reduced blue oak seedling emergence. Emergence was 45% on plots where grasses were controlled with herbicides and hoeing compared to 29% on plots without grass control. After 3 years, blue oak seedling survivorship was significantly less on uncontrolled plots compared to plots with grass control (P≤0.01) [3].  On sites in Santa Barbara and Monterey counties, Callaway [43] found blue oak seedling establishment was least frequent in open annual grassland and most frequent beneath coastal sage scrub species. Causes of recruitment failure differed between annual grassland sites and sites with shrubs. Blue oak seedling mortality from drought was most common in annual grassland, whereas acorn predation was the most common reason for blue oak recruitment failure under shrubs [43].

Blue oak is regenerating successfully on some sites despite competition from nonnative annuals. A 1990 resurvey of plots in San Benito and Monterey counties, originally inventoried in 1932, showed net gains in blue oak basal area and small tree density (4-11 inches (10-28 cm) DBH). Blue oak/annual grass and blue oak-gray pine/annual grass communities had significant increases in both blue oak basal area and cover of annual grasses (for example, ripgut brome and wild oat (Avena fatua), while blue oak basal area and grass cover in interior live oak-blue oak/perennial bluegrass (Poa spp.) communities were similar to the original survey [122]. Some blue oak establishment may occur in annual grassland even with drought. At the Hastings Natural History Reservation, unirrigated blue oak seedlings in annual grassland showed 33% survivorship 1.5 years after acorn plantings. The area was experiencing a severe, prolonged drought [184,185].

Annual recruitment of seedlings is not necessary for a long-lived species such as blue oak [18]. Because of a flush of blue oak establishment that occurred statewide from 1850 to 1900 [177,244,252], some suggest that recruitment of this species occurs in episodic bursts [19,81,196,244,252]. Episodic bursts may only occur when many factors favoring blue oak establishment coincide: high acorn production, low acorn predation, protection from desiccation during germination, above-average fall precipitation, low competition from neighboring plants, and limited seedling and sapling browsing [164]. A convergence of favorable conditions may occur only once or twice in a century but still be sufficient for successful recruitment in a long-lived species such as blue oak. Since blue oak can live for 200+ years, sporadic, sometimes widely spaced recruitment pulses are probably enough to replace aging trees [92,196]. While episodic bursts in recruitment have occurred on some sites, however, other sites show an historic pattern of steady recruitment over decades [169,176,177]. Tree-ring age analysis of trees in Kern County showed blue oak recruitment was fairly continuous from 1570 to 1850, when a seedling flush occurred [176,177]. It is unclear if episodic recruitment was historically the norm or if blue oak relied on both episodic establishment pulses and steady recruitment [196]. McCreary [169] calls for research on blue oak stand dynamics, including mortality rates for all size classes, to determine if there are "enough" seedlings and saplings for adequate blue oak regeneration.

Sapling and pole recruitment: Although blue oak seedlings are plentiful on many sites, saplings and pole-sized trees are generally rare [169,199,219,226]. Even seedling regeneration is poor in some areas [33,164], and there is concern that there will not be enough juvenile replacements when mature blue oaks die [169]. Lack of sapling and pole recruitment has been attributed to livestock [73,105,219], mule deer [92,158,165], and pocket gopher [7,92] herbivory, drought [199], interference from nonnative annual grasses [7], fire [17,41,244], and/or fire exclusion [176]. McClaran and Bartolome [165] found that blue oak requires 10 to 30 years to transition from the seedling to sapling stage.

Causes of blue oak recruitment failure vary spatially and temporally. At the San Joaquin Experimental Range, few blue oak have reached sapling size despite cessation of livestock grazing since 1934: Lack of sapling recruitment there is attributed to wildlife herbivory [69]. McClaran and Bartolome [165] suggest that seedlings must grow quickly enough to surpass the browse line in 10 to 13 years for blue oak sapling recruitment, and that this may not be possible during periods of prolonged drought. A study monitoring ages and growth rates of blue oak seedlings in southern California was undertaken during a period of extended drought. The study found 68.5% survivorship and a mean total growth rate of 0.02 inch (0.5 mm) of blue oak seedlings over 6 years. Many blue oak seedlings died back to their root crowns in summer. Slow growth (and hence, lack of recruitment to the sapling stage) was attributed to the 6-year drought. Blue oak seedling age ranged from 1 to 26+ years, with most seedlings <10 years old [199].

Protection from browsing may promote blue oak sapling recruitment. A study on a Shasta County ranch found nonnative Himalayan blackberry (Rubus discolor) presence increased the number of blue oaks recruited to the sapling stage. Blue oak seedlings and saplings grew in Himalayan blackberry thickets more often than expected based on area covered by the thickets (P=0.01). Blue oak seedlings and saplings in thickets were significantly taller and thicker in basal diameter compared to open-grown blue oaks (P=0.05). The researchers attributed the differential survivorship and growth to absence of cattle browsing in Himalayan blackberry thickets [255].

In a statewide study, blue oak sapling establishment varied with geographical location and site characteristics. In the northern Sierra Nevada, the steepest slopes supported the greatest number of saplings. Along the Sacramento and San Joaquin deltas 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 [186]. In a survey in the southern Sierra Nevada, presence of blue oak seedlings and saplings was positively associated with tree cover (P<0.01). Seedling recruitment was negatively associated with grazing (P<0.01), but grazing was nonsignificant for saplings [218]. Standiford and others [219] found blue oak saplings in Madera and Kern counties were more common on relatively high-elevation sites than on low-elevation sites that received less rain, and suggested that moisture may limit blue oak sapling recruitment on dry, low-elevation sites. In a 13-county survey, Swiecki and others [226] found blue oak sapling recruitment was positively associated with fire, canopy gaps, presence of shrubs, insolation, and altitude, and negatively associated with grazing. The majority of sites surveyed had few or no blue oak saplings, although seedlings were numerous. The researchers concluded that current blue oak sapling recruitment was insufficient to offset losses of mature blue oaks [225,226].

Climate effects: Effects of long-term climate patterns on blue oak are unclear. A state-wide study of blue oak acorn production and growth patterns found synchrony over large geographic scales, suggesting that large-scale climate patterns are important in determining rates of blue oak reproduction and growth [146]; however, a tree-ring chronology study in the Tehachapi Mountains found that precipitation was not correlated to blue oak stem recruitment [81]. See Climate for further details of the Tehachapi Mountains study.

Vegetative regeneration: Blue oak produces root crown or bole sprouts after top-kill by cutting or burning [21,172,173]. Sprouting ability varies with tree age, site, postdisturbance precipitation, andwhen the disturbance was firefire severity (see Plant Response to Fire). Consequently, blue oaks may fail to sprout on some sites [66,102]. Some root crowns initially support more sprouts than others, but number of sprouts/root crown generally equalizes within a few postdisturbance years. Pruning or light browsing may initially encourage growth but probably makes no long-term impact on sprout growth. In a study to determine pruning effects on growth rates of sprouts on harvested blue oaks, sprouts of stumps pruned to 2 sprouts/root crown showed increased growth rate for 2 postharvest years compared to sprouts of unpruned root crowns. After that, sprout growth rates were similar on pruned and unpruned stumps [21]. Sprout growth is often rapid, so blue oak sprouts have a higher probability of survival to sexual maturity than true seedlings [165]. At the University of California's Sierra Foothill Range and Field Station, coppice sprouts grew rapidly from experimentally-cut trees measuring 4 to 36 inches (10-91 cm) in diameter. Seventeen years after cutting, sprouts averaged 13 feet (4 m) in height, ranging from 9 to 17 feet (3-5 m) tall [135]. Frequent top-kill, however, may result in bushlike or stunted trees [66].

Blue oaks that retain some live bole tissue may show a stronger sprouting response than blue oaks that are killed back to the root crown. In an across-state harvesting experiment, the percentage of blue oaks that sprouted after cutting was significantly greater for trees cut 35 inches (90 cm) above ground (x=75%) compared to trees cut at ground level (x=45%). Stumps of small-diameter trees (≤6.1 inches (15.5 cm)) produced significantly more sprouts than stumps of large-diameter trees. Harvest date (winter, spring, summer, or fall) did not affect the number of sprouts produced, although stumps of trees cut in spring produced significantly shorter sprouts than stumps of trees cut in other seasons (P<0.05 for all measures) [172]. Mensing [175] stated that winter cutting or burning generally results in faster sprout growth than tree removal in other seasons.

Sprouting ability declines with age. Mature trees produce more bole than root crown sprouts. Bole sprouts grow more slowly and have higher mortality rates than root crown sprouts [96]. Very old trees either do not sprout or produce only bole sprouts [173].

SITE CHARACTERISTICS:
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].

SUCCESSIONAL STATUS:
Blue oak is moderately shade tolerant [116,186,191]. Seedings and saplings can persist in shade but require release to become pole-sized trees [169,226].

Oak woodland to other types: Blue oak woodland, chaparral, and annual grassland boundaries are dynamic, and mechanisms causing shifts from one type to another are not fully understood. Field and greenhouse experiments show that chaparral shrubs are sometime nurse plants to blue oak, facilitating blue oak seedling establishment and probably, as blue oaks grow and shade out the shrubs, eventual conversion of shrub-dominated sites to blue oak woodlands [43]. Callaway and Davis's [45] study of shifts in coast live oak woodland coverage may also apply to blue oak woodlands. Using GIS layers to analyze vegetation shifts at Gaviota State Park, they found coast live oak, coastal sage scrub, and annual grassland types were relatively stable on undisturbed landscapes, with each type losing little total cover over 42 years (1947-1989). Fire or grazing generally lowered transitional rates among these types, but fire resulted in a high conversion rate from coast live oak woodland to annual grassland and from coastal sage scrub to annual grassland. Transition rates varied with topographical position and soil substrate. Callaway and Davis concluded that fire, grazing, and site interactions determine type-shift rates among coast live oak woodland, coastal sage scrub, and annual grassland. At the landscape level, only portions of these types shifted, with some patches undergoing rapid transitions with fire or grazing, and other patches remaining static as edaphic or topographic climax communities [45]. Similar studies are needed to determine type shifts and successional patterns among blue oak woodlands, chaparral, and annual grasslands.

Old fields: In an old-field succession study on the Hastings Natural History Reservation, blue oak was present on untilled rangeland but did not appear on old fields until 29 years after field abandonment [252].

Fire exclusion has resulted in unprecedented, dense basal areas in some blue oak woodlands. In Annadel State Park, coast Douglas-fir is invading blue oak-California black oak-coast live oak communities, changing what was historically a savanna to a densely canopied woodland [20]. In Sequoia National Park, a comparison of contemporary blue oak woodland structure with that noted in historical records from the settlement period showed a large increase in blue oak cover and density. Vankat and Major [244] suggest that increased density of blue oak woodlands is due to a combination of fire exclusion and past livestock grazing. For example, the blue oak-California buckeye phase of the blue oak woodland type is characterized by a partially-closed canopy, and frequent surface fires probably maintained blue oak as the canopy dominant. In the absence of fire or other top-killing disturbances, California buckeye is successionally replacing blue oak on some sites in Sequoia National Park, with the blue oak woodland communities succeeding to closed-canopy California buckeye-blue oak forests [18].

SEASONAL DEVELOPMENT:
Blue oak acorns germinate in fall and emerge in winter. Acorns planted at the Hastings Natural History Reservation emerged from late February to late March [95]. Most active growth occurs from March through May, when soil moisture and blue oak water uptake are high and air temperatures are warm [18]. On the Hasting Natural History Reservation, leaves expanded from late March to 26 April and abscised in October [46]. In Sequoia National Park, leaf expansion occurred simultaneously with stem elongation on 1 site but began when stem elongation slowed on 2 other sites [18]. Blue oak flowers in late winter or early spring [72]. Baker and others [18] found blue oak failed to flower following a 2-year drought, even though precipitation was above normal in the spring of study. Acorns disperse from late summer to late fall [169]. Phenological development of blue oaks in Sequoia National Park was as follows [18,19]:

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

Blue oak undergoes premature leaf abscission during summer drought [1,168,173,188]. When blue oaks drop their leaves in response to summer drought, the trees go partially dormant until soil moisture increases in spring, when blue oaks produce a flush of new leaves [18,196]. Trees that drop their leaves in summer usually continue to develop and fill their acorns [196]. On the Hastings Natural History Reservation, blue oaks began leaf drop in late August in a dry year and in late November in a wet year [93]. Site characteristics and stand structure influence degree of leaf drop. On the Sierra Foothill Range and Field Station, blue oaks dropped leaves for 2 successive years in mid-August in response to drought. Trees on shallow, rocky soils or south-facing slopes lost more leaves than trees on valleys or swales, and trees growing in clumps lost more leaves than trees growing alone. Trees that defoliated in summer leafed out earlier in spring than trees that did not defoliate in summer (r=0.42). In this 3-year study, summer leaf fall had no short-term effect on tree mortality [168].

FIRE ECOLOGY

SPECIES: Quercus douglasii

 

Photo courtesy of Univ. of CA, Davis, Agricultural Experiment & Cooperative Extension.
FIRE ECOLOGY OR ADAPTATIONS:
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] (see Discussion and Qualification of Plant Response).

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 Grassland
California grassland Replacement 100% 2 1 3
California Shrubland
Coastal 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 Woodland
California 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].

POSTFIRE REGENERATION STRATEGY [223]:
Tree with adventitious buds, a sprouting root crown, sobols, and/or root suckers
Crown residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on-site or off-site seed sources)

FIRE EFFECTS

SPECIES: Quercus douglasii
IMMEDIATE FIRE EFFECT ON PLANT:
Low- or moderate-severity fire generally top-kills blue oak seedlings and saplings [78,124,165,173,176]. The bark of young blue oaks catches fire and burns easily, providing little protection from fire [173]. Mature trees are resistant to top-kill by low-severity surface fires and most moderate-severity surface fires, but are top-killed or killed by severe fires or the sustained heat of most chaparral fires [173,200]. Because the bark is thin [66,235], the boles of mature blue oaks scar easily. Wildfires in Sequoia National Park have scarred large blue oaks even where fire severity was low [100,200] (see Plant Response to Fire).

Because summer and early fall wildfires occur during periods of high air temperatures and are more severe, they generally kill more blue oaks than prescribed fires, which are usually of low severity [11,192,214,235].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
Prescribed October fires on a cattle ranch in Madera County had little effect on mature blue oaks (>4 inches (10 cm) DBH) with an annual grass ground layer, but cover of mature blue oaks was reduced from prefire levels when a chaparral understory was present. The site was a mosaic of interior live oak/chaparral, blue oak woodland/chaparral transition (blue oak with minor amounts of chaparral and shrubby interior live oak), and blue oak-interior live oak/annual grass communities. Interior live oaks were selectively crushed prior to prescribed burning. This prefire site preparation reduced blue oak cover only in interior live oak/chaparral communities. The fire management objective was to reduce woody cover on sites with a chaparral understory and increase herbaceous production in all 3 plant communities. Prescribed burning reduced blue oak seedling density in all 3 communities compared to prefire numbers [174,235,235].

Blue oak canopy cover (%) before and after site preparation and prescribed fire in plant communities with varying woody fuels [174,235]

Community Pretreatment
(1986)
Postcrush treatment
(1987)
Postfire month 2
(1987)
Postfire year 1
(1988)
Postfire year 2
(1989)
Postfire year 8
(1995)
interior live oak/chaparral 15a* 15a 3b 1b 10a 2b
transition 37a 40a 35a 35a 28a 35a
blue oak-interior live oak/grass 23a 23a 24a 29a 22a 23a

Blue oak seedling densities (trees/acre) before and after site preparation and prescribed fire [235]

interior live oak/chaparral 23 17 0 7 ---** ---
transition 54 51 0 6 --- ---
blue oak-interior live oak/grass 1 18 0 0 --- ---

Changes in blue oak wood production (cords/0.2 acre) in blue oak transition communities [174,235]

interior live oak/chaparral 1.21 0.72 0.72 --- --- ---
transition 0.80 0.80 0.80 --- --- ---
blue oak-interior live oak/grass 2.56 2.56 2.56 --- --- ---
*Values in a row followed by a different letter are significantly different (P>0.05).
**No data.

Pre- and postfire growth of 24 small blue oaks on the cattle ranch was monitored with and without cattle and mule deer browsing until postfire year 13. At that time, there was no evidence that burning had stimulated blue oak growth: greatest growth gain occurred on unburned, unbrowsed sites. Browsing reduced blue oak growth rate more than fire. On browsed sites, the browse line extended to 60 inches (150 cm) above ground, with 80% of meristems on marked trees getting browsed. Some blue oaks on burned, browsed sites grew above the browse line [24].

Mean height (cm) of blue oak saplings in postfire year 13 [24]
  Browsed Unbrowsed
Burned 150 260
Unburned 175 275

PLANT RESPONSE TO FIRE:
Blue oak sprouts from the root crown or bole after top-kill by fire [21,78,172,173,200]. Postfire sprouting occurs even in drought years [100,124]. Large trees that have developed a relatively thick bark layer are likely to survive low- and moderate-severity fires with crown scorch, bole scars, or no damage. Mature blue oaks with severe crown scorch usually produce bole sprouts after fire [100,200]. Although moderate-severity fire usually leaves basal scarring, blue oaks generally grow bark over small fire scars within a few postfire years. Mature blue oaks often show no evidence of internal fire scars, which get covered as bark grows around them [100].

Top-killed blue oak seedlings and saplings sprout during the first postfire growing season following low-severity fire: Even first-year seedlings can sprout. Sprouts may grow above the browse line more rapidly than true seedlings, and therefore have a greater probability of survival to maturity [164,165]. However, this response is variable and depends on physical and biological site characteristics and pre- and postfire site management. Bartolome and others [24] found prescribed fire and livestock grazing reduced blue oak growth compared to blue oaks on unburned, ungrazed sites.

Blue oak shows rapid postfire height gain on favorable sites. In a dendrochronology study, McClaran [167] reported that 70% to 85% of blue oaks in stands on the Sierra Foothill Range and Field Station probably originated as sprouts that emerged within 1 year of fire. Growth rates of blue oaks establishing within 1 year of fire were significantly greater than growth rates of blue oaks establishing at other times (P<0.01) [167].

Vertical growth rates of blue oak establishing on the Sierra Field Station within 1 year after fire vs. trees establishing at other times [167]
Tree height (cm)

Mean growth rate (cm/year)

Trees establishing within 1 year after fire Trees establishing in nonfire years
0-60  33.9a* 16.5b
60-135 13.8c 10.1d
*Values followed by different letters are significant at P<0.01.

However, McClaran found a flush of blue oak establishment did not follow every fire, and about 10% to 30% of blue oaks on the Sierra Field Station established in nonfire years. Many factors may combine to suppress blue oak establishment after fire [167] (see Barriers to regeneration).

Effects on recruitment: Although it is widely accepted that blue oak woodlands evolved under a regime of frequent surface fires [8,101,164,165,212], there is little consensus regarding the effects of fire on contemporary blue oak populations, which encounter environmental conditions severely altered from historic times (see Fire exclusion in the postsettlement period). Some studies found that prescribed burning benefited blue oak populations. For example, McClaran [164] found small-diameter blue oaks grew taller after fire compared to unburned small-diameter blue oaks. Based on fire modeling, Anderson and Pasquinelli [17] concluded that periodic prescribed fire benefited blue oak regeneration, but that wildfires tended to reduce blue oak regeneration. Other studies, however, found negative effects of frequent fire on blue oak populations. Bartolome and others [24] and Swiecki and others [225,226] found frequent fire reduced blue oak seedling and sapling numbers, while infrequent fire had a neutral to positive effect on seedling and sapling density.

The interactive effects of prescribed fire, livestock grazing, site quality, and other factors affecting blue oak survivorship and growth are complex and not completely understood. Harvey [110] concluded that a history of fire, or lack of fire, did not explain differences in blue oak recruitment on his study sites in San Luis Obispo and Santa Barbara counties.

Sapling response— Blue oak saplings are well adapted to survive moderate-severity fires. Top-kill and subsequent sprouting after fire can prolong the juvenile period, however, which may reduce pole recruitment [228,241]. In a state-wide survey, Swiecki and others [225] found frequent fire was negatively associated with sapling recruitment, while infrequent fire was not associated or only slightly associated with blue oak sapling recruitment.

Tall saplings with thick stems are most likely to survive moderate-severity fire. Following a low- to moderate-severity wildfire in Vacaville, 9% of blue oak saplings died within 5 postfire years. Of blue oaks saplings that were completely top-killed, 76% were significantly smaller than those only partially top-killed (P<0.001). Generally, saplings taller than 79.1 inches (201 cm) or with stem diameter >2.2 inches at 12 inches stem height (5.6 cm at 30 cm stem height) were partially top-killed. Twenty percent of top-killed blue oaks regained their prefire height by postfire year 5. Sprout height gain was greatest in postfire year 1, with stem growth slowing afterwards. Meadow voles browsed new sprouts, further slowing blue oak postfire growth [228]. Swiecki and Bernhardt [228] concluded that moderate-severity fire negatively affected blue oak regeneration.

Tietje and others [235] concluded that low-severity prescribed fire neither set back nor benefited blue oak sapling recruitment. A September prescribed surface fire was set at Camp Roberts, San Luis Obispo and Monterey counties, in a blue oak-coast live oak woodland. Based on flame lengths, fire intensity was estimated as low to moderate. The fire was patchy, burning only about 250 (100 ha) acres of a 500-acre (200 ha) treatment area. Most blue oak saplings were top-killed. In postfire year 1, overall survivorship of blue oak saplings tagged before fire was 75%. Saplings that failed to sprout tended to have heavier prefire fuel loads within 3.3 feet (1 m) of their stems than saplings that survived, and the researchers concluded that the fire killed the saplings' roots. Across the study area, however, percentage of blue oak saplings that sprouted was similar for sites with light, medium, and heavy fuel loads (P=0.745). Prefire sapling height was not related to postfire survivorship: blue oak saplings that died averaged 45.6 inches (101 cm) in height before fire, and saplings that survived averaged 43.3 inches (110 cm) before fire. In postfire year 1, mean length of the longest sprouts on individual root crowns was 24.8 inches (63 cm), with a mean of 15.8 sprouts/root crown. Although the fire had little short-term effect on blue oak growth, the researchers concluded that frequent, low-severity prescribed fire would benefit the Camp Roberts blue oak population by reducing annual grasses, recycling nutrients, and reducing the risk of severe fire [235].

Crown scorch: Mature trees crown-scorched by surface fires often replace their leaves the next year with no apparent ill effects [173]. Mature blue oaks with most of their leaves scorched may be top-killed or die back to the bole, however. After moderate-severity prescribed burning on Mt Hamilton in Santa Clara County, most blue oaks with 100% crown scorch sprouted from the root crown, although a few died [78].

On sites in the Sierra Nevada, most mature blue oaks survived surface wildfires even with 100% crown scorch. For mature trees incurring bole damage, bole sprouting was more common than basal sprouting. Basal sprouting from large, top-killed trees occurred on 10 of 11 burn sites but was infrequent (approximately 10% of trees with bole damage). Basal sprouting occurred mostly in blue oaks <5.9 inches (15 cm) in DBH and did not occur in trees >24 inches (60 cm) in DBH [103].

In a related study, recovery of blue oak after a severe surface arson fire in Sequoia National Park was monitored for 2 years. Precipitation for the 2 study years was below average [100]. Medium-sized blue oaks (4-15 inches (11-39 cm) DBH) were mostly top-killed, with root crown sprouts appearing in postfire year 1 [102]. Some large, mature trees escaped basal scarring but sustained crown scorch. A mean of 65% large, crown-scorched blue oaks died back to the bole and grew bole sprouts. Bole sprouting was most common in trees with >50% crown scorch [100]. For detailed information on this study, see the Research Paper by Haggerty.

Across 11 sites on the Kaweah River watershed in Tulare County, blue oak mortality rate was highest in the largest (≥15 inches (40 cm) DBH) and smallest (≤4 inches (10 cm) DBH) size classes. Top-kill was highest in seedlings and saplings, while crowns of most large trees survived. Top-kill rate was slightly higher than mortality rate in blue oaks ≤2 inches (5 cm) in DBH [102].

Survival (number of live trees) of crown-scorched blue oak after a 1987 wildfire in Sequoia National Park [102]
Year

Crown scorch (%)

25 25-50 50-75 100
1987 39 20 22 36
1988 39 19 22 36
1989 39 19 21 30
Mortality 0% 5% 4.5% 16.7%

Basal scarring occurred most often in saplings [102]. Across 4 Kaweah River watershed sites, fire severity and damage to blue oak was greatest on the ridgetop, and the ridgetop site supported the greatest number of sprouting blue oaks after fire [100]. Fire damage and postfire response of blue oak are shown in the table and figures below.

Fire damage to and response of blue oaks 2 years following wildfire in Sequoia National Park [100]
Transect Mean crown scorch (%) 100% crown scorch (n) Mean scorch height (cm) Mean bole char height (cm) Fire scars (%) Aboveground mortality (%) Basal sprouting (%)
Northwest slope 27a 3 36a 20a 65 0 7
Ridge 90b 28 58b 60ab 59 19 32
Lower southeastern slope 45ac 9 50ab 50a 64 5 8
Upper southeastern slope 54c 6 50ab 100b 40 9 24
*Within columns, numbers with different letters are significantly different (P<0.05).


Crown sprouting in scorched blue oak wildfire in Sequoia National Park [102] New scar formation on blue oaks after a wildfire in Sequoia National Park [102]

Postfire seedling establishment is apparently rare for blue oak. Fire probably plays an important role in seedling establishment, but its role is not well understood. Postfire blue oak establishment from acorns may occur only when a suite of favorable factors coincide (see Barriers to regeneration).

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Blue oak may recover slowly following moderate-severity fire, and some ecotypes may fail to sprout following even low-severity fire [66,102]. Based on tree size and fire effects, Horney and others [124] present regression models to predict blue oak survivorship after wildfire.

Mortality of juvenile blue oaks (seedlings and saplings) can be high when a wet growing season results in dense annual grass growth, which fuels severe surface fire when dry. Fuel buildup from a combination of dense annual grasses and decades of fire exclusion led to a moderately severe surface fire on an undeveloped valley oak-blue oak-coast live oak/wild oat-Italian ryegrass (Lolium multiflorum) woodland at Stanford University. The oak woodland had been closed to grazing since 1989 or before, on some sites. Oak sapling recruitment was low, so study plots were established in 1990 to monitor oak regeneration. Heavy spring rainfalls in 1991 and 1992 caused an "unusually lush growth of grasses, forbs, and other understory plants". Annual grasses were dense and tall, reaching 8 to 10 feet (2-3 m) in height on some plots. In the absence of fire, small- and large-diameter fallen and standing deadwood had accumulated, and shrubs were encroaching into the oak woodland. The woodland burned in a July 1992 wildfire that spread onto some of the study plots. Mortality rate of juvenile blue oaks in postfire year 1 (1993) was 14% on burned plots and 8% on unburned plots. Rodents girdling juvenile blue oaks in 1992 and 1993 reduced blue oak growth on both burned and unburned plots. Prefire height of blue oaks that survived the fire tended to be more than prefire height of blue oaks that the fire killed. For all oak species combined, 32% of oaks >10 inches (25 cm) tall survived, while only 9% of oaks less than 10 inches tall survived (data for blue oak alone are unavailable). However, for oaks that sprouted after fire, there was no significant correlation between prefire height and postfire growth rate [214].

Pre- and postfire mean annual growth rates (inches) of blue oaks before and after a 1992 wildfire at Stanford University [214]
Plot type Prefire growth rate (inches/year) Postfire growth rate (inches/year)
Blue oaks killed by fire 1.4a* not applicable
Surviving blue oaks on burned plots 2.4a 3.2a
Blue oaks on unburned plots 7.3b 2.0a
*Within columns, numbers with different letters are significantly different (P=0.002).

Oaks on fire plots were generally younger and smaller than oaks on unburned plots. Therefore, the above results are not directly comparable but show a general trend of similar growth rates for unburned blue oak juveniles and juveniles top-killed by fire. The researchers suggested that the significant difference in blue oak growth in 1990 to 1991 was due to higher rodent browsing on plots that later burned compared to plots that did not burn [214].

A Kern County study illustrates possible interactive effects of fire and browsing on blue oak recruitment. Blue oak recruitment on the study site was low, but relatively continuous, under the frequent, low-severity surface fire regime in place before European settlement around 1842. Fires were frequent during the 1843 to 1865 settlement period, and blue oak had a regeneration peak in 1856. Nearly half the 1856 cohort had double stems, suggesting they originated as postfire sprouts. Hunting pressure from soldiers in nearby Fort Tejon probably minimized postfire mule deer browsing of blue oak sprouts. The study area became a cattle ranch in 1866, and a fire-free interval of 70 years followed. The 1856 cohort has matured to a dense, even-aged stand, with almost no blue oak recruitment since then. The author concluded that a change in fire regime and subsequent canopy closure contributed to lack of blue oak recruitment, but many other factors may also be contributing. Since 1856, nonnative annuals have invaded the understory and are likely outcompeting blue oak seedlings for water, and browsing pressure from mule deer and cattle has greatly increased [178].

Grazing does not always reduce blue oak recruitment. On the Hopland Field Station, moderate domestic sheep grazing and low-severity prescribed fire had no significant effects on blue oak seedling recruitment (P>0.10). Blue oak seedlings established in similar numbers on grazed, burned, and grazed-and-burned plots [11]. See Fire Case Studies for further details on this study.

FIRE MANAGEMENT CONSIDERATIONS:
Prescribed fire: Urban development in blue oak ecosystems makes prescribed burning programs difficult to implement [221], and it is unclear if prescribed fire is needed for conservation management of blue oak [100]. Important questions include how often prescribed surface fires are needed—if they are needed at all—to maintain blue oak types [95,100]; whether fire can increase blue oak recruitment [100,165]; and how fire affects succession in blue oak woodlands [12,100]. Some suggest that frequent surface fires help maintain the open character of blue oak savannas [18,100]. There is general consensus that wildfires burning dry fuels under high air temperatures and low relative humidities reduce blue oak density [11,187,192,214,235,242].

Prescribed fire may help control nonnative annual grasses in blue oak ecosystems, although it may also increase cover of nonnative forbs. Fire eliminates the thatch layer that inhibits blue oak emergence, and may reduce annual grass establishment. In a blue oak savanna in Sequoia National Park, either a single prescribed spring fire, repeat spring fires (2 or 3 successive fires), or repeat fall prescribed fires (3 successive fall fires) increased the diversity and relative dominance of native and nonnative forbs to nonnative annual grasses compared to an unburned control. Wild oat and ripgut brome showed greatest reduction in response to successive spring fires (12.4% reduction), while nonnative Maltese starthistle (Centaurea melitensis) showed the greatest increase (46.3%) after successive fall fires. Nonnative annual grasses regained prefire biomass in 2 to 3 years when prescribed burning was stopped, so the researchers stated that prescribed fires need to be repeated regularly for annual grass control [195]. For detailed information on this study, see the Research Paper by Parsons and Stohlgren [195].

Prescribed burning is conducted after annual grasses have dried in spring—usually in May—or after the first rains of fall. Such burning does not mimic the natural fire regime of frequent summer and fall surface fires under which blue oak evolved; however, since foothill woodlands have undergone a type shift from blue oak/perennial grass to blue oak/annual grass, it is impossible to recreate historic fuel conditions [8]. Because sprouting ability of blue oak may vary with ecotype or site [66,102], managers may want to use small prescribed fires to test the sprouting capability of juvenile blue oaks on their site before conducting prescribed burning over large areas.

Fire research methods: McClaran [164] found that using both cat-faced and unscarred blue oaks gave the best estimate of fire-return intervals on the Sierra Field Station. Using only cat-faced blue oaks and excluding trees with heal-ever scars, mean fire-return interval was recorded as 6.4 years across 2 study sites. Using only healed-over blue oaks with internal scars, mean fire-return interval was 4.2 years. On 1 of the sites, mean fire-return interval was longer on small plots (0.05 ha, 17.7-year mean fire-return interval) than on large plots (0.1 ha, 13.0-year mean fire-return interval). The difference in fire-return intervals across small and large plots was significant at P<0.025 [164].

Fire effects on small animals: Low- to moderate-severity October prescribed burning in blue oak and valley oak-coast live oak-blue oak woodlands on Camp Roberts, San Luis Obispo and Monterey counties, had little overall effect on small animals. Relative abundance of small mammals, breeding birds, reptiles, and amphibians did not change from prefire levels after burning, and oak canopy cover did not change after burning [247,248]. See Sapling response for further details of this study.

FIRE CASE STUDIES:

Blue oak seedling response to fire and grazing on the Hopland Field Station, California

FIRE CASE STUDY CITATION:
Fryer, Janet L., compiler. 2007. Blue oak seedling response to fire and grazing on the Hopland Field Station, California. In: Quercus douglasii. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [ ].

SPECIES INCLUDED IN THE STUDY:

Common name Scientific name
blue oak Quercus douglasii

FIRE CASE STUDY REFERENCES:
Unless otherwise indicated, the information in this Fire Case Study comes from the following study:

Allen-Diaz, Barbara H.; Bartolome, James W. 1992. Survival of Quercus douglasii (Fagaceae) seedlings under the influence of fire and grazing. Madrono. 39(1): 47-53. [11].

STUDY LOCATION:
This study was conducted on the University of California's Hopland Field Station in Mendocino County.

SITE DESCRIPTION:
The study sites were located on 2 wildland pastures, each around 30 ha in size. Domestic sheep have grazed the pastures since before 1951. Annual precipitation was below normal during the 4 years of study, ranging from 600 to 720 mm. Mean annual precipitation at Hopland Field Station is 950 mm.

PREFIRE PLANT COMMUNITY:
The study site is a blue oak woodland-annual grassland mosaic. Seventy-six percent of the study site is blue oak woodland with mature blue oaks and an understory dominated by nonnative annual grasses. A pretreatment vegetation survey found blue oak seedlings in the woodland understory, but there were no blue oak saplings. Understory cover and production of the annual grasses average 65% and 1,500 kg/ha, respectively. The open annual grassland occupies 18% of the study site; the other 6% is a dense stand of interior live oak (Q. wislizenii). Three 0.5 ha study plots were set up within the pastures. Within the study blocks, the overstory was composed of blue oaks >10 cm DBH, providing 50% canopy coverage.

Study sites are classified in the following plant community and likely experienced the historic fire regime described below:

Fire regime information on the plant community in which blue occurred in this study. Fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Model [150]. This vegetation model was developed by local experts using available literature and expert opinion as documented in the PDF file linked below.
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
California oak woodlands Replacement 8% 120
Mixed 2% 500
Surface or low 91% 10
*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.
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].

SPECIES PHENOLOGY:
Not stated. Blue oak was likely shedding leaves during the October fire treatments.

FIRE SEASON/SEVERITY CLASSIFICATION:
Fall/low severity

FIRE DESCRIPTION:
Fire management objective: The objective of this study was to document the effects of prescribed fire and domestic sheep grazing on naturally regenerating blue oak seedlings.

Fire prescription and behavior: Four treatments were randomly applied to each 0.5 ha study block:

1) prescribed fire with domestic sheep grazing
2) prescribed fire without domestic sheep grazing
3) domestic sheep grazing without fire
4) no domestic sheep grazing and no fire (control)

Domestic sheep—mostly dry ewes—used the grazing-treatment pastures from 15 May 1985 to 15 October 1985: the dormant season for annual grasses at Hopland Field Station. Stocking rates were adjusted to produce residual understory fuels of around 600 kg/ha. The domestic sheep were returned from 15 December 1985 to 15 February 1986 and from 15 May 1986 to 15 October 1986.

Prescribed burning was conducted at 12:01 PM in October 1986 after the first fall rains. Fuels were a mixture of dry grass, blue oak litter, and a small amount (about 50 kg/ha) of green grass.

Burning conditions for the blue oak woodland prescribed fire at Hopland Field Station [11]
Air temperature Relative humidity Wind speed Total understory fuel load
18 °C 40% 5-10 km/hour 750 kg/ha

The fires did not completely consume the dry grass and oak litter.

FIRE EFFECTS ON TARGET SPECIES:
All the blue oak seedlings on the study plots were top-killed. Growth of sprouting blue oak seedlings was monitored from the spring of 1987 through May 1990 (posttreatment growing seasons 1-3).

Neither prescribed fire nor domestic sheep grazing had significant effects on blue oak seedling recruitment compared to control plots. The only factor that approached statistical significance was year of seedling establishment (F=4.60, P<0.10), with highest rate of blue oak seedling establishment in 1988 (postfire year 2). Neither fire, grazing, nor a combination of fire and grazing had significant effects on the number of blue oak seedlings observed on study plots compared to control plots. Some seedlings were sprouts of seedlings that had established before treatments; others germinated after treatments.

Overall blue oak seedling density was low in postfire year 1, increased greatly in postfire year 2, then declined to near postfire year 1 levels in postfire years 3 and 4. Overall blue oak seedling mortality rate was about 50% per year. For seedling sprouts, mortality was not significantly associated with plant age before treatment, number of postfire sprouts, or plant size. Blue oak germinants continued to establish in each of the 4 posttreatment study years, including 1989 and 1990, which were droughty. Results from fire and grazing treatments were pooled because they were not statistically different. Blue oak seedling mortality was 49% in postfire year , 43% in postfire year 2, and 48% in postfire year 3.

Number of sprouts, leaves, and height of blue oak seedlings, measured each May [11]
Year Age (yr) Sprouts (n) Leaves (n) Height (cm) n
Fall 1987 cohort
1988 (postfire year 1) 1+* 1.74 (0.96)** 5.99 (2.78) 4.33 (1.74) 110
1989 (postfire year 2) 2+ 2.84 (1.10) 5.66 (2.24) 5.08 (1.70) 56
1990 (postfire year 3) 3+ 2.94 (1.03) 5.09 (1.91) 3.73 (1.16) 32
Fall 1988 cohort
1989 (postfire year 2) 1 2.63 (0.91) 5.33 (2.71) 4.75 (1.93) 63
1990 (postfire year 3) 2 2.76 (0.82) 4.51 (1.86) 3.65 (1.67) 33
Fall 1989 cohort
1990 (postfire year 3) 1 2.54 (0.65) 4.73 (1.76) 2.95 (1.18) 11
For seedlings that died, number of sprouts, leaves, and height of blue oak seedlings by cohort, measured the May prior to seedling death [11]
Year Age (yr) Sprouts (n) Leaves (n) Height (cm) n
Fall 1987 cohort
1988 (postfire year 1) 1+* 1.76 (1.00) 6.12 (3.43) 4.56 (2.03) 54
1989 (postfire year 2) 2+ 2.58 (0.74) 5.04 (2.67) 4.69 (2.03) 26
Fall 1988 cohort
1989 (postfire year 2) 1 2.59 (0.96) 5.00 (1.80) 5.09 (2.24) 29
*True seedling ages are known for 1988 and 1989 cohorts but not for the 1987 cohort.
**Standard deviations are in parentheses.

FIRE MANAGEMENT IMPLICATIONS:
Fire outcome: This study shows that blue oak seedling establishment is potentially compatible with prescribed fire and/or domestic sheep grazing.

Limitations of this study: The study does not show the effects of fire and/or domestic sheep grazing on blue oak saplings, a critical and vulnerable life stage for blue oak (see Sapling recruitment) that was absent from this study site. Results of this study may not apply to all blue oak woodland sites, as site characteristics, particularly soil water, vary greatly across blue oak's distribution. Allen-Diaz and Bartolome [11] concluded that blue oak seedling recruitment is adequate at the Hopland Field Station. Although blue oak seedling numbers fluctuate and their spatial distribution is irregular, seedlings are always present in the understory. Blue oak seedlings are maintaining a constant mortality and replacement rate over time. With canopy closure of 50% on the study sites, blue oak seedlings may require some canopy removal for successful release. Further prescribed fire and fire surrogate studies (for example, selection cutting of overstory blue oaks) are needed to determine how managers can successfully transition blue oaks from the seedling to sapling stage.

MANAGEMENT CONSIDERATIONS

SPECIES: Quercus douglasii

IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Blue oak is an important species for livestock and wildlife [196]. Blue oak woodlands are the most heavily utilized cattle rangelands in California: 65% of California's total livestock forage is provided within blue oak-gray pine woodlands [38].

Blue oak woodlands are important habitat for small mammals and birds [62,152]. A 3-year study in the central Sierra Nevada foothills showed that 92 species of birds utilized blue oak woodland, with 60 species nesting in the woodland [30]. For inventories of small mammals, birds, and herptiles using blue oak habitats, see Block and others [31] and Verner and Boss [245].

Blue oak woodlands are important mule deer habitat. Use is particularly heavy during fall acorn drop and into winter, when annual grass green-up occurs [140]. On the North Coast Ranges, mule deer used blue oak woodlands significantly more than chaparral (P<0.001) except for one growing season after a prescribed fire in the chaparral [141].

Predators frequenting blue oak habitats include mountain lions, coyotes, bobcats, gray foxes, northern raccoons, American badgers, and skunks [62]. A night-camera survey in a mixed-oak riparian woodland in Napa County's wine country showed that striped skunks, bobcats, coyotes, gray foxes, and mountain lions used the oak woodland corridors extensively. Predator usage was significantly greater in wide, undisturbed corridors compared to narrow or denuded corridors (P=0.03). Blue oak was one of the dominant oaks in the woodland [115].

Several rare or threatened species use blue oak woodland habitat. Bald eagles, golden eagles, peregrine falcons, and California condors inhabit blue oak woodlands [245]. A study on the Sierra National Forest found California spotted owls used live blue oaks for nesting [220]. Purple martins in the Tehachapi Mountains, where what are probably the last remaining purple martin populations in the state occur, also use blue oaks for nesting [254]. The state-endangered foothill yellow-legged frog inhabits blue oak woodlands [31], and the western spadefoot is restricted to blue oak woodlands [245]. Blue oak woodland was prime habitat for the now-extinct California grizzly bear, the largest of the brown bear subspecies [164].

Livestock, mule deer, lagomorphs, and rodents browse blue oak. The acorns are eaten by at least a dozen species of songbirds, several upland game birds, rodents, mule deer, feral and domestic pig, cattle, and all other classes of livestock [5,68,213]. The acorns are a critical food source for mule deer, which migrate from dry, high-elevation summer ranges to blue oak woodland for fall and winter forage [28,173]. Blue oak acorns accounted for about 15% of the total volume of food consumed by mule deer on the Tehama County winter range [28,213].

Band-tailed pigeons and acorn woodpeckers preferentially select small blue oak acorns over large acorns [9,79], which may positively affect blue oak seedling establishment by leaving large acorns available for establishment.

Palatability/nutritional value: Blue oak browse and acorns are highly palatable to livestock and wildlife. Ungulates generally prefer browsing blue oak's spineless leaves over spiny leaves of associated live oaks [164]. Blue oak sprouts are palatable to all classes of browsing wildlife and livestock. Sampson and Jesperson [213] gave mature blue oak foliage the following browse ratings:

Columbian black-tailed deer: excellent to good
domestic sheep: fair to poor
domestic goats: fair to poor
cattle: poor
horses: poor to useless

Blue oak browse is relatively high in protein and low in tannins compared to levels in associated oaks [164]. The crude protein content of young, partially expanded leaves of blue oak on the San Joaquin Experimental Range averaged 30%, while that of fully developed leaves averaged 11%. The ratio of calcium to phosphorus is nutritionally satisfactory for cattle in young leaves (2.2:1.0) but disproportionate in mature leaves (15:1). Blue oak acorns are low in crude protein but high in crude fiber, fat, and oils [84,213]. For detailed nutritional analyses of blue oak browse and acorns, see Sampson and others [84,213].

Cover value: Blue oak canopies provide important shade cover for a variety of animals during California's hot summer months. Beef cattle gain more weight on rangelands where blue oak provides relief from summer heat compared to rangelands without blue oak cover [120].

Blue oak provides cover for cavity-nesting birds. A breeding bird survey on the Hopland Field Station showed that cavity-nesting species dominated the avian community within blue oak woodlands. Violet-green swallows and plain titmice were the 2 most common of 72 bird species that used blue oak woodland habitats [257].

Since blue oak is the dominant—and sometimes the only— oak in the foothills surrounding the Central Valley, many species of birds use blue oak for cover. Blue oak provides preferred nesting, foraging, and escape cover for the Nuttall's woodpecker, plain titmouse, and white-breasted nuthatch [30]. On the Sierra Foothill Range and Field Station, Nuttall's woodpeckers foraged on blue oaks more often than associated oaks or gray pine, with this preference extending across all seasons [29]. A San Luis Obispo survey of red-tailed hawk nest sites revealed that red-tailed hawks used blue oaks as their nest tree 74% of the time [233].

Downed blue oak woody debris provides cover for small mammals such as dusky-footed woodrats [247] and herptiles. Lizards used downed blue oak logs and branches as cover on the Sierra Foothill Range and Field Station [32].

Except in riparian zones, xeric conditions may make blue oak woodlands poor habitat for many amphibians in summer; however, salamanders including 3 Species of Concern (yellow-blotched, Tehachapi slender, and Kern County slender salamanders) utilize blue oak cavities and downed logs for thermal, foraging, and mating cover [30,222]. Reptiles are frequently encountered in blue oak woodlands [30], using downed blue oak logs and branches for foraging, thermal, and mating cover [36]. Several species (for example, the western fence lizard and western skink) prefer the drier environment of blue oak and other deciduous oak woodlands, and seldom use the more mesic evergreen oak woodlands [30]. Borchert and others [36] provide an inventory of herptiles found in blue oak communities of the Southern Coast Ranges.

Blue oaks overhanging water courses provide shade in fish habitats. Water temperatures increase when riparian-zone blue oaks are removed, lowering habitat quality for salmonids. Blue oak woody debris affects the physical structure of rivers and streams by creating pools and reducing the sediment load. To protect fisheries, Giusti and Merenlender [82] call for increased protection of and management guidelines for low-elevation hardwoods in riparian zones.

VALUE FOR REHABILITATION OF DISTURBED SITES:
Blue oak is planted for restoration of wildlife habitat, riparian zones, and watersheds. The roots bind soil of steep watersheds, reducing the incidence of mass soil movement downslope into permanent and ephemeral streams [173]. Trees are established from acorn plantings or from transplanting nursery stock. Blue oak was successfully used to revegetate upper streambanks in Almaden Valley [83] and elsewhere; however, restoration on some rangelands where blue oak was removed for "rangeland improvement" (see Management Considerations) is sometimes difficult. Sites where oaks were removed are subject to increased rates of soil erosion compared to sites where oaks were not removed [40].

A case study from northwestern California illustrates how oak removal was accomplished and why blue oak has difficulty regenerating on some sites where oaks were removed. An inventory of a tree-removal site on the Hopland Research Station showed that evergreen oaks (coast and interior live oaks and scrub oak (Quercus berberidifolia)) were regenerating successfully on tree-removal sites. Deciduous oaks (blue oak and California black oak), however, were not regenerating. Tree removal was conducted from 1959 to 1965, using herbicide spraying followed by prescribed fire. The burn was seeded to nonnative clovers (Trifolium spp.), nonnative perennial pasture grasses, and nonnative soft chess (Bromus hordeaceus). A few mature oaks were left for aesthetics and to provide thermal cover for domestic sheep, and a few oaks sprouted after being sprayed and burned. These few remaining oaks are the parents of current oak regeneration. Aerial and GIS surveys show that 52% of the area was blue oak woodland prior to tree removal. In 1996, blue oak woodland covered 2.5% of the area, showing the greatest cover loss of 5 oak species. Blue oak is now regenerating only in riparian zones and in clusters beneath parent blue oaks. Dry soils, browsing pressure from domestic sheep, mule deer, small mammals, and insects, and/or competition from nonnative herbaceous species are implicated in blue oak's failure to regenerate on the site. The authors concluded that deciduous oaks, particularly blue oak, required artificial plantings given shade and protection from browsing for successful restoration [40].

Field and greenhouse experiments show that blue oak establishment is highest beneath shrub canopies or under shade cloth [43,184]. Protection from herbivory and full sunlight are generally recommended to optimize establishment of artificial blue oak regeneration [6,40,171,229]. Propagation techniques are discussed in the following sources: [2,5,168,168,183,192,234]. See these sources for acorn harvest methods [148], evaluation of herbivory protection devices [60,148,171], shade devices [171], control methods for annual grasses growing with blue oak seedlings [171], and planting methods for acorns and containerized blue oak seedlings [148,216]. Rehabilitation of a site on the Sierra Foothill Range and Field Station where blue oaks had been completely removed in the 1960s was finally successful—after 2 attempts were thwarted by grasshopper and rodent browsing—by using a combination of cattle grazing to reduce rodents, which compete poorly for herbaceous forage against cattle, and tree shelters to protect artificial blue oak regeneration from the cattle [229].

OTHER USES:
Blue oak is a valuable landscaping ornamental. A study was conducted in Mendocino and Sonoma Counties to assess the value of blue oak on lands undergoing subdivision. It showed that the aesthetic and amenity values of blue oaks at a density of 40 stems/acre resulted in a 21% to 27% increase in land value when compared to sites with no blue oaks [217].

Native peoples processed blue oak acorns into meal [16]. They used blue oak sprouts for making baskets [14], acorn leachate for dying baskets, and blue oak wood for making bowls and other implements [53,237].

OTHER MANAGEMENT CONSIDERATIONS:
Blue oak occurs mostly on private lands. Blue oak woodland covers 8% of California's total land area [5,19]. Seventy-five percent of this resource is in private ownership, 14% is in the National Forest System, and 11% is in various other public ownerships [33].

Management of this species has been controversial. From the late 1950s through the early 1970s, many studies showed that palatability and production of forage in the understories of blue oak ecosystems were low compared to forage on open sites without blue oaks [111,132,135,136,189,190]. Murphy and Crampton [190] and Kay and Leonard [136] found blue oak removal increased productivity of annual grasses. As a result of these studies, statewide "rangeland improvement" was recommended, involving removal of blue oak from livestock rangelands [238,243]. This recommendation resulted in the loss of more than 1 million acres (0.4 million ha) of blue oak woodland from combinations of cutting, prescribed burning, and herbicide spraying [33,243].

In contrast, other studies found forage production was 15% to over 100% higher under blue oaks than in open grassland [64,68,76,77,118,119,202], and that herbaceous plants beneath blue oak were higher in protein, nitrogen, phosphorus, and potassium and lower in lignin and fiber than herbs growing in open grassland. Forage under blue oak started growing earlier and remained green after surrounding forage had dried [42,119].

Blue oak removal may result in decline of soil quality and fertility. In a study on the Sierra Foothill Range and Field Station, soils where blue oaks were removed 21 years prior had less soil microbe biomass and bulk density, less total carbon, nitrogen, and phosphorus, and higher pH compared to soils with blue oaks [49]. Callaway [48] found nutrient deposition under blue oak canopies was 5 to 10 times greater than in open annual grassland. Nutrients came mainly from tree litterfall and precipitation/dew throughfall. Nutrient deposition varied seasonally. Nitrogen was mostly deposited in litterfall from September through December. Phosphorus, potassium, and magnesium deposition occurred mainly from rain throughfall in winter and spring. Blue oak canopies play an important role in nutrient cycling [48]. A study on the Hasting Natural History Reservation found that by throughfall, epiphytic lichens on blue oak deposited a mean of 2.85 kg/ha/year of atmospheric nitrogen and 0.15 kg/has/year of atmospheric phosphorus [143].

Duncan and Clawson [68] reported that cattle prefer forage beneath blue oak to that of open grassland, even in summer after forage in both areas has dried. Holland [118] found that death or removal of blue oak resulted in a gradual decline in forage production and quality. Supporting this, another study showed an increase in unpalatable tarweed (Madia gracilis) following blue oak removal [136].

Plant-water-site interactions may explain the discrepancies between studies showing that blue oak decreases forage production and those showing that blue oak enhances forage production. One study [23] suggests that forage production beneath blue oaks is relatively high on dry sites but is low on more mesic sites. Another study found that blue oaks with shallow, fine roots inhibited production of understory herbs compared to production of herbs beneath deeply-rooted blue oaks [47] (see General Botanical Characteristics for information on how blue oaks develop roots). This inhibition may be partially attributable to allelopathic blue oak root exudates as well as competition for water and nutrients. Variations in root morphology may also partially explain differences in understory production in blue oak ecosystems. Climate fluctuations and degree of canopy closure also influence understory forage production. Annual grass production varied significantly among years (P<0.05) on the Sierra Foothill Range and Field Station, with high rainfall years favoring growth of groundlayer vegetation under blue oak canopies, and low rainfall years depressing forage production beneath blue oak canopies [57]. In the North Coast Ranges and Sacramento Valley, blue oaks on sites receiving less than 20 inches (500 mm) mean annual precipitation had no effect on or enhanced understory production, while blue oaks on sites receiving more precipitation generally suppressed understory production [75].

Blue oak probably has both positive (facilitation) and negative effects (allelopathy) on understory herbs, with both influencing groundlayer plant species composition. A study on the Hopland Field Station found ripgut brome tended to dominate under blue oak canopies, while soft chess dominated open grassland. The researchers credited better performance of ripgut brome beneath blue oak canopies to increased nutrient availability there [207]. However, blue oak can retard growth of annual grasses. Reciprocal transplant experiments in the field and greenhouse indicate that some blue oak root exudates are allelopathic to ripgut brome. This effect probably also extends to other annual grass species.

Blue oak's negative effects on annual grass productivity may be at least partially ameliorated by blue oak's "root pump", which facilitates annual grass growth. Blue oak's fine roots leak water and nutrients into surrounding soil [42,127]. Callaway [42] suggests that blue oak roots deep in the soil extract nutrients from low soil layers, then transport and exude the nutrients into upper soil layers where they are available for groundlayer herbs [48]. Holland [118] and others [119] also found these facilitative effects of blue oak on annual grasses, with soil nutrient levels remaining higher under blue oak canopies than in open grassland for 10 or more years after tree death.

Although blue oak is now generally regarded as a desirable species, blue oak populations continue to decline. Continued clearing of blue oak on rangelands and poor natural regeneration were 2 major management concerns identified by the Hardwood Task Force of the California Board of Forestry [5]. Road construction and residential and commercial development [196], lowering water tables  [246], and use of blue oak for fuelwood [173] all contribute to blue oak decline. Since most blue oak woodlands are on private lands, development is probably the largest threat to blue oak habitats [196]. Development accounted for 46% of blue oak loss between 1973 and 1985, surpassing the loss from rangeland clearing before that.

Damaging agents: Blue oak is vulnerable to several species of fungi. The most serious of these are Inonotus dryophilus, Laetiporus sulphureus, and Armillaria mellea, all of which cause heart and root rot [173]. Compacted soil may increase blue oak susceptibility to root-rot fungi [59]. Like other oaks in the white oak subgenus, blue oak is resistant to the fungus-like water mold causing sudden oak death disease [210].

Pacific mistletoe (Phordendron villosum) infects blue oak [230].

A large number of insects infest blue oak. One study showed that 38 species of insects inhabit blue oak, attacking every part of the tree. The most damaging of these pests in terms of regeneration are the acorn feeders, which include cynipid wasps, the filbert weevil (Curculio uniformis), and the filbert worm (Melissopus latiferreanus). These insects can destroy large portions of a year's acorn crop [173].

Raabe [201] provides a general review of diseases and insect pests on blue oak and other California oaks. See Swiecki and others [224,225] for more detailed descriptions of insect and diseases affecting blue oak.

Quercus douglasii: REFERENCES


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