Total basal area, density, mortality, and
ingrowth of California black oak on unburned and burned plots
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (for example, [66,98,100]).
Form: California black oak is typically 30 to 80 feet (9-25 m) in height and 1 to 4.5 feet (0.3-1.4 m) DBH at maturity. Large trees may exceed 120 feet (36 m) in height and 5 feet (2 m) DBH. On open, fertile sites form is tall and straight, with a clean bole and a broad, rounded crown [188]. The champion tree, on the Siskiyou National Forest of Oregon, is 124 feet (38 m) tall, 338 inches (859 cm) in circumference, and has a 115-foot (35 m) spread [9]. California black oak's lower branches may nearly touch the ground, form a browse line, or be clear from the bole for 10 to 40 feet (3-12 m) above ground [188]. In closed stands, the crown is narrow and slender in young trees and irregularly broad in old trees. Trunks are usually free of branches for 20 to 40 feet (6-12 m) in closed stands. Old trees often have forked trunks that are decayed and hollow [193]. California black oak sometimes grows in scrub form on dry, infertile sites [42,147].
Aboveground morphology: California black oak has thin, smooth bark when young. Bark becomes moderately thick, deeply fissured, and platy with age. Bark of mature trees is 0.75 to 2 inches (1.9-5.1 cm) thick [98,170]. California black oak is the only western oak species in the red oak subgenus that is deciduous [275]. The deeply lobed leaves are typically 4 to 8 inches (10-20 cm) long [170]. Flowers are unisexual. Staminate flower clusters (catkins) emerge from the leaf axils of the previous year's growth [33,147]. Pistillate flowers are solitary and develop from current-year leaf axils [33]. Acorns are relatively large in this species, from 1 to 1.2 inches (2.5-3 cm) long and 0.6 to 0.7 inch (1.5-1.8 cm) wide [6,98,170].
Oracle oak resembles California black oak. It is readily identified in winter by its sparse evergreen foliage, which contrasts with California black oak's winter-deciduous character [147].
Roots: California black oak grows from one to several taproots that may penetrate to bedrock. It has large, laterally spreading roots that extend from vertical ones, and many surface roots. Trees on shallow soils tend to have more vertical roots than trees on deep soils [149]. California black oaks on shallow or fine-textured soils may have vertical roots and no apparent taproot. Taproots of trees on clay or other fine-textured soils usually decay [149]. Roots often graft when root crown sprouts are thick, forming a dense, interclonal mass of roots [149,209].
Physiology: California black oak is highly drought tolerant. A Hastings Natural History Reservation study found that among 5 associated oak species, California black oak and blue oak were the most drought tolerant based on xylem water potentials [120].Stand and age class structure: California black oak occurs in pure stands, mixed stands, and scattered groves. Pure stands usually indicate sites unfavorable to conifer growth or repeated disturbance such as fire or logging [62,147]. In pure stands, California black oak is usually even-aged, originating from sprouts after a top-killing event such as logging or fire. Such stands are usually less than 125 years old. Some seedlings may occur in these mostly even-aged stands [148].
© Br Alfred Brousseau, Saint Mary's CollegeAge class structure varies in mixed stands with a California black oak component. In stands where fire has been excluded, there is a trend towards mostly mature (>50 years) California black oaks, few seedlings and sprouts, and almost no saplings [17]. In a Placer County, California, mixed-conifer-California black oak stand, mean California black oak age was 148 years, ranging from 58 to 356 years [75]. Disturbance history of the site was not available. Age class structure is not always skewed toward old trees, however. Tree coring on the Hall Canyon Research Natural Reserve in the San Jacinto Mountains of California showed a California black oak age class structure dominated by true seedlings (not sprouts), with a mean frequency of 290 seedlings/900 m². The next largest age class was 60- to 79-year-old "saplings", with mean frequency of 8.5 saplings/900 m². Trees in the 120- to 139-year-old age class, the oldest represented, were slightly more abundant than trees in the sapling class (mean frequencies of 2 trees/900 m² and 2.5 trees/900 m² for 120- and 139-year old trees, respectively). There were no dead California black oaks. Study sites were logged in the early 1900s, and fire had been excluded since logging [213] (see Size class recruitment for further information).
Individual California black oaks may live 500 years or more [42,147,227], although 100 to 200 years is more typical [225].
California black oak is native to Oregon and California [147,194]. It grows in the foothills and lower mountains of southern Oregon and California, with a north-south range of about 780 miles (1,260 km) [149]. It is contiguously distributed from the North Coast Ranges in Lane County, west-central Oregon, south through the Coast and Cascade ranges, the Sierra Nevada, and the Transverse and Peninsular ranges to just north of the Mexican border in the Agua Tibia Mountains of San Diego County, California [100,147,170]. One California black oak population extends into the Great Basin in Lassen County, California [275]. An isolated population occurs in the Sierra de San Pedro Mártir of northern Baja California [188]. The US Geological Survey provides an online distributional map of California black oak. Young and others [275] provide a detailed description of California black oak's distribution in California. Gaman and Casey [71] provide remote sensing basal area data for California black oak in California.
Oracle oak is the most common and widespread California black oak hybrid [147]. Oracle oak hybrid swarms occur on lower slopes of the western Sierra Nevada and on the inner Coast Ranges from Shasta and Siskiyou counties south to Ventura County, California. Rarely, they are found in southwestern Oregon. California black oak à coast live oak swarms occur on the Central Coast Ranges [147,275].
Fire adaptations: California black oak has several adaptations that enable it to survive fire. Its thick, insulating bark [98,170] protects it from low- and most moderate-severity surface fires. If top-killed, it sprouts from the root crown [4,64,69,148,208,231,261]. Its extensive root system has large nutrient and water reserves and is capable of supporting prolific sprouting [42,64,148,159,193,275]. California black oak also establishes from acorns after fire [64,110,191]. Dispersal of off-site acorns by seed-caching animals probably allows for burn colonization [64,110]. For fires that do not top-kill all parent trees, California black oak seedlings likely establish from crown-stored acorns as well as from off-site seed sources.
Fire prepares a site in several ways that promote California black oak seedling establishment and growth. Seedbed preferences (mineral soil or light duff) match those produced by fire [110]. California black oak is an early seral species that requires light for rapid growth (see Successional Status). Surface fires thin the canopy and create small openings for California black oak seedling establishment by killing fire-intolerant, late successional conifers [191]. Mixed-severity and stand-replacing fires provide large openings where California black oak stands may establish.
FIRE REGIMES: All of the ecosystems where California black oak is important are fire dependent. Forests and woodlands with California black oak historically experienced short-return interval surface fires and/or mixed-severity fires [110,251,251]. Fire altered woodland and forest structure by thinning shrubs and late-successional, fire-sensitive conifers while retaining large, fire-resistant trees including California black oak [36,260,261]. Reconstructive studies suggest that low-elevation forests and woodlands with California black oak had mostly open canopies. At low elevations, where California black oak is most frequent, fires were of mostly low severity. There were probably exceptions to this general pattern, however. Chang [48] stresses that "variation and scale matter" when describing California's FIRE REGIMES. Pioneer naturalists Muir [168] and Jepson [105] described open, parklike forests in low elevations of the Sierra Nevada, but others noted dense patches of vegetation within otherwise open stands [132,244]. Gruell [89] provides a series of repeat photographs, starting in 1849, that document extensive, open ponderosa pine-California black oak woodlands in the Sierra Nevada in the mid-1800s. The series also shows denser mixed-conifer forests, with a higher proportion of white firs and Douglas-firs, on north- and east-facing slopes than on more open south- and west-facing slopes [89]. Some speculate that at midelevation, California black oak and other drought-tolerant trees historically prevailed on dry aspects, while shade-tolerant conifers were more prevalent on moist aspects. Mixed-severity fires at midelevation probably helped create that mosaic [65,261].
Severe fire was probably infrequent in presettlement mixed-conifer forests of Oregon and California, but mixed-conifer forests experienced severe fires occasionally. Patchy stand-replacement fire was important in maintaining California black oak stands [147]. Long-term data from giant sequoia stands suggests that severe fire historically occurred in small patches [185,236], while both widespread and localized surface fires were common [45]. There is little historical information on the size and distribution of high-severity fires in lower montane forests of the Sierra Nevada [261]. Pioneer accounts of persistent California black oak and chaparral stands―which are generally maintained by stand-replacement fire―within ponderosa pine and mixed-conifer forests suggest that patchy, stand-replacement fire occurred occasionally in low- and midelevation forests [132,263]. Patchy, severe fire may have helped maintain California black oak stands within conifer forests.
In a study of fire histories from 1650 through 1920, Stephens and Collins [230] found a trend of increasingly late-season or increasingly early growing-season fires from the southern Cascade Range south to the southern Sierra Nevada. Widespread fires were associated with drought or a wet year just before the fire year. They provide the following compilation of FIRE REGIMES in forests where California black oak occurs [230]:
Summary of fire histories from the southern Cascades and Sierra Nevada before European-American settlement
Area and cover type Median FRI*A more detailed summary of historic FIRE REGIMES in ecosystems where California black oak is important follows.
Ecosystems with mostly surface fires:
California black oak forests and woodlands historically experienced very frequent understory fire [220]. Holland [101] describes California black oak woodland as "an obvious fire type": without very frequent fire California black oak cannot maintain dominance on most sites, and is successionally replaced by conifers [101,255]. Fire-return intervals of less than 10 years are required to stop successional replacement of California black oak by conifers [2,3]. Historically, live understory fuels in California black oak stands were mostly herbaceous species with scattered shrubs. Although voluminous, California black oak litter decomposes relatively rapidly (see Fuels). Total fuel accumulation in California black oak communities was historically low, and low-severity fires rarely killed mature California black oak branches and stems. California black oak's open crown did not often support crown fires in the California black oak type [220]. A century of fire exclusion and successional replacement in the Sierra Nevada has greatly reduced the extent of pure California black oak stands [186].
California black oak occurs in mixed-oak woodlands at Annadel State Park and other sites in the North Coast Ranges of northern California. Historic fire regime in these mixed-oak woodlands was frequent surface fires. Heart rot in oaks precludes tree coring and exact fire dating, but fire-return intervals were probably less than 10 years. In the absence of fire, Annadel State Park oak woodlands are being invaded by understory Douglas-fir and California bay (Umbellularia californica), which are successionally replacing California black oak and other oaks. Most Douglas-firs and California bays in the oak woodlands established after 1910, and most are 50 years of age or less [26], suggesting that prior to 1910, frequent surface fires killed young Douglas-firs and California bays in the understory.
Ponderosa pine-California black oak forests had frequent, low-severity surface fires [19,203] until fire exclusion began in the late nineteenth century [203]. Contemporary forests are increasingly dominated by understory Douglas-fir, white fir, and incense-cedar [186,203]. Fire-return intervals for ponderosa pine forests with a California black oak component ranged from 6 to 22 years in the Cascade Range of southern Oregon and northern California (review by [203]). Fire-return interval in ponderosa pine-California black oak forests historically increased with increasing elevation in the Sierra Nevada [45], with a tendency towards shorter mean fire-return intervals (5-15 years) on dry, west- and south-facing slopes and longer fire-return intervals (15-25 years) on mesic, east- and north-facing slopes. Midelevation forests typically had mixed-severity fires that created patchy mosaics [65].
Other yellow pines: California black oak is an associate in Jeffrey pine and Washoe pine forests. Return intervals for surface fires in these relatively dry yellow pine ecosystems were probably slightly longer than in the moister ponderosa pine-California black oak types, which had higher fuel loads and greater fuel continuity. Many Jeffrey pine and all Washoe pine communities are east of the Cascade Range-Sierra Nevada rain shadow. In a review, Riegel and others [203] report that fire-return intervals for Jeffrey pine and mixed Jeffrey pine-ponderosa pine forests ranged from 2 to 56 years in the northern Warner Mountains and 3 to 38 years in south-central Oregon. Smith [222] estimates that historic fire-return intervals ranged from 5 to 20 years for northeastern California Jeffrey pine types. A fire history study in the San Bernardino Mountains showed mean fire-return intervals of 14 and 10 years in Jeffrey and ponderosa pine forests, respectively, during Native American (before 1860) and European settlement (1860-1904) periods. From 1905 to 1974, mean fire-return intervals became significantly longer (P=0.05), at 66 years and 32 years in Jeffrey and ponderosa pine forests, respectively [145].
Washoe pine likely had a different fire regime than either ponderosa pine or Jeffrey pine. Washoe pine communities experienced surface fires, but average intervals between fires were probably longer than for the other western yellow pines. The only fire scar data from Washoe pines (as of 2007) show fire-return intervals ranging from 3.5 to 40 years for a Warner Mountains population (Smith, S., unpublished data cited in [203]).
Native Americans increased historical fire frequency in yellow pine types, including ponderosa pine, through intentional use of fire [11,14,16,127,133,142,202,220]. Very frequent fire-return intervals converted the forests to oak woodlands on some sites [11,127,142,202]. Along with patchy stand-replacement fire, Native American fires were critical in maintaining California black oak as a cover type. Among oak species, Native Americans targeted California black oak stands in particular for frequent underburning. California black oak acorns were a valuable food source (see Other Uses), so Native Americans maintained California black oak stands through active fire management [11,16,133,202]. Kilgore and Taylor [116] concluded that natural ignitions alone would not support the very short fire-return interval (2-3 years) that fire chronologies show for some mixed-conifer stands in the Sierra Nevada. Historical records indicate that Yosemite Valley Miwoks burned annually in order to stop successional replacement of California black oak and other oaks by ponderosa pine [16]. Native American burning had at least 6 effects that benefited the tribes. Burning reduced acorn pests; facilitated fall acorn collection; maintained open forest structure useful for hunting; promoted edible and basketry grasses; increased edible mushroom production; and promoted sprouting woody species used in basketry [11,13,14]. In a review, Arno [19] states "sizable areas of open woodland dominated by ... California black oak ... were characterized by frequent understory fires largely due to deliberate burning by Native Americans". Atzet and Wheeler [22] suggest that Native American burning in the Siskiyou and Klamath mountains historically influenced FIRE REGIMES there, although the extent and effects of Native American fires there are mostly unknown.
Mixed-conifer montane forest: At higher elevations, Sierra Nevada mixed-conifer forests had historically longer fire-return intervals compared to lower-elevation ponderosa pine forests. The fire regime was mostly low-severity underburns at intervals ranging from 7 to 16 years [261]. Studies on the Feather River-San Joaquin River watershed of the Sierra Nevada show a historic fire-return interval ranging from 7 to 9 years in the mixed-conifer zone [268]. A Lake Tahoe study showed a range from 5 to 15 years from 1649 to 1921 [232].
In giant sequoia groves, where California black oak is a common hardwood, understory fires historically reoccurred at 2- to 39-year intervals [45,116,261]. Restoration thinning, prescribed underburning, and wildland fire use in some giant sequoia forests has helped reduce the potential for large, stand-replacement fires [119,261]; however, fire is largely excluded in most mixed-conifer montane forests (review by [261]). With fire exclusion, giant sequoia, ponderosa pine, sugar pine, and California black oak are being successionally replaced by incense-cedar and white fir, and tree density is increasing [174,186]. White fir in particular is increasing fuel loads and vertical continuity of fuels in giant sequoia-mixed conifer-hardwood forests. A 1997 study in Sequoia-Kings Canyon National Park showed that giant sequoia-mixed forest had greater total downed fine and woody fuel accumulation, as well as greater potential for crown fires due to understory late-successional conifers, compared to adjacent lower-elevation ponderosa pine forest [186].
Ecosystems with mixed-severity fires:
California black oak is a component of some redwood forests. These forests historically experienced mostly patchy, moderate-severity surface fires. Fires would occasionally crown, especially in the southern and eastern edges of the redwood forest type [242]. Fire-return intervals in redwood forests historically ranged from 20 to 500+ years. Veirs [264] found a range of 50 to 500 years near Crescent City, California, while researchers in Redwood State Park [63,241] and Muir Woods [104], California, reported fire-return intervals of 20 to 31 years. Some report intervals of 125 to 500 years where redwood merges with wetter Sitka spruce (Picea sitchensis) forests (review by [242]). Stuart [241] reported significant differences (P<0.05) between fire-return interval means for presettlement (before 1875, x=31 years), settlement, (1875-1897, x=15 years), and postsettlement (after 1897, x=14 years) in Redwood National Park. Fire sizes were not correlated with fire frequency or settlement period [241].
Mixed-evergreen and dry Douglas-fir forests of southern Oregon and northwestern California, which have a California black oak component, historically experienced mixed-severity fires [4,135,242]. Agee and Edmonds [4] define a mixed-severity fire as one that top-kills 20% to 70% of a stand. Fire-return interval in the Klamath-Siskiyou region ranged widely, from 3 to 116 years, with return intervals in the shorter range for low-elevation forests where California black oak is most prevalent (review by [135]). Mean fire-return intervals were longest, and fires most severe, on north- and east-facing slopes [251,251]. In a review of Agee's previous work, Agee and Edmonds [4] report a decreasing fire-return interval from coastal to inland forests and from north to south, from 90 to 150 years in coastal southwestern Oregon to about 50 years in central-southwestern Oregon. Dry Douglas-fir types in the Siskiyou Mountains had a mean fire-return interval of 20 years prior to European settlement; presettlement fire-return intervals in the Salmon River watershed of northern California ranged from 10 to 15 years. After stand-replacing fire, dry Douglas-fir types of southern Oregon and coastal northern California generally had "several low to moderate severity fires" that thinned the understory of young white firs and Douglas-firs and favored California black oak and other sprouting hardwoods. Agee and Edmonds [4] speculate that presettlement stand structure of mixed-evergreen and dry Douglas-fir forests differed from present conditions, with fewer small trees, snags, and logs due to frequent fires, and suggested that these forests usually reached old-growth stage (about 250 years) before the next stand-replacing fire [4]. At the landscape level, stand-replacing fire was patchy, creating a mix of old-growth forest and younger forest stages [4,135] that favored California black oak. Taylor and Skinner [250] found a historic pattern of mixed-severity and stand-replacement fires, including several severe fires, on the Klamath National Forest in the mid-19th century. For Douglas-fir-white fir-sugar pine stands, fire-return intervals were shorter during the presettlement period (1627-1849, x=14.5 years) than during the fire exclusion period (1905-1992, x=21.8 years). Mean fire-return interval from 1627 to 1992 was 12 years, with a range of 6 to 35.5 years [250].
Current period and fire exclusion: California black oak populations have declined since presettlement times. The decline is due to several factors (see Management Considerations), with fire exclusion among the most important. Major structural changes in woodlands and conifer forests of California and southern Oregon have occurred with fire exclusion, with dense stands of conifers or dense, mixed stands of conifers and shrubs dominating the once open stands where California black oak flourished. Fuel loading in these forests represents an unprecedented buildup of live woody fuels, snags, and downed woody materials [4,203,251]. When these forests burn, the dense understory produces a ladder effect that often results in crown fire [4,251]. This results in high-consumption, mixed-severity or severe fires that are sometimes fatal to California black oak [110].
Fire has been excluded for decades in most of the southern Cascade Range and the Sierra Nevada, so fire-dependent communities have missed several fire cycles. Contemporary FIRE REGIMES include less frequent, larger fires compared to presettlement times [135,156,261]. In the Siskiyou-Klamath region, for example, 5 fires larger than 20,000 ha occurred between 1987 and 2002. All 5 fires occurred during drought under extreme fire weather conditions. Most closed-canopy, old-growth forests that burned had not experienced fire since at least 1911, when fire records were first started [135]. Van Wagtendonk and Fites-Kaufmann [261] summarize that "Fuel accumulations, brush, small trees, and dense forests produce very different conditions for the inevitable fire that occurs, whether from lightning or human sources. Some headway is being made in wilderness areas and areas where prescribed fire can be applied safely and effectively" [261]. Schmidt and others [216] assessed Douglas-fir forests of the Siskiyou-Klamath region as "moderately or significantly altered" from historic conditions, and at risk of "losing key ecosystems components" without fuel hazard mitigation. In contrast to historically small fires, fuel build-up with fire exclusion has increased fire size in the dry Douglas-fir forests of the Siskiyou and Klamath National Forests to hundreds, sometimes thousands, of acres [4,7,7,24,24,251]. As in the past, these large fires are mostly of mixed severity but are larger in total area burned [7,24,88]. In the 1987 Hayfork Fire on the Hayfork District of the Shasta-Trinity National Forest, for example, crown fire (>50% of crown consumed) burned 5% of the landscape. Twenty-five percent of the landscape was >50% scorched; 32% was 10% to 50% scorched; and 38% was <10% scorched (Weatherspoon and Skinner (n.d.), unpublished document cited in [4]).
In mixed-evergreen forests, fire exclusion may not increase fire severity on all sites or in all stages of succession. In a fire history study of Douglas-fir-tanoak forests with a California black oak component on the Klamath National Forest, Odion and others [177] found a trend toward large fires (>1,500 ha) in recent decades compared to fire sizes before the 1970s, which averaged around 500 ha. However, 1 fire of the Klamath Wildfire Complex in 1987 was a large (100,000 ha), mixed-severity fire of mostly of low severity. Fire mix was 59% low, 29% moderate, and 12% high severity. In this wildfire, fire was generally most severe in shrubland and open forest and lowest in multiaged, closed-canopy forests. For closed-canopy forests, fire severity was lower where fire had been excluded since 1920 compared to sites that had burned since 1920. The authors concluded that fuel loads in the closed-canopy, mixed-evergreen forests had decreased, not built up, in the long absence of fire. Fuel build-up did occur in open forests and on plantations, however [177].
Fuels: Mature California black oaks contribute large annual loads of leaf litter and small woody debris. They also add substantially to large woody debris fuel loads [147]. Average annual litter accumulation under a 75-year-old stand in the Sierra National Forest of California was 0.6 ton/acre, and total litter accumulation was 6.2 tons/acre [117]. California black oak litter decomposes "rapidly", however [220]. One old California black oak (75-100 years) contributed 1,380 pounds of litter/year on the Sierra National Forest. Beneath its crown, total soil organic matter averaged 30,110 pounds/acre [147].
Standing dead fuel load of California black oak is also substantial. An inventory of dead crown fuels in ponderosa pine-California black oak stands in Placer County, California, showed that California black oak had significantly (P<0.001) more dead branches/tree (x=1.33 dead branches/tree) compared to ponderosa pine (x=0.45 dead branch/tree). Large California black oaks infected with Pacific mistletoe (Phoradendron villosum) had the greatest number of dead branches/tree. More California black oaks were infected with Pacific mistletoe (19% infection rate) compared to ponderosa pines infected with western dwarf mistletoe (Arceuthobium campylopodum) (4%) [76]. California black oak snags are rare [130,228]. An inventory on the Modoc and Lassen National Forests showed a total of 4 California black oak snags in 24 five-ha plots [130]. Fuel load estimates for Pacific madrone-California black oak-tanoak clearcuts on the Challenge Experimental Forest of Yuba County, California, range from 7 to 50 tons of slash/acre [147]. A photo series is available to help quantify fuels in ponderosa pine-California black oak stands and ponderosa pine stands with a component of California black oak [30].
Fire exclusion has resulted in a major increase in understory fuels in most communities with California black oak [261]. Stephens [229] compiled 1899 stand structure data, including basal area, density, and DBH, from 4 mixed-conifer stands in the northern and central Sierra Nevada. Two unlogged stands may provide reference condition information.
Several models are available for estimating standing California black oak fuels. Hann [92] provides a model for predicting crown width of California black oak and other overstory trees in mixed-evergreen forests. Sundahl [246] provides a model for predicting crown fuel load of California black oak. Snell [224] provides tables for predicting crown fuel loads and dry weight mass of California black oak [224]. Paine and Hann [183] give maximum crown-width equations for estimating densities of California black oak and other southwestern Oregon trees.
Sudden oak death disease may affect fuel loads and FIRE REGIMES in areas of heavy infection. Increased dead fuel loads, more open canopies, and changes in fuel moisture, understory composition and postfire successional pathways can follow oak mortality [54]. (See the discussion in Fungi and water molds for details of the disease.) Condeso and Meentemeyer [50] speculate that in Sonoma County, California, fire exclusion may have increased the area and density of oak woodlands relative to historic conditions, creating conditions favorable to rapid spread of sudden oak death disease. As of this writing (2007), studies are underway to assess potential impacts of sudden oak death disease on fuel loads and the fire ecology of California black oak and other western oaks [54].
The following table provides fire return intervals for some of the plant communities and ecosystems where California black oak is important. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) California chaparral Adenostoma and/or Arctostaphylos spp. <35 to <100 [189] California montane chaparral Ceanothus and/or Arctostaphylos spp. 50-100 Jeffrey pine Pinus jeffreyi 5-30 Pacific ponderosa pine* Pinus ponderosa var. ponderosa 1-47 [19] coast Douglas-fir* Pseudotsuga menziesii var. menziesii 40-240 [19,167,204] Pacific coast mixed evergreen Pseudotsuga menziesii var. menziesii-Lithocarpus densiflorus-Arbutus menziesii <35-130 [19,49] California oakwoods Quercus spp. <35 [19] coast live oak Quercus agrifolia 2-75 [84] canyon live oak Quercus chrysolepis <35 to 200 blue oak-foothills pine Quercus douglasii-P. sabiniana <35 Oregon white oak Quercus garryana <35 [19] California black oak Quercus kelloggii 1-30 [16,19,189] interior live oak Quercus wislizenii <35 [19] redwood Sequoia sempervirens 5-200 [19,63,241] *fire return interval varies widely; trends in variation are noted in the species reviewPrescribed and wildland fire are recommended as "the obvious and most feasible substitutes for filling the ecological role of historic fires" for wildland ecosystems with California black oak [19]. California black oak occurs in many vegetation types, all of which are fire-dependent, so fire or fire surrogate treatments may be needed to maintain or increase its coverage [110]. However, altered stand structure and composition, including a buildup of understory fuels, make it difficult to restore historic stand structure and plant community composition of montane forests using prescribed fire alone, so thinning is usually conducted before prescribed underburning to restore open structure and dominant seral trees (review by [19]). Evidence suggests that trends toward large, severe surface and crown fires will continue in montane ecosystems without reintroduction of fire and/or silvicultural treatments. Agee and Edmonds [4] suggest that mixed-conifer stands managed solely for softwood timber production will have more severe fires than stands also managed for hardwoods such as California black oak. They recommend repeat underburns at approximate 10-year intervals to reduce fuel loads and vertical fuel continuity [4]. Thomas and Agee [254] caution that in mixed-conifer stands with heavy fuel loads, a first fire "consumes much fuel, but also creates a lot." A second fire within 10 years consumes woody fuels from the first fire and reduces fuel loads below that of the first fire for at least 10 more years [254]. For yellow pine forests, Kauffman and Martin [110] recommend low- to moderate-consumption prescribed fire to promote California black oak and other hardwoods, targeting forest floor reduction to less than 8 to 16 tons/acre. This may require several fires [110].
In 1996, the National Park Service began mechanical hazard fuel reduction and prescribed burning in ponderosa pine and mixed-conifer forests in Yosemite National Park. Target conditions are fuel loads of 10 to 30 tons/acres; 5 to 25 pole-sized conifers/acre; and 90 to 150 overstory conifers/acre. Minimizing mortality of pole-sized and larger California black oak and other oaks is also a management objective. Treatment results are preliminary as of this writing (2007). Initial thinning reduced total fuel loads by 26% and pole-sized conifers by 76%; however, 1-hour and 10-hour flashy fuels were only reduced 17%. Total fuel reduction was below target goal with thinning alone, but follow-up prescribed burning is expected to further reduce fuel loads [184]. Van Wagtendonk [262] discusses the response of California black oak and other vegetation to prescribed fire in Yosemite Valley. See the Research Project Summary of van Wagtendonk's [260] study for further information.
Agee and Edmonds [4] recommend fuelbreaks to reduce fire severity in mixed-evergreen and dry Douglas-fir forests. Placed along ridgelines with otherwise continuous fuels, fuelbreaks provide a zone where fire severity is reduced between stands with heavier fuels and potentially higher fire severities. A high occurrence of California black oak and other sprouting hardwoods in these forest types means that fuelbreaks require maintenance every 10 years or less. They recommend fuelbreak design in Oregon Caves National Monument as a good example of a functional but visually pleasing fuelbreak [4]. Caution is warranted, however, because fuelbreaks provide a corridor for invasive nonnative plants, and monitoring is recommended if fuelbreaks are used [160].
Fuels management studies: A few studies provide results from fuel reduction treatments using thinning and/or prescribed burning in mixed-conifer forests with a California black oak component. Sweeny and Biswell [248] compare fuel load reductions after 4 February and March fires in a ponderosa pine-California black oak-Douglas-fir forest in Lake County, California. Knapp and others [119] compare fuel reduction and fire behavior on early season (June) and late-season (September-October) prescribed fires in mixed-conifer forest in Sequoia National Park. Stephens and Moghaddas [233,234] discuss results of a Blodgett Research Station fuels reduction study using crown thinning and thinning from below followed by mechanical treatment (mastication of understory trees), mechanical treatment followed by prescribed fire, prescribed fire alone, and a no-treatment control. They discuss benefits of retaining some coarse woody debris and creating snags with prescribed fire for wildlife benefit, and describe treatment impacts to forest structure [234].
While useful for the long-term goal of reducing fire severity potential, fuel mastication may increase fire severity on some sites in the short term. On Whiskeytown National Recreation Area in northern California, mastication did not reduce fuels. Instead, mastication converted standing live fuels into dead, small-sized surface fuels. When burned under prescription, mastication units had significantly (P<0.001) longer flame lengths and higher fire temperatures compared to unmasticated burn units. Mortality of pole-sized oaks, including California black oak, and pines was greater in masticated burn units compared to unmasticated burn units. For California black oak, average mortality of overstory trees was 23% and 0% on masticated and unmasticated units, respectively. Mortality of pole-sized California black oaks was 47% and 17%, respectively. The researchers describe techniques for reducing fire severity for managers interested in using mastication and prescribed fire to reduce fuel loads [38].
Prescribed fire in a mixed-conifer-California black oak forest near the Plumas National Forest successfully reduced fuel load. When a wildfire burned through the site previously burned under prescription, fire severity and fire suppression costs were less compared to adjacent land where fire had been excluded [165]. For further information on this study, see the Research Paper by Moghaddas [165].
A fall prescribed fire in the Tharp Creek Watershed of Sequoia National Park produced 50.0% average annual California black oak mortality on a white fir-mixed conifer site monitored for 5 years after fire. Mortality was concentrated in the subcanopy. The fire burned from 23 to 26 October 1990. Relative humidity during the day was 21% to 30% and at night was 30% to 40%. Fuel moisture levels in the litter and duff averaged 28%. For 100-hour and 1,000-hour fuels, moisture levels were 14% and 64%, respectively. At the time of ignition, air temperatures were 50 to 61 °F (10-16 °C) and winds were calm. The fire was a combination of backing and strip headfires with flame lengths of 0.16 to 7.9 feet (0.05-2.4 m). One-hour, 10-hour, and 100-hour fuels were reduced by 96%, 77%, and 60%, respectively. Tree (≥4.6 feet (1.4 m)) mortality was evaluated before and after fire as well as from an unburned reference site [171]. For more information, see the entire Research Paper by Mutch and Parsons [171].
Under prescribed burning conditions, moist leaf litter from large California black oaks may locally reduce combustibility of the forest floor. In Yosemite National Park, soil temperatures were monitored during a restoration prescribed fire in a ponderosa pine community. The fire consumed almost none of the duff layer beneath a large California black oak with a thick layer of loose leaves at its base. Both litter and duff layers were consumed 15 feet (4.6 m) away from the oak's canopy, where ponderosa pine litter prevailed. Fuel loadings were 13 tons/acre under the California black oak and 58 tons/acre away from the oak. During burning, soil temperature 2 inches (5 cm) below the soil surface was 65 ºF (18 ºC) beneath the California black oak, while soil temperature was 162 ºF (72 ºC) 15 feet (5 m) away from the oak's canopy. Forest floor consumption was more uniform where a smaller California black oak was growing on a steep incline and oak litter sloughed downslope. The fire burned to the base of the smaller California black oak, and the forest floor was completely consumed. Mean forest floor reduction was 61 tons/acre [211]. Further data were not available from this preliminary study. The authors noted that "additional work is needed to quantify the effects of prescribed burning on California black oak growing in association with ponderosa pine" [211].
Models: Miller and Urban [161] provide a model for predicting changes in stand structure and succession after prescribed burning or thinning in mixed-conifer or ponderosa pine forests with California black oak.
Wildlife: Prescribed fire can benefit wildlife, and it is sometimes conducted with wildlife habitat improvement as a major fire objective [118]. In Trinity County, California, prescribed burning was conducted in a foothills pine-Oregon white oak-California black oak community to improve Columbian black-tailed deer habitat by increasing available browse species, including California black oak, and opening the canopy. A research note on the fire documents that in postfire year 1, Columbian black-tailed deer used the burned area significantly more (P=0.05) than an adjacent unburned area. Use was based on pellet counts in burned (3,313 pellet groups/ha) and unburned plots (1,966 pellet groups/ha) [112].
Alternatives to prescribed fire: Prescribed fire cannot be used for fuel reduction on all sites. Dulitz and others [58] successfully used commercial thinning and slash treatment to reduce fuels in a mixed ponderosa pine-sugar pine-California black oak stand on the Mountain Home Demonstration State Forest of Tulare County, California. Large trees and some snags were left for wildlife habitat. The authors provide thinning costs and pre- and postharvest fuel loads [38].Soils: California black oak is most common on dry, well-drained soils. McDonald [147] states that "probably the most important single soil variable that limits the occurrence of California black oak is internal drainage. Black oak is not found growing 'with its feet wet' " [147]. California black oak may be a poor competitor on moist soils. A study in the Siskiyou Mountains of southwestern Oregon found that in mixed-conifer forests, California black oak was competitively excluded by white fir unless water stress exceeded 2,000 kPa [21].
California black oak has no strict soil texture preference [188]. Soil textures supporting California black oak range from sandy loams to gravelly clay loams [42,147]. Maximum growth rates are attained on deep, well-drained, slightly acid loams and clay loams [42,147]. Heavy clay topsoils rarely support California black oak. California black oak tolerates shallow, rocky soils, but tends toward a shrubby form, or is less frequent, on such sites [147]. California black oak grows on diverse parent materials including granite, basalt, and sandstone [42,147]. It is rare on serpentine soils [275]. McDonald [147] provides a list of principal soil series where California black oak occurs in California. Knops and Koenig [121] provide a fertility analysis for soils beneath California black oak vs. soils beneath other oaks species on the Hasting Natural History Reservation.
Aspect influences California black oak development. North- and east-facing aspects favor California black oak development in central and southern portions of California black oak's range, while south- and west-facing slopes are most favorable in California black oak's northern limits. A Tehama County, California, study showed differential California black oak density, depending upon soil moisture and aspect. On west-facing slopes, California black oak basal area was greatest on the driest soils, while on east-facing slopes, California black oak basal area was greatest on relatively mesic soils [85]. Aspect is not important to development in deep canyons [147]. California black oak is most common on the west slope of the Cascade Range and Sierra Nevada, but also occurs on dry, east-slope sites [275].
Terrain supporting California black oak ranges from level valley floors to alluvial slopes, rocky ridges, and steep slopes. Steep slopes are most typical, with landscapes strongly dissected by first- to fourth-order streams [147]. California black oak often dominates over ponderosa pine on dry ridges and in rocky canyons [40].
Elevation: California black oak's overall elevational range is 200 to 8,000 feet (60-2,400 m) [66,98]. It has the widest elevational range of any of the western oaks [147,154]. In Oregon, California black oak ranges from 450 feet (100 m) near Eugene to 3,000 feet (900 m) in the Klamath Mountains [147]. In California, elevation ranges from 200 feet (60 m) in Napa and Santa Rosa valleys to 8,000 feet (2,440 m) in the San Jacinto Mountains [42,147]. Overall range in the mountains of southern California is 3,600 to 8,000 feet (1,000-2,400 m) [80].
Climate supporting California black oak is mediterranean, with wet, mild winters and hot, dry summers. Eighty to ninety percent of the annual precipitation falls from November to April [159]. Mean annual precipitation ranges from 30+ inches (800 mm) in southwestern Oregon and 30 to 100 inches (800-3,000 mm) in northwestern California to 12 to 15 inches (300-400 mm) in northeastern California. Snowfall comprises 0% to 80% of total precipitation, ranging from 10% to 50% where California black oak is most productive. This snow zone, where California black oak is most abundant and reaches its greatest size, is on west-facing slopes of the southern Cascade Range and on west-facing slopes of the Sierra Nevada from Shasta County south to Tulare County. Mean maximum and minimum temperatures in that snow zone are 103 ºF and 5 ºF (39 ºC and -20 ºC), respectively [147].
California black oak mostly grows on the warmest sites at the northern edge of its range. Graham and others [83] rated California black oak as the best predictive indicator of warm sites on the Hungry-Pickett area of southwestern Oregon of 16 relative-temperature indicator species. It is an indicator species for relatively hot sites in the southern Umpqua Basin of southern Oregon [163].
Shrub form and site: A shrubby form of California black oak, once described as dwarf black oak (Quercus kelloggii forma cibata) [157], occurs in Shasta, Trinity, and Tehama countries, California. It occurs at elevations of 3,000 to 6,550 feet (910-2,000 m) on dry, shallow, rocky soils. These soils are often unstable, resulting from old slides or composed of colluvial material, and slopes are steep (70-80%) [149].
SRM (RANGELAND) COVER TYPES [219]:
109 Ponderosa pine shrubland
110 Ponderosa pine-grassland
201 Blue oak woodland
202 Coast live oak woodland
203 Riparian woodland
208 Ceanothus mixed chaparral
209 Montane shrubland
Some of the information in pages that follow comes from reviews by Burns and Honkala
[42] and McDonald [147].
Mature California black oaks usually survive fires of all severity. Low-severity fire generally top-kills saplings, while stem and branch tissue of larger California black oak survives [249]. Low-severity fire causes some cambium damage to trees pole-sized and smaller [25,110]. By about 60 years of age, California black oak bark is thick enough to survive low-severity fire [148]. Regardless of fire severity, spring fire corresponding to the active growing season results in greater tissue damage than fire in other seasons [110].
Moderate-severity surface fire top-kills pole-sized trees, but thick, insulating bark usually protects larger trees from fire kill [225,249]. Approximately half of all young California black oaks in a stand are killed by moderate-severity fire [110]; most other young California black oaks are top-killed [61]. For California black oaks large enough to escape top-kill, moderate-severity fire typically causes localized charring and cambium death in the trunk [193,225,249]. Plumb [192] states that "California black oak is the only (oak) species that develops bark sufficiently thick to resist low- to moderate-intensity fire in larger trees" (>6.3 inches (16 cm) DBH). The outer trunk bark is thinner and chars more readily than bark of associated pines, so California black oak trunks are more fire-sensitive to moderate-severity fire than associated ponderosa and sugar pines of similar size [162,220]. California black oak's cambium may incur heat damage even where bark is >0.5 inch (1.3 cm) thick [170,180].
Crown or severe surface fire usually top-kills mature California black oaks [162,180,220,225,249,275]. Crown or severe surface fire that burns into the root crown kills even large trees [41,110,147,155]. Mortality of pole-sized and smaller trees is common after crown fire [61,110]. Complete kill may occur in California black oaks of all size classes when individual trees or clumps of trees are surrounded by or adjoining brush [180]. Severe surface fire often kills shrubby California black oaks [110].
California black oak is a critical species for wildlife. Oaks (Quercus spp.) may be the single most important genus used by wildlife for food and cover in California forests and rangelands [60,253], and California black oak occupies more total area in California than any other oak species [31]. Its acorns are the largest of the western oaks [27], and are heavily consumed by livestock, mule deer, feral pig, rodents, mountain quail, wild turkey, jays, and woodpeckers [87,139]. California black oak and mixed-oak woodlands are important wildlife habitat. In riparian oakwoods of the Central and North Coast Ranges, for example, California black oak and associated oaks form riparian corridors within agricultural areas. A night-camera survey in a mixed-oak riparian woodland in Napa County's wine country showed that many predators, including striped skunk, bobcat, coyote, gray fox, and mountain lion, used the oak woodland corridors extensively. Predator usage was significantly greater (P=0.03) in wide, undisturbed corridors compared to narrow or denuded corridors [99].
The leaves and berries of Pacific mistletoe, which commonly infects California black oak, are important foods for a variety of birds and small mammals (review by [76]).
Cattle make heavy use of California black oak for food and cover [180,212]. They browse California black oak leaves and stems, and forage for acorns on trees and on the ground [212]. California black oak-mixed oak woodlands are important rangelands [245]. A Blodgett Forest Research Station study of forage use by free-range cattle found consumption of California black oak peaked in June (17.8% of diet), with use extending to August (10.2% of diet) [113]. Cattle may consume much of the California black oak acorn crop in some years. To reduce competition for acorns between mule deer and cattle, Kie and Loft [114] recommend removing cattle from oak rangelands during acorn drop.
Mule deer: California black oak woodlands and forests provide mule deer (Columbian black-tailed deer) cover and habitat [118]. Mule deer browse California black oak [207,212], and acorns are important mule deer forage. Mule deer use of California black oak browse peaks in summer and is lowest in winter [131]. They rely on the acorns after acorn drop: California black oak acorns constitute an average of 50% of the fall and winter diets of Columbian black-tailed deer in high mast years [195]. Fawn survival rates generally increase or decrease with the size of California black oak acorn crops [42]. Mule deer eat the acorns from fall through spring. On the Tehama Deer Range of California, California black oak acorns constituted a mean of 27.8% of the October mule deer diet and 21% of the April diet [147]. California black oak acorns are especially important in early spring just before and during snowmelt [147], when new green forage is still scarce.
Other large mammals: American black bear use California black oak forest types heavily for spring, summer, and fall cover [42]. American black bears and northern raccoons eat California black oak acorns [139]. Ponderosa pine-California black oak forests provide habitat for mountain lions and other large predators [221]. Foothill oak woodlands with a California black oak component provide prime mountain lion hunting grounds [52].
Small mammals: Chipmunks and squirrels make heavy use of acorns (Quercus spp.) [139]. California black oak acorns comprise about 50% of the fall and winter diets of western gray squirrel during high mast years [195]. Plant communities with a substantial California black oak component are good habitat for rodents. California black oak and white fir are the principal tree species associated with northern flying squirrels in the San Bernardino Mountains, where the squirrels are at the southern edge of their range. On the Plumas National Forest, 37% of long-eared chipmunk, 54% of Townsend's chipmunk, 36% of yellow-pine chipmunk, and 50% of golden-mantled ground squirrel habitat use was on early seral conifer burns where the dominant vegetation was California black oak and mixed chaparral species [253]. California black oak provides cover for dusky-footed woodrats, a major prey item for California spotted owls [270].
Birds: Many seed predators consume California black oak acorns, and California black oak is a preferred foraging substrate for many birds [5,166]. Acorn-consuming songbirds include blackbirds, chickadees, crows, goldfinches, larks, Clark's nutcrackers, nuthatches, sapsuckers, and thrashers [139]. California black oak acorns form a majority of the acorn woodpecker's diet in some areas [137,205]. The acorns are important in the fall diet of band-tailed pigeons [173,223], comprising 3.2% of their September diet and 7.7% of their November diet [147]. All of 68 bird species observed in oak woodlands of the Tehachapi Mountains of California used California black oak for part of their foraging activities. The acorn woodpecker, the northern oriole, and the Nashville warbler showed greatest preference for California black oak [29]. In a mixed-conifer forest on the Blodgett Research Station, red-breasted and chestnut-back chickadees foraged on California black oak significantly more than expected (P<0.05) based upon California black oak's frequency [1,39]. A Blodgett Forest Research Station study of bark-foraging birds showed the pileated woodpecker made high use of California black oak branches while hawking [166]. As a guild, insect-gleaning birds preferred (preference value L=0.05) California black oak more than expected based on availability [5]. Morrison and others [39] express concern that managing only for pines in mixed-conifer forests may reduce population densities of chickadees and other gleaning birds that depend on California black oak and other hardwoods for food and cover.
Protected animals: California black oak woodlands are important habitat for several rare and threatened species. California black oak provides diversity and nesting and other cover for California spotted owls [164,228,265] and flammulated owls [146]. A Sierra National Forest-Sequoia Kings Canyon National Park study showed California spotted owls in mixed-oak woodlands nested in California black oaks more often than expected (P<0.05) based on California black oak's relative abundance [175]. In a study of Sierra Nevada fisher populations, fishers in mixed-conifer-California black oak habitat had smaller home ranges than fishers in Sierra lodgepole pine (Pinus contorta var. murrayana) or Jeffrey pine habitats. The authors suggested that fisher home range requirements were smaller in mixed forest because California black oak and other montane oaks increased habitat quality [270,276]. Stewart and others [237] discuss habitat needs of sensitive-listed southern California herptiles in mixed-conifer forests with a California black oak component.
Arthropods: The bole and limbs of California black oak are habitat for many arthropods species. Some of these arthropods are important to gleaning and other insectivorous birds [39]. Schowalter and Zhang [217] provide a compilation of arthropod family assemblages found on branches of California black oak and associated woody species on the Teakettle Experimental Forest.
Tree cavities filled with rainwater or detritus form unique habitat for aquatic- or detritus-based invertebrate communities. Mosquitoes are among the insects that live in rain-filled California black oak cavities during the larval stage. Some invertebrates are largely or entirely restricted to cavities in California black and other oaks during a portion of their life cycle. Woodard and others [274] detail invertebrate species presence and food chains in cavities of California black and other oaks in Mendocino County, California.
Palatability/nutritional value: California black oak browse is highly palatable to cattle and mule deer, but is less valuable for other ungulates [212]:
Palatability of California black oak foliage
Animal Rating mule deer good to excellent cattle poor to good domestic sheep poor to fair horses useless to poorCalifornia black oak acorns have a high concentration of lipids [149]. They provide little nutrition for herbivore growth and bone-building but are an excellent source of energy due to the high fat content [212]. California black oak leaves may provide the highest food value of all the western oak browse for fattening cattle [136].
California black oak acorns are highly palatable to livestock and mule deer. In a study in the Cuyamaca Mountains of southwestern California, mule deer preferred California black oak acorns over any other forage. Mule deer would search through forest litter for acorns even during spring, when new, palatable sprouts and herbs were readily available [37]. Mean nutritional values of California black oak acorns are given below (review by [59]):
Percent (%) composition of California black oak acorns
crude protein 3.43 crude fiber 14.07 fat 11.05 ash 1.14 calcium 0.09 phosphorus 0.06 tannins 1.81Scrivner and others [218] provide a mineral analysis of California black oak acorns collected on the Hopland Field Station.
Cover value: Wildlife use California black oak heavily for cover. California black oak provides valuable shade for livestock and wildlife during the hot summer months [59]. Cavities in California black oak provide den or nest sites for owls, woodpeckers, tree squirrels, and American black bears [60,271]. Cavity nesters also use oracle oak [271]. For further information, see Importance to Livestock and Wildlife.
California black oak is most common in Pacific ponderosa pine (Pinus ponderosa var.
ponderosa; hereafter, ponderosa pine) and mixed-conifer forests, where
it is more often an associated than a dominant species [135,188]. California
black oak cooccurs with ponderosa pine throughout nearly all of California
black oak's distribution [61,148]. White fir (Abies
concolor), incense-cedar (Calocedrus decurrens), and sugar pine (Pinus
lambertiana) are additional important associates in mixed-conifer forests [188].
California black oak is a common associate in dry-site coast Douglas-fir
(Pseudotsuga menziesii var. menziesii;
hereafter, Douglas-fir) forests in Klamath-Siskiyou region of
southwestern Oregon and northwestern California. The Klamath-Siskiyou area is
one of the most biologically diverse regions of the world, so California black
oak has many overstory plant associates there [135]. McDonald [147] provides a list of
associated trees and shrubs across
California black oak's distribution.
The California black oak cover type occurs sporadically and in
relatively small extent across California black oak's distribution. It is best
developed in southern Oregon and the northern Sierra Nevada [148,149,187,261].
More often, California black oak stands form a transition zone between mixed
chaparral and mixed-conifer forest in Sequoia National Park [185] and other
locations. The California black oak cover type is disturbance-dependent on most
sites and was more widespread before European settlement. Historically extensive
California black oak stands in Yosemite Valley,
for example, are attributed to California black oak
cultivation and very frequent understory burning by Miwoks to
promote California black oak over ponderosa pine [16,70] (see Native American fires and Other Uses
for further information).
California black oak is common to dominant in some oak (Quercus spp.) woodlands, which
have annual grassland, shrub, or mixed shrub-annual grassland understories. In
mixed-oak woodlands on the North Coast Ranges of northern California,
California black oak grows in association with coast live oak
(Q. agrifolia), blue oak (Q. lobata), Epling's oak (Q.
à eplingii), and/or tanoak (Lithocarpus
densiflorus) [26,148]. California black oak
is an associate or rare dominant in mixed-oak riparian woodlands of
coastal northern California [99]. In
low-elevation canyons throughout much of its range, California black oak codominates with canyon live oak
(Q. chrysolepis), with canyon live oak usually
dominant on dry sites and California black oak assuming dominance on mesic sites [90,149]. A
California black oak-coast live oak/interior live oak (Q. wislizenii var.
frutescens) woodland is described from Cuyamaca Rancho State Park near
San Diego, California [37].
California black oak occurs in a few plant communities dominated by narrow endemics. It
is an associate in Washoe pine (Pinus washoensis) forests in Modoc County,
California [275], giant sequoia (Sequoiadendron giganteum) forests in
the Sierra Nevada [261], and bigcone Douglas-fir
(Pseudotsuga macrocarpa) woodlands in the mountain ranges of southern California [80].
East of Los Angeles in the San Jacinto Mountains, California black oak codominates in
California black oak-Coulter pine (Pinus coulteri)-canyon live oak/chaparral whitethorn-manzanita
(Ceanothus leucodermis-Manzanita spp.) forests [273].
California black oak joins a unique western hardwood-Great Basin shrub transition community in
the southern Cascade Range. A California black oak population in Lassen County,
California, transitions into a basin big sagebrush (Artemisia tridentata var. tridentata)
community on pluvial Lake Lahontan [275].
Published classifications naming California black oak as a dominant or indicator species,
from north to south, are as follows:
California black oak woodland of Willamette Valley
California black oak and California black oak-Oregon white oak (Q. garryana)
association of western Oregon [240]
ponderosa pine-California black oak-Oregon white oak/wedgeleaf ceanothus (Ceanothus cuneatus)
community of western Oregon [178]
ponderosa pine-California black oak association of the Medford District and the Rogue National Forest [23]
California black oak/ceanothus (Ceanothus spp.) and ponderosa pine-California
black oak/ceanothus series in the Siskiyou Mountains [22]
California black oak-Douglas-fir vegetation type of the Siskiyou Mountains (review of
classifications by [68])
an indicator species (100% frequency) of the Douglas-fir/poison-oak/Pacific hound's tongue
(Toxicodendron diversilobum/Cynoglossum grande) association on the Dead Indian Plateau [20]
California black oak/poison-oak and California black oak/purple needlegrass (Nassella pulchra)
association at Castle Rock, California [243]
nonconiferous California black oak phase of mixed-evergreen forest in northern California [25]
the Coast Ranges hardwood series:
California black oak-Pacific madrone (Arbutus menziesii)-coast live oak subseries
mixed oak-coast live oak/poison-oak subseries
California black oak-coast live oak-shore pine/ocean spray (Pinus contorta var. contorta/
Holodiscus discolor) subseries
California black oak-valley oak/grass (Poaceae) subseries [8]
Coulter pine-California black oak forest of the Coast Ranges [101,255]
California black oak communities on Bennett Mountain, Sonoma County:
California black oak/California fescue (Festuca californica)
California black oak-California bay (Umbellularia californica)
California black oak-Douglas-fir
California black oak/poison-oak
California black oak/sweet cicely (Osmorhiza berteroi) [256]
the coast live oak-California black oak-Pacific madrone phase of mixed-evergreen forest
on the Hastings Natural History Reservation [86]
montane California black oak upland hardwood vegetation type of the central and southern coastal foothills
Jeffrey pine-California black oak and ponderosa pine-California black oak phases of the
montane conifer vegetation type in the central and southern coastal foothills [235]
the Coast Ranges and Sierra Nevada hardwood series:
California black oak/poison oak subseries
California black oak-canyon live oak/poison-oak subseries
canyon live oak-California black oak subseries
California black oak/grass subseries
the Sierra Nevada hardwood series:
California black oak/poison-oak-snowdrop bush-Ithuriel's spear
(Styrax redivivus-Triteleia laxa) subseries
California black oak/deer brush (Ceanothus integerrimus) subseries
California black oak/deer brush-poison-oak/bracken fern (Pteridium aquilinum) subseries
California black oak/greenleaf manzanita (Arctostaphylos patula) subseries [8]
mixed-evergreen California madrone-tanoak-California black oak forests and pure California black oak
phases of mixed-evergreen forests in the Sierra Nevada [148]
mixed-evergreen California madrone-canyon live oak-California black oak forest on the Yuba
River watershed [91]
California black oak phase of ponderosa pine and Jeffrey pine (Pinus jeffreyi)
forest vegetation types [210,255]
California black oak phase of the broadleaf forest subformation in southern California [190]
California black oak woodlands of the San Bernardino Mountains [102]
Coulter pine-California black oak forest of the Transverse and Peninsular ranges [101,255,266]
California black oak
populations have greatly decreased from historic sizes. This is due
to a number of factors including drought, animal foraging, logging, and fire
exclusion [60].
Softwood production:
California black oak serves as a nurse tree to conifers. Ponderosa pine,
Douglas-fir, and incense-cedar seedlings often establish under crowns of
large California black oak while adjacent ground remains unproductive [42].
Damaging agents, biological―
Many pathogens may infect California black oak. Hecht-Poinar and others [97] provide
a general review of agents that infect or infest California black oak
and other western oaks, and discuss disease control
methods. A discussion of specific pathogens follows.
Fungi and water molds:
California black oak is highly
susceptible to wood-decaying fungi. Heart rot is mainly
caused by two fungal pathogens: Inonotus dryophilus
and Laetiporus sulphereus. Another fungus, Armillaria mellea,
causes root and butt rot in old or fire-damaged trees [42,149].
Sprouts arising from infected boles are susceptible to heart rot [147].
Summer irrigation encourages growth of root rot fungi [198]. McDonald [147] provides a review of major fungus
species that California black oak hosts.
California black oak is also susceptible to several leaf diseases, such as
oak leaf fungus (Septonia quercicola) and oak anthracnose
(Gnomonia veneta). These agents
are usually not serious unless defoliation is repeated [147].
Burns and Honkala [42] and McDonald [147] provide reviews of
leaf-damaging fungal pathogens.
California black oak and
other oaks in the red oak subgenus are vulnerable to sudden oak death disease
[18,56,72]. Both the origin and life cycle of
the fungus-like water mold (Phytopthora ramorum) causing this
disease are unclear. Based on genetic studies that showed a "limited gene pool" for sudden oak death
mold populations in North America compared to European populations, Garbelotto
and others [72] speculate that the mold is "an introduced organism, but
its actual origin and global genetic structure remain unknown." They
provide a tentative explanation of the mold's complex life cycle, which
requires an alternate host to complete [72,73].
Sudden oak death disease is fatal to most red oak species.
Red oak saplings often die within weeks of
infection. Mature trees may take several years to die. Among red oaks,
California black oak is very susceptible to sudden oak
death mold infection [18,241]. In
laboratory seedling and sapling inoculation studies of Texas and California
oaks, California black oak had the highest infection rate of
14 red oak and white oak (subgenus Leucobalanus) species [18].
Field monitoring in Marin
County, California, showed a progression from total infection rates (all oak
species) of 39.0% in 2000
to 62.4% in 2003, and a consummate rise in mortality from 3.8% to 9.4%.
There are many alternate plant hosts for the mold, including some species
that show symptoms of infection
and some that do not. Garbelotto and others [72] and Swain [247] provide
lists of known sudden oak death mold hosts including California bay, redwood, tanoak, toyon
(Heteromeles arbutifolia), and
Pacific rhododendron (Rhododendron macrophyllum) [72,247].
Actual methods of mold spore dissemination were not documented as of 2007.
Suspected mechanisms include wind, water, soil, sap-sucking insects
and other animal vectors, and transported
firewood [18,53,73]. Bark beetles often attack infected trees [158],
but they are apparently
secondary agents, not sudden oak death vectors [73].
Sudden oak death mortality of California black oak in the Central Coast
Ranges is widespread [54,96]. Oak mortality from sudden oak death disease may affect fuel loads [54].
Guides for identifying infected oaks [73,239],
collecting pathogen samples for laboratory analysis, and
removing infected trees to minimize disease spread [239] are available. The website Monitoring Sudden Oak Death
provides county and regional maps of infestations in
California. The Pacific Southwest Region of the
US Department of Agriculture, Forest Service, provides annual
monitoring updates on their Forest Pest Conditions
website. Fungicides for controlling sudden oak death
mold are being investigated. Federal and State
agencies in Oregon are testing a clearcut-and-burn treatment for infected sites
in southwestern Oregon [72]. Control treatment results were not available as of
2007.
Interior live oak is the least sensitive of the Californian oaks in the red oak subgenus [56],
so oracle oaks may be genetically less susceptible than California black oak to sudden oak death
mold compared to their California black oak parents.
Parasitic plants:
This species is frequently infested with Pacific mistletoe [42,147].
Insects:
Many insect species feed on California black oak. McDonald [147]
reports that insect damage to California black oak is "usually secondary,
reducing growth but seldom killing the tree." He provides a review of
California black oak insect pests [147].
Damaging agents, physical―
Heavy, wet snow often breaks California black oak branches. Broken
limb edges become portals for fungal infection [147].
Native American use: California black oak was profoundly important to Native American tribes living within California black oak's distribution. At one time, California black oak acorns were a tribal food staple [66,142]. Many Native American villages were located near California black oak groves, and acorns―particularly California black oak acorns―were a primary source of tribal wealth. California black oak acorns were a much-desired trade item for desert tribes without ready access to acorns [16,35]. Oak (Quercus spp.) woodlands were managed as orchards on many sites (Yosemite Valley, for example): Trees were pruned, and stands thinned and burned (see Native American fires), to enhance acorn production [16,34,142,202].
Tribes preferred California black oak acorns over those of other oak species for making acorn meal because of California black oak meal's superior taste and pudding-like texture when cooked [27,55]. California black oak meal was often mixed with other oak meals to improve meal flavor and texture [27]. Native Americans used California black oak meal to make soup, bread, and pudding. Moldy California black oak flour was used to treat boils and sores [149]. Some Miwoks still harvest and process California black oak acorns for food [10,11].
The Western Monos used epicormic sprouts of burned California black oaks in basketry [12]. The Yosemite Miwoks used California black oak branches for making household utensils, and the trunks as support columns for roundhouses. The Wintu made black dye from the bark [188].
Commercial use:
Wood products― California black oak is a valuable timber tree. The wood is used for making cabinets, furniture, flooring, high-grade lumber, pallets, and industrial timbers. It is also used as fuelwood [42,138,267]. Forks of open-grown California black oak were used in late 1800s as "naturally assembled" ship keels and ribs [149].
Other products― California black oak is a valuable ornamental. The deep shade and aesthetic appeal of California black oak make it a highly desirable landscaping tree [32,33].
Cork oak (Q. suber) scions were grafted onto California black oak rootstalk in the 1940s to produce cork during World War II [42].
Seasonal Development for California black oak
Event Period acorn germination February-April [143,155] seedling emergence 8 April-8 July on the Challenge Experimental Forest [155] catkins mature mid-March to mid-May [42,147] leaf drop mid-August to mid-September [42] acorns mature mid-August to mid-September of 2nd year [42] acorns drop mid-August to early November of 2nd year [42,147] leaf fall September-October; usually coincides with acorn drop [147]California black oak establishes from sprouts and acorns after fire.
Sprouting: California black oak sprouts from the root crown or bole after top-kill by fire [64,64,147,148,149,162,194,220,231,249]. Trees arising from sprouts often have multiple trunks. Young and others [275] report that single-stemmed California black oaks are rare on the east slope of the Sierra Nevada, and suggest that most east-side California black oak stands resulted from postfire sprouting. Ability to sprout extends across age classes, with seedlings [147] and mature trees showing ability to sprout after fire. Partially burned trees may produce epicormic sprouts on the bole or branches [12,108,149,154,162]. Stephens and Finney [231] found that after prescribed fire in a mixed-conifer stand in Sequoia National Park, 90% of burned California black oak sprouted from the root crown. Neither percent forest floor consumption, percent crown volume scorched, nor crown scorch height were good predictors of California black oak mortality, although those variables successfully predicted conifer mortality. For California black oaks that died, mortality occurred within 1 postfire year [231].
McDonald [148] reports that sprouting starts in the first postfire growing season, with "up to 100 sprouts bursting forth from the most vigorous stumps". Postfire sprouts usually grow rapidly [275]. Within a few weeks following fire, most surviving California black oaks sprout from the root crown and/or undamaged portions of the trunk. This response is independent of the rainy season; new shoots draw upon water and nutrient reserves in the root system and appear soon after spring, summer, or fall fire. Sprouting is "vigorous" in saplings and young trees [159]. Very old trees may fail to sprout or produce only epicormic sprouts [159]. Epicormic sprouts are less "vigorous" than root crown sprouts, with a greater tendency to die back or become infected with wood-decaying fungi while still young [149].
Season of burning affects California black oak sprout density. Individual California black oak grew significantly more sprouts after early fall and early spring prescribed fires compared to late fall and late spring prescribed fires in ponderosa pine and mixed-conifer forests on the Blodgett Forest Research Station, the Challenge Experimental Forest, and the Plumas National Forest. See the Research Project Summary of Kauffman and Martin's [108,109,110,111] study for further information on response of California black oak and other plants to those fires.
California black oak seedling after the Cedar Fire in Cuyamaca Rancho State Park.Seedling establishment: McDonald [148] stated that after fire, acorns provide a secondary reproductive mechanism for California black oak establishment, allowing the species to move into newly created openings and new areas. Seedlings may establish as early as the first postfire growing season [110,148]. Seedlings probably establish from acorns cached by animals (see Seed dispersal), and establishment from crown-stored acorns is also likely. Fire favors seedling establishment in several ways. It prepares a favorable seedbed not only by removing litter, but also by killing damaging molds and insects present in the litter layer. Sapling mortality from root rot is less on recently burned than unburned sites [25,155]. See Discussion and Qualification of Plant Response below for more information.
California black oak seedling numbers increased significantly (P>0.05) following a low-severity prescribed fire in a Jeffrey pine-California black oak forest in Cuyamaca Rancho State Park, California [129,140,141,141]. For further information on this study, see the Research Project Summary of Martin and Lathorop's [129,140,141] study.
California black oak reproduces by sprouting from the root crown and establishing from acorns. Sprouting accounts for most reproduction, while seedling establishment helps maintain existing stands and allows the species to expand into new areas [148]. However, because sprouting depends upon fire or some other top-killing disturbance, it is less important than seedling establishment on undisturbed sites. Even on disturbed sites, seedlings sometimes outnumber sprouts. Garrison and others [78] found that California black oak seedlings were more prevalent than sprouts after logging in Placer County, California. Standiford and others [226] provide a model for predicting probabilities of regeneration (seedlings and sprouts combined) for California black oak and other oaks in the southern Sierra Nevada.
Vegetative regeneration: California black oak sprouts from the root crown or bole after fire, logging, frost, or other top-killing events [42,69,148,153,194,275]. Edwards [61] and McDonald [148] state that most California black oak stands originate from sprouts. Sprouting is California black oak's primary method of reproduction after top-killing events like fire [147,148,194]. Even seedlings sprout after top-kill [147,155], and sprouting ability is retained until trees are "very old and moribund" [147]. Old California black oaks may not sprout if perennating buds are covered by thick bark [155]. Trees originating from root crown sprouts are often multistemmed [275]. Number of sprouts/bole tends to decrease over time; pole-sized clonal clumps generally contain 1 or 2, or occasionally up to 4, sprouts. Multistemmed colonies are more common on dry, infertile sites than on productive (high timber yield) sites [147].
Open structure and dormant-season disturbances promote initially high sprout densities. On the North Coast Ranges, stumps of California black oaks cut from December through May had a mean sprouting rate of 95%, while stumps cut from June through November had a sprouting rate of 45%. California black oak produced more sprouts, which grew faster, in clearcuts compared to partial cuts (review by [155]). Similarly, clearcutting on the Challenge Experimental Forest in the southern Cascade Range of California resulted in greater sprout production compared to shelterwood cutting [194]:
Mean number of California black oak sprouts/bole and sprout growth for 10 postharvest years
Postharvest year Sprouts/bole Sprout height (feet) Clearcut Shelterwood Clearcut Shelterwood 0 55+ 28 ...* 2.0 2 55+ 23 ... 2.0 4 35 17 7.5 3.9 6 23 15 12.5 4.9 8 18 13 16.1 5.9 10 15 12 19.7 7.0 *no data.California black oak produces epicormic sprouts on the bole and branches [108,147,154]. Epicormic sprouts reduce California black oak's timber value [154], but may be important to California black oak regeneration when fire or other disturbance kills it back to the bole.
Regeneration from seed:
Pollination: California black oak is wind pollinated [15,42,125]. Pollen is apparently not limiting for reproduction. In Sequoia-Kings Canyon National Park, California, California black oak pollen was "overrepresented" in pollen rain samples based on California black oak's relative frequency [15].
Breeding system: California black oak is monoecious and outcrossing [33,147]. Levels of genetic diversity within and among California black oak populations are unclear. A genetic study found higher levels of genetic diversity within California black oak populations than expected for a wind-pollinated species. The authors caution, however, that their sample size was small (n=9 populations) and call for further studies on the population genetics of California black oak [56]. Common garden provenance trials on the Challenge Experimental Forest showed few differences in survival, growth, leaf flush, and leaf fall among 8 California black oak populations planted on an elevational gradient [155]. Animal acorn dispersers (see Seed dispersal) probably influence California black oak distribution and population genetics [275]. Further studies are needed to determine genetic diversity and gene flow patterns for California black oak.
Seed production: Although there are generally more California black oaks of sprout than seedling origin [148], California black oak "can produce adequately from seed" [147]. California black oak acorns require 2 years to develop and ripen [98,134]. Abortion of first-year acorns is common, especially during drought [155]. California black oaks begin sporadic acorn production at about 30 years of age or 39 feet (12 m) in height [33,42], with large crops produced beginning at 80 to 100 years of age [42]. Abundant production continues until trees are 200 or more years of age, with reproduction declining as tree senesce [147]. Average production for a 150- to 200-year-old tree was 6,500 acorns [42]. Large trees produce more acorns across time and space―and therefore parent more seedlings―than small California black oaks. In a Placer County, California, study, California black oak seedling density increased with mean stand DBH [78].
California black oak is a masting species, with high acorn production in some years and low production in others [123,134,147]. It would be useful for fire and wildlife managers to predict years of high acorn production, but to date (2007), such models remain elusive for California black oak. A 7-year study of a Carmel Valley, California, population showed that 55% of California black oaks produced no acorns, while 28% produced "bumper" crops [46]. Oaks (Quercus spp.) tend to mast synchronously; however, this tendency is weak in California black oak and does not occur in all populations [122,123,124]. Since California black oak requires 2 years for acorns to mature, its masts do not coincide with associated oaks in the white oak subgenus, which have acorns taking 1 year to develop [121,122,134]. Koening and others [123,124] suggest that because California black oak acorns take longer to develop, environmental variables affecting acorn maturation are more complex, and acorn production less predictable, for California black oak compared to oaks with acorns that mature in 1 year. In a 12-year study on the Hastings Natural History Reservation, Koenig and others [125] reported complete acorn crop failure for California black oak and canyon live oak, which also takes 2 years to develop acorns, in 1 of the study years (1991). Associated white oaks did not experience crop failure [125]. Studies spanning several decades, rather than 10 or so years, may be needed to better understand and predict variation in California black oak acorn production [122].
A few acorn-production trends, however, are noted across California black oak's distribution. Some individual California black oak trees or groves produce seed every year, but abundant acorn crops for entire California black oak stands generally occur every 2 to 3 years [147]. Some populations have mast cycles longer than 2 or 3 years, however. Koenig and others [122] suggested a 6-year masting cycle for California black oak on the Hastings Reservation of west-central California. For the 5 oak species on the Reservation, climatic variables only correlated with masting in California black oaks (r=0.3, P<0.001) and canyon live oaks. Both oaks tended to produce large crops in years following "particularly cold winters" [122]. In a 24-year study on the Challenge Experimental Forest, McDonald [151] reported that California black oak produced 5 acorn crops in 30 years: 3 medium (many acorns on 25-50% of trees) to heavy (many acorns on >50% of trees) crops and 2 light (few acorns on >25% of trees) to very light (few acorns on <25% of trees) crops. There was no discernible pattern to variations in California black oak acorn production [151]. Garrison and others [77] and McDonald [151] provide protocols for visually estimating California black oak acorn production.
Acorn depredation: Many animal species consume California black oak acorns (see Importance to Livestock and Wildlife), including small and large mammals, birds, and insects. Litter molds also kill acorns [42,147].
Seed dispersal: McDonald and Tappeiner [155] rate California black oak acorn dissemination as "good". Animals and gravity disperse the acorns [42]. Since California black oak acorns are heavy, those not dispersed by animals tend to fall beneath the parent tree's crown, which suppresses seedling growth [147]. Seed-caching animals are the most important acorn dispersers. Unretrieved seed buried in caches is more likely to germinate and establish than seed in litter or on the ground [42,147]. A few hours of direct sunlight or sustained indirect heat kills California black oak embryos [155]. California ground squirrels, western gray squirrels, Steller's jays, and scrub jays are important California black oak acorn cachers [42]. Western gray squirrels and scrub jays are most critical for California black oak regeneration because they move acorns away from the parent tree before burial [110,147,148]. Chances of seed dispersers selecting California black oak acorns over acorns of other oaks are good. Acorn-caching dispersers are more likely to select large acorns, such as those of California black oaks, than smaller acorns of other oaks. Further, the nutrient reserves in large acorns give California black oak a better change of establishing compared to oaks with smaller acorns [6]. For California black oak acorns that are not cached, those falling beneath the parent tree show better emergence than those in the open. Shade, deep litter, and a high proportion of moist organic material in the soil beneath large California black oaks are "probably an asset" to California black oak development. As seedlings, California black oaks grow slowly when shaded by their parent. However, such shaded seedlings respond well to release, often forming clumped groves after parent death or harvest [155].
Seed banking: California black oak has a short-term seed bank. Acorns in the field lose viability rapidly when warm fall or spring temperatures desiccate the embryo, although California black oak acorns remain viable for 2 to 5 years under laboratory storage conditions [155]. Acorns in litter and buried in animal caches form a temporary seed bank and may survive several months. California black oak probably cannot form a longer-term seed bank, however, because seed nutrient reserves deplete rapidly with warm temperatures. In an artificial regeneration study on the Challenge Experimental Forest, California black oak acorns were planted 2 inches (5 cm) deep in the fall of 1969. A few acorns were later buried to 4-inch (10 cm) depths by winter storms that displaced soil. While most California black oak seedlings emerged in spring 1970, the deeply buried acorns emerged in the summer or fall of 1970. At the same stages of development, the deeply buried summer and fall germinants showed weak taproot development compared to the shallowly buried spring germinants. Most late-emerging germinants died from drought in their first year, while most spring germinants survived [147].
Germination: Most California black oak acorns are dormant, requiring overwinter stratification to germinate [103,147]; however, this species apparently produces some acorns that are capable of immediate germination [215]. Germination is hypogeal [42,147]. In common garden studies comparing germination characteristics of several California oak species, most California black oak acorns sown in October germinated within a short period in February regardless of population origin. This synchronized germination contrasted with germination of 6 other oak species, which slowly progressed through winter [144]. California black oak acorn viability varies greatly [103,147], generally ranging from 30% to 95% [147]. In a greenhouse study using acorns from the San Bernardino National Forest in southern California, both population and parent sources affected California black oak acorn germination rate (percent germination range=20-93%). Parents producing acorns with high moisture contents had fewer viable acorns, and acorns from high-elevation stands (6,300 feet (1,900 m)) had lower germination rates than acorns from low-elevation stands (4,500 feet (1,400 m)) [103]. California black oak acorns with relatively high moisture contents generally show poor emergence because they are susceptible to fungal infections [42].
Seedling establishment: A moist, open mineral or light duff seedbed favors California black oak establishment [110], but seedbed requirements are not rigid. California black oak also establishes in undisturbed leaf litter [147]. McDonald [150] noted presence of both seedlings and sprouts on a clearcut on the Challenge Experimental Forest. Seedlings were concentrated where mature California black oaks had stood prior to tree harvest. Sprouts were more scattered in distribution but "outgrew all other vegetation" where present [150]. More seedlings established in large openings than in small openings after a 1963 group-selection cut on the Challenge Experimental Forest [152]:
Mean density of California black oak seedlings by size of group-selection opening at postharvest year 11 (1974) Opening size (ft) 30 60 90 seedlings/acre 166 588 550Deterrents to establishment: Unfavorable site conditions and foraging animals reduce seedling recruitment. Seedlings cannot establish on heavy clay soils or soils compacted by logging. Drought kills many seedlings [42]. Mule deer and California pocket gophers eat acorns and kill or reduce the growth rate of young California black oaks [42,155]. In a study at Cuyamaca Rancho State Park, mule deer consumed 85% of 1 season's mast. The following spring, mule deer consumed 100% of emerging seedlings. Partially because of acorn and seedling predation, there had been almost no seedling recruitment in Cuyamaca Rancho State Park for 25 years prior to the study [37]. Other browsing animals adversely affecting seedling establishment include lagomorphs, grasshoppers, and other insects [42].
Seedling and sprout growth: California black oak's shoot and horizontal root development are slow for the first 6 to 7 years, with most initial growth concentrated in vertical roots [42]. On favorable sites, California black oak seedlings are 2 to 4 inches (5-10 cm) in height, and have a 9-inch (20 cm) taproot, 28 days after emerging. Lateral roots are slower to develop than taproots [147]. First-year seedlings are typically 2 to 6 inches (5-20 cm) tall, and their taproots may extend 3 feet (0.9 m) below ground [155]. Conifer seedlings grow faster than and outshade California black oak seedlings, but California black oak seedlings often grow through chaparral shrubs [42]. California black oak seedlings on open sites grow faster than seedlings beneath the canopy [78]. California black oaks respond to release after logging, fire, or other top-killing events open the canopy.
With a fully developed root system, sprouts grow faster than seedlings [159,275]. After logging in Yuba County, California, California black oak sprouts reached a mean of 1.9 feet (0.6 m) in postharvest year 1 and averaged 4.7 feet (1.4 m) in height by postharvest year 4. In postharvest year 1, average increase in stem diameter was 270% greater than stem increase the year prior to harvest. California black oak sprouts grow faster than conifer seedlings [147].
Until about age 25, California black oak saplings rapidly gain height growth but have thin stems. Afterwards, trees on open sites gain larger diameter:height ratios. Sixty-five-year-old trees on open sites average 13 to 14 inches (33-36 cm) in diameter, while 65-year-old trees on more crowded sites average 7 to 9 inches (18-23 cm) diameter. Growth is maximized on level sites with deep soils. After about 40 years of age, trees on east aspects sometimes gain more height growth than trees on other aspects. Growth rate declines at around age 65, and most trees are "fully mature" at age 90 [147]. Several models are available for predicting diameter growth [93], height-diameter growth and volume [74,128,147], and mortality [94] for California black oaks and other western hardwoods. Site index curves are also available for estimating rate of California black oak growth [196]. Plumb and McDonald [194] present a 6-year volume-growth and mortality table for California black oak thinned to 7 decreasingly dense basal areas.
Size class recruitment: California black oak may establish seedlings in large numbers on some sites, but have only a few trees graduate into the sapling class [17]. Standiford and others [227] identified the sapling stage as critical to California black oak recruitment in the southern Sierra Nevada, with many trees in seedling and adult stages compared to sapling and pole stages. A study on the Hall Canyon Research Natural Reserve east of Los Angeles also showed high seedling recruitment for California black oak but little recruitment to sapling and pole stages. Most of the parent trees established as sprouts after logging in the early 1900s. The authors characterized California black oak's overall recruitment as "slow but steady" on the harsh site, which was experiencing a severe, prolonged drought, ozone damage to conifers, and conifer die-off from bark beetles [213].
California black oak may be present from early to late succession [150,153]. It is moderately shade tolerant in early life, growing best in full sun but persisting in dense shade [42,147,225]. Pole-sized California black oak are less shade tolerant than seedlings and saplings. Pole-sized California black oaks often grow tall and thin until reaching a position in the canopy where light is received. Large trees are shade intolerant [42,149,225].
Forest succession: Once California black oak matures, it requires decades without disturbance before successional replacement by understory conifers. However, as California black oak's crown grows upward, enough light eventually filters through to the forest floor to allow shade-tolerant conifers to establish [148,149]. California black oak is a nurse tree in such situations, ameliorating soil temperature and moisture for conifer seedlings [149]. California black oak cannot grow as tall as associated conifers, so conifers usually overtop California black oak in the absence of disturbance [249]. Once California black oak is overtopped, conifers replace California black oaks of all size classes [148]. In the Klamath Mountains and Coast Ranges, California black oak is mostly replaced successionally by Douglas-fir [243]. California black oak in the Siskiyou Mountains and southward is also seral to ponderosa pine [22]. In the absence of disturbance in the Sierra Nevada, California black oak is slowly replaced (40+ years) by understory ponderosa pine and Douglas-fir at low elevations or by ponderosa pine, sugar pine, incense-cedar, and white fir in midelevation mixed-conifer forests [148,149,182]. However, at very low elevations (≤2,500 feet (760 m)) young ponderosa pines usually fail to overtop California black oak without hardwood logging [149]. Fire exclusion has allowed succession to shade-tolerant conifers (see Sudden oak death disease and other sections of Current period and fire exclusion for further information). California black oak responds to release when conifers are removed from the overstory [149]. Logging has favored California black oak on some sites by selective removal of ponderosa pine and sugar pine [148]; however, other management practices have favored conifers. Plantation herbicide spraying and grubbing have promoted conifer seedlings at the expense of California black oaks and other sprouting species [149,153].
Pure California black oak stands grow on low-productivity sites or sites with a history of multiple or severe past disturbances [249]. McDonald [148,149] stated that California black oak is a "persistent subclimax species" in midelevation montane forests, with frequent fire or logging maintaining its dominance.
Chaparral: California black oak often successionally replaces Ceanothus and Manzanita species in montane chaparral [61]. California black oak may also grow through shrub stands on relatively mesic, low-elevation chaparral sites [149]. Cooper [51] suggested that California black oak is a late-successional species on some chaparral-conifer forest ecotones, forming a stable, transitional hardwood forest between chaparral and coniferous forest.
The scientific name of California black oak is Quercus kelloggii Newb. (Fagaceae).
It is in the red oak subgenus (Erythrobalanus) [66,82,98,100,107].
California
black oak hybridizes with other oaks in the red oak subgenus [275]. Natural hybrids where distributions of
California black oak and other western oaks overlap are:
California black oak is used for wildlife habitat restoration, and its well-established roots reduce erosion and protect watersheds [147,176]. Mature trees are windfirm [149].
Artificial regeneration of California black oak is exacting. Ripe acorns are harvested from trees in late summer or early fall. Acorns collected after midfall are frequently unviable due to fungal infection. Seedlings establish best when acorns are planted in fall [176,249]. Even so, mortality due to acorn predation and herbivory is usually high, and replacement plantings are necessary for good stand establishment. Protecting seedlings with wire caging extending 36 inches (92 cm) aboveground and 18 inches (46 cm) belowground reduces the need for replacement plantings [176]. Artificial regeneration requires a seedbed free of other trees and shrubs [249]. There is an acorn collecting and planting protocol for California black oak [258]. For additional acorn collecting, storage, and planting tips, see [32,33]. There are few reports on the success of outplanting programs. Fritzke [70] discusses techniques used in successful restoration plantings in Yosemite Valley; Roberts and Smith [206] do the same for an experimental California black oak plantation near Idyllwild, California.
