This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identifying California's oak species are available in these sources: [68,96]. However, identifying oaks is often difficult due to hybridization, and interior live oak hybrids are common. Tucker [199] pointed out that scrub oak hybrids do not "key down" well. Brophy and Parnell [45] provide a key to help identify interior live oak-coast live oak hybrids.
The varieties of interior live oak are distinguished by their growth form. The typical variety (Q. wislizeni var. wislizeni) grows as a tree, and scrub interior live oak (Q. wislizeni var. frutescens) grows as a shrub [96]. The typical variety reaches from 33 to 75 feet (10-23 m) tall [96,159]. Open-grown trees have a dense, rounded crown [155,164], with branches that may extend to the ground [164]. Trunks are one to several [164]. Scrub interior live oak typically reaches 7 to 20 feet (2-6 m) tall [96] and is intricately branched [137]. In Tehama County, interior live oak is typically 8 to 10 feet (2-3 m) tall and shrubby in form [24]. Limited water in the substrate may be a factor driving the shrub or scrub form [89], although frequent fire may produce the same result. Interior live oak typically has numerous, short branches, regardless of form. In a study comparing leaf and branch architecture of 6 cooccurring sclerophyllous tree species in Mendocino County, interior live oak had more densely packed branches and leaves than Pacific madrone, canyon live oak, tanoak, giant chinquapin (Chrysolepis chrysophylla), and California bay (Umbellularia californica); this was true for both sun- and shade-grown interior live oaks [102].
Interior live oak wood is strong, dense, and close-grained [137]. The bark is relatively thin [78,164] on most trees and is composed mainly of live cambium that is susceptible to fire damage. Bark of a 3-inch (7 cm) diameter interior live oak was 0.1 inch (0.3 cm) thick with a very thin layer of outer bark; bark of a 12-inch (30.5 cm) diameter tree was 0.3 inch thick with a "small amount of dead bark" on the outer surface [164]. Bark of large trees can be up to 3.0 inches (7.5 cm) thick [137].
The leaves and fruits of interior live oak are relatively small. The leaves are evergreen and sclerophyllous; the margins may be spine-toothed to entire [96,164]. The leaves are elliptical and about 1 to 3 inches (2.5-8 cm) long [155]. Male catkins are about as long as the leaves [102]. The smaller, female flowers are born in the leaf axils in clusters of 2 to 4 [159]. The fruits are acorns, a type of nut [96]. They are about 0.3 to 0.5 inch (0.8-1.3 cm) wide [164].
Interior live oak is deep-rooted. In a review comparing maximum root depths of sclerophyllous species around the globe, interior live oak had greatest average root depths of all oaks and most other species that were compared; only Eucalyptus had greater maximum root depths [48]. A study in Placer County found interior live oak roots extended at least 24.3 feet (7.4 m) feet through fractured rock before reaching groundwater [124].
Interior live oak is apparently not long-lived. Trees may live 150 to 200 years, although studies of interior live oak's longevity are few [164]. Because interior live oaks sprout, their root systems may be several generations older than their trunks [164].
Interior live oak does not tolerate flooding. When the Terminus Reservoir near Visalia flooded, interior live oaks died if water covered the soil around their trunks for more than 1 week [92].
Interior live oak is a native California-Baja California endemic. It occurs over about 16% of California's landscape [165]. Interior live oak is most common in the Inner Coast Ranges, the foothills of the southern Cascade Range [181], and the Sierra Nevada [125,126]. It also occurs on Santa Cruz Island [125]. Both varieties are common in northern California. Scrub interior live oak is most common in southern California, especially in the Transverse and Peninsular ranges [155]. The distributions of both varieties extend to Baja California Norte [75].
As of 2011, only one English-language publication provided information on interior live oak populations in Baja California Norte, so except for that source [143], all geographical locations referred to in this review are in California.
States and provinces [125,200]:
United States: CA
Mexico
Fire is a vital component of chaparral and woodland communities with interior live oak. Frequent fires can encourage new growth of interior live oak and other sprouting species on rangelands [24,24]. Where oak woodland/annual grassland communities form mosaics or blend with chaparral, fires at 20- to 25-year intervals may best balance the regeneration requirements of sprouting species and those that regenerate solely from seed, such as wedgeleaf ceanothus [24].
Chaparral is not usually burned under prescription because of the high flammability of many chaparral species. Green [77] noted that chaparral can rarely be burned successfully under prescribed weather conditions because under the prescription window for weather, the shrubs are usually too moist to burn. Typically, litter and small twigs are consumed but larger stems are not, and the prescribed fire skips over large patches of brush [77]. If prescribed burning is planned and reducing oak cover is a fire management goal, he recommended prefire preparation that top-kills and desiccates the brush, such as crushing or herbicides, with herbicides most effective on oaks and other species with thick, stout stems. See his 1977 publication [77] for detailed instructions on these prefire treatments, and his 1980 publication [78] for recommendations on preparing a prescription for burning in chaparral.
Plumb and MacDonald [165] consider fire an "almost inescapable occurrence" in California oak woodlands and state that trying to exclude fire from these woodlands is not practical. Periodic surface fires in oak woodlands reduce fuel loads, especially the shrub understory, and help prevent severe wildfires that can be lethal to oaks. Hence, they recommend allowing or prescribing frequent, low-severity surface fires in oak woodlands to reduce fuel loads and interference with oak growth from associated shrubs [165].
Fires in oak woodland-chaparral communities can favor mule deer. Near Clear Lake, does averaged higher rates of ovulation on brushlands burned under prescription compared to unburned brushlands, and bucks were heavier. Blue oak-interior live oak-gray pine and chamise chaparral communities formed a mosaic in the area [26].Interior live oak is adapted to stand-replacing fires in chaparral [84] and frequent surface fires in oak and oak-pine woodlands ([98,180,183], review by [49]). Relatively frequent, recurring crown fires help maintain interior live oak chaparral [49]. In both chaparral and oak woodlands, most wildfires historically burned down from higher-elevation conifer ecosystems [70,201]. Lightning ignitions are infrequent in chaparral and oak woodlands; historically, American Indians, miners, and ranchers were probably responsible for most fires in these communities [70]. With a long history of fire use by American Indians and then European settlers, it is difficult to separate natural and anthropogenic FIRE REGIMES in oak woodlands [183]. Interior live oak woodlands, and blue oak [180,183] and oak-conifer ([98], review by [49]) woodlands with a substantial interior live oak component, historically experienced mostly short return-interval surface fires, although these woodlands may also experience mixed-severity fires [156].
Chaparral: Chaparral ecosystems have short to moderate intervals between stand-replacement fires [113,211]. Minnich [138] describes a "smolder and run" behavior of chaparral fires. The fire cycle is irregular due to variations in weather and stand configurations of annual grassland-chaparral-oak woodland mosaics, but chaparral remains "remarkably stable under a wide range of FIRE REGIMES" that can vary from 20 to 100 years between fires [138]. Fire intensity is generally high but varies with fuels and weather. Most fires occur in summer, although Santa Ana winds can drive large wildfires in autumn [111].
Because fire scar records are rare to lacking in chaparral ecosystems, it is difficult to determine historic fire-return intervals. They may range from 10 [175] to as long as 60 ([113], reviews by [49,70]) or 100 [138] years. Rundel [175] pointed out that chaparral vegetation can burn after only a few years of postfire growth. Kittredge [114] reported that an interior live oak chaparral site on the San Dimas Experimental Forest reburned 3 years after a previous wildfire.
Short fire-return intervals favor sprouting species such as interior live oak, while relatively long fire-return intervals favor a mix of sprouters and obligate seeder species such as wedgeleaf ceanothus [24] and common deerweed (Lotus scoparius) [113]. Pioneer accounts of fire patterns in southern California chaparral suggest that before 1919, chaparral fires varied in severity across the landscape, with the low fuel loads of recent burns supporting less severe fires than the higher fuel loads of sites that had not burned in decades [142].
Fire exclusion may have had little effect on either fire frequency or fire size of chaparral, although experts disagree on this. Minnich [141,142] claims that in chaparral, fire size, rate of spread, and severity during extreme fire weather conditions have increased since attempts at fire exclusion. With the more even-aged structure of contemporary chaparral, Santa Ana winds tend to drive fires without the reductions in fire severity historically provided by young chaparral stands [142]. However, Keeley and others [112] contend that neither fire size nor severity have increased with attempts at fire exclusion in chaparral ecosystems. Their analyses of chaparral in southern California found fire frequency increased during the last half of the 20th century, but average fire size decreased. They attributed these changes to increased anthropogenic ignitions—mostly from arson—and fire suppression. Keeley [111] suggests that the 30- to 40-year fire-return interval typical of California chaparral during the last half of the 20th century is more frequent than fire-return intervals of the past.
Oak woodlands: Oak woodlands, including interior live oak and blue oak-interior live oak communities, have a long history of intentional burning by American Indians and ranchers [187]. Interior live oak woodlands and forests historically experienced mostly frequent understory surface fires [211]. Fire-scar evidence is difficult to obtain from interior live oak and other oaks due to the prevalence of heart rot in old oaks, so fire-scarred conifers growing in oak communities are usually used to obtain fire histories [187]. Fire-scarred ponderosa pines recorded the fire history of an interior live oak-canyon live oak-California black oak/whiteleaf manzanita (Arctostaphylos viscida)-toyon woodland in El Dorado County. From 1850 to 1952, fire-return intervals on 3 sites ranged from 2 to 18 years and averaged 7.8 years. Stand structure was likely open during that period. There was no significant difference in mean fire-return intervals among the 3 sites despite large differences in slope (5%, 30%, and 55%). Cattle ranching was the primary land use during the time studied, and the author surmised that fires were set frequently by ranchers to improve cattle forage. Before the mid-1800s, the area had been a community center for the Miwoks; unfortunately, there were no ponderosa pine trees or stumps old enough to record the fire history of that time. By the 1990s, successional changes with fire exclusion had led to a dense stand structure of 1,635 trees/ha; 75% of the basal area was oaks [187]. Roy and Vankat [174] claim that excluding fire from oak woodlands can lead to a shift in species composition, with successional replacement of decadent overstory oaks by understory chaparral shrubs.
California's oak/grass woodlands historically experienced surface fires every 5 to 25 years [183]. These frequent fires burned at low severities, which tended to kill shrub seedlings and keep the shrub layer short [88,202]. Grasses likely fueled these mostly fast-moving fires [88]. Occasional mixed-severity fires also occurred [156]. Because these communities form a mosaic with or lie between chaparral and low-elevation ponderosa pine woodlands, chaparral shrubs or conifers formed pockets where fire crowned, resulting in more lethal effects to vegetation, especially nonsprouting species [202].
Yosemite National Park's fire records from 1930 to 1983 show that lightning ignitions were relatively infrequent in the canyon live oak-interior live oak-chaparral ecosystem, but when fire occurred, it was "very intense". Fire occurrence was disproportionately low in the ecosystem (4.2% of the Park but 1.9% of fires), with a fire-return interval of about 20 to 30 years. Excepting fires <10 acres (4 ha) in size, area burned averaged 177.5 acres (71.8 ha). Because canyon live oak-interior live oak chaparral-woodlands lie outside wilderness areas of the Park, fires in this ecosystem were suppressed during the time under investigation [201].
Oak-conifer woodlands: Frequent fires are needed to maintain the oak component of California's oak-conifer ecosystems (for example, [187]), although as of 2011, information on FIRE REGIMES in interior live oak-conifer ecosystems in particular were lacking. Ponderosa pine-oak woodlands with an interior live oak component historically experienced mostly short-interval, low-severity surface fires that favored both pines and oaks (review by [49]). Scrub interior live oak is prominent on new burns in bigcone Douglas-fir woodlands [5]. Little fire history was available on bigcone Douglas-fir communities as of 2011. However, bigcone Douglas-fir communities lie next to California's chaparral belt and burn often. Bigcone Douglas-fir generally survives and sprouts after surface but not after crown fires [139], so surface fires likely help maintain bigcone Douglas-fir communities. Walter and others [208] suggest that fire-return intervals in Coulter pine communities are variable. Areas going 100 or more years without fire may develop into open forests with an overstory of Coulter pine, canyon live oak, and interior live oak and an understory of chaparral whitethorn, Eastwood manzanita, and other chaparral species [208].
Because California's oak-conifer communities usually occur near chaparral or conifer forest ecotones and often have chaparral species in the understory, they may experience mixed or stand-replacement fires. Knobcone pine communities, in which interior live oak and other scrub oaks are often important components of the vegetation [5,12], primarily have stand-replacement fires at intervals long enough that the knobcone pine can establish and produce its serotinous cones before the next fire [98]. Knobcone pines must be at least 10 years old to produce cones [206].
See the Fire Regime Table for further information on FIRE REGIMES of vegetation communities in which interior live oak may occur. Find further 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".
Fire adaptations: Interior live oak has adapted to fire by sprouting from perennating buds on the root crown [88,138,202]. It may sprout even in the seedling stage [88]. Among large-fruited taxa that grow in chaparral, interior live oak is one of the most successful postfire sprouters on north-facing slopes, where it typically shades out most obligate seeders in early postfire years [138]. Plumb and MacDonald [165] summarize the need of interior live oak and other California oaks for frequent fire:
"Although fire is anathema to individual oak trees, it is essential for continuation of oak stands under natural conditions, especially on commercial timber sites where inherently taller conifers are more competitive. By destroying the conifers, the oaks are free to sprout. Because of rapid sprout growth, the oaks capture the area and are perpetuated."
Although the relationship between fire frequency and Quercus regeneration is unclear, several studies show that frequent fire favors oak regeneration, reduces ladder fuels in the understory, and helps control acorn predators such as the filbert weevil and filbert worm (review by [183]).
Plant response to fire: Interior live oak sprouts from the root crown after top-kill by fire [24,25,28,69,83,87,98,109,181]. Postfire recovery is usually rapid [98]. Keeley [109] classified interior live oak as an "obligate resprouter" after fire. Biswell and Gilman [24] rated it a "vigorous" sprouter after fire, showing a stronger sprouting response than associated deciduous oaks such as California black oak and blue oak. Interior live oaks often have multiple stems as a result of repeated top-kill by fire and postfire sprouting [157]. Top-killed interior live oaks may sprout soon after winter, spring, or summer fires (see Vegetative regeneration). With summer fires, sprouts may appear as early as postfire week 3, but with late fall fires, sprouting does not usually begin until the next spring [24].
Large, old trees may survive fire without being top-killed [87] but more often, large trees are located in areas that have not burned for 50 to 100 years [157]. Large trees may produce epicormic sprouts after surface fire [87] that scorches the branches.
Fire may kill interior live oak in areas with heavy fuels, particularly in chaparral or communities with a chaparral understory. In a blue oak-interior live oak-gray pine/wedgeleaf ceanothus woodland in Madera County, a prescribed 5 August fire killed 75% of interior live oaks. In postfire year 9, interior live oak comprised 15% of total woody plant species composition. A similar prescribed fire in Madera County resulted in 90% kill of interior live oak. In postfire year 7, interior live oak comprised 15% of total woody plant species composition. Chaparral whitethorn and wedgeleaf ceanothus dominated the community [25]. Prefire composition of these plant communities was not provided.
Interior live oak may establish from acorns after fire, but postfire sprouting is far more important [87]. One year following a stand-replacement wildfire on Quail Ridge Reserve near Lake Berryessa, density of interior live oak seedlings was not significantly different between burned and control plots. It ranged from 7 to 100 seedlings/ha. However, basal sprout regeneration was significantly greater in burned than in control plots (P<0.05) [10]. Surveys of 91 interior live oak-dominated plots on the San Bernardino Forest found no interior live oak seedlings in interior live oak chaparral, while interior live oak forests averaged 10 interior live oak seedlings/0.1 ha. The authors suggested that longer fire-return intervals on forest plots allowed formation of the forest stand structure and establishment of interior live oak seedlings [211]. Minnich [138] stated that because chaparral taxa do not rely on off-site seed dispersal onto burned sites, they are not vulnerable to fire size.
Fire scars can be ports of entry for heart-rot fungi. To date (2011), however, little research had been conducted on the relationships between fire, oaks, and heart-rot fungi [165].
Postfire recovery: A qualitative study on the Los Padres National Forest found interior live oak sprouted from the root crown after the Marble-Cone Wildfire of August 1977. The fire burned 178,000 acres (72,000 ha); most of this acreage was mixed chaparral. Scrub interior live oaks "were seldom completely consumed by the chaparral crown fires; they usually remained as charred trunks, perhaps five to ten feet tall, standing above the ashes". Within a month after the wildfire, they were sprouting from the root crowns and by November, the sprouts were "several feet tall". A portion of the higher-elevation, mixed-evergreen canyon live oak-tanoak-interior live oak forest also burned in the Marble-Cone Wildfire, with a mix of surface and crown fire that varied in severity from low to high. Scrub interior live oak also "sprouted readily" from the base after top-kill in this mixed-evergreen forest [83].
No interior live oak mortality was observed in postfire month 10 (July) after severe wildfire in September 1947 on the Tehama Deer Winter Range. All interior live oaks were top-killed, with an average sprout height of 24.9 inches (63.2 cm) in postfire month 10. Mule deer browsed the sprouts heavily the 2nd winter after the wildfire [24].
Prescribed fire and clearcutting may result in similar interior live oak coverage. Eight years after a moderate-severity, prescribed September fire in the Santa Ynez Mountains, interior live oak had similar densities—10 sprouts/900 m²—on burned plots and on clearcut, unburned fuelbreaks [36].
Although interior live oak sprouts may be dense in early postfire years, stem density usually decreases with succession. Many sprouts of chaparral species do not survive if the site burned when root crowns and roots were water-stressed and/or had low carbohydrate reserves [175]. Heavy postfire browsing may reduce or eliminate interior live oak postfire regeneration [95], especially on small burns. After a 1,100-foot² (100 m²) test plot in interior live oak chaparral near Santa Cruz was burned under prescription, mule deer browsed interior live oak and California coffeberry (Rhamnus californica) sprouts so heavily that many plants of both species died, and bigberry manzanita, which was not browsed, became dominant [80].
Two studies, one in Sequoia National Park and the other in Madera County, show a short-term reduction in interior live oak after fire, with interior live oak showing rapid recovery in early postfire years.
In Sequoia National Park, a 26 June 1987 arson fire reduced interior live oak abundance for at least 2 postfire years. Fire conditions were "extreme", with a mean daytime air temperature of 86° F (30° C), relative humidity of 17%, and fine fuel moisture of 3.5%. Slopes ranged from 20° to 39°; mostly, dry annual grasses carried the wildfire [87]. Fire severity was mixed, varying from low to high [88]. Fire severity became moderate after midnight, when relative humidity rose to 50%. Fire effects and postfire responses were measured the fall after the wildfire and in postfire year 2. As measured that fall, postfire mortality of interior live oak was low: only one "very small diameter" stem had been killed. Crown scorch of interior live oaks and blue oaks combined ranged from 18% on west-facing slopes to 61% on ridgetops; bole char height ranged from 8 inches (20 cm) on west-facing slopes to 39 inches (100 cm) on east-facing slopes. Nine interior live oak seedlings were found on study sites; all were determined to have established before the fire. All 9 seedlings sprouted after the fire, but 1 seedling had died by postfire year 2 [87].
In postfire year 2, all large (82.6-133.4 inches (32.5-52.5 cm) diameter), crown-scorched interior live oaks had live crowns and had produced epicormic sprouts, but most smaller trees were dead [87]. Most crown-scorched interior live oaks were <82 inches in diameter, so mortality was highest in smaller size classes [88]. Mortality also increased with degree of crown scorch; overall, all interior live oaks with 100% crown scorch were dead, while none with <51% crown scorch had died [87]. Some surviving crown-scorched individuals grew both epicormic and basal sprouts. Chances of interior live oak stem survival (vs. top-kill) increased with tree size (P<0.001), and 86% of large trees bore scars from previous fires. Over half of top-killed interior live oaks (n=154 individuals) had basal sprouts [88].
Mortality was higher for interior live oaks than for blue oaks in postfire year 2: 11% of tagged, burned interior live oaks and 6% of tagged, burned blue oaks were dead. Survival rates of postfire sprouts were higher for interior live oak than for blue oak [87], however, and interior live oak had more sprouts/root crown [88]. More than half of interior live oaks that sprouted the fall after fire had surviving sprouts in postfire year 2, while only 2 top-killed blue oaks still had live sprouts [87].
The author concluded that the wildfire reduced interior live oak density in the short term due to aboveground mortality of small trees, but because most large trees survived, there was little change in interior live oak's basal area [87]. See the Research Paper of this study for further details on fire effects on and postfire responses of interior live oak and blue oak.
Mechanical and prescribed fire treatments reduced interior live oak cover for about 6 years in Madera County. On the Ellis Ranch, a private cattle ranch spanning elevations from 2,500 to 3,250 feet (750-975 m), 600 acres (240 ha) of interior live oak and blue oak woodlands were thinned, then the shrub understory crushed, in July 1986. During thinning, all interior live oaks were cut for firewood but most blue oaks were retained for shade. After mechanical treatments, the site was burned under prescription in August 1986. The goals were to increase browse available for cattle and wildlife, reduce canopy cover of interior live oak, and reduce understory fuels [71,135].On 2 of 5 plots, these treatments significantly reduced interior live oak cover in postfire year 1 compared to pretreatment cover (P<0.05) [71].
Interior live oak cover, density, and firewood volume after thinning, crushing, and prescribed fire in Madera County. Data are means, calculated from 5 interior live oak-blue oak or blue oak-interior live oak stands [71,135]. Variable PretreatmentIn the short term, interior live oak canopy cover and volume were reduced the most on sites where interior live oak was dominant before treatments; this was attributed more to cutting than burning. Crushing and burning successfully reduced shrub density, cover, and height, so more browse was available as forage [135]. Interior live oak was returning to pretreatment density by postfire year 2, particularly on plots where it dominated before treatments. On all sites, wedgeleaf ceanothus and yerba santa comprised about half of the new canopy by postfire year 3 [71,135]. A follow-up prescribed fire in 3 to 4 years was recommended to once again reduce abundance of interior live oak and the shrubs [71]. Repeat burning was not accomplished, however, so by postfire year 8, canopy cover of shrubs was similar to pretreatment levels. Interior live oak regeneration had not regained tree size, so on sites where interior live oak dominated before treatments, stand structure had shifted from an overstory of interior live oak trees to an overstory of shrubs. Blue oak was the sole overstory dominant in former blue oak-interior live oak stands [135].
Postfire successional patterns: Fire generally favors interior live oak [181] successionally. In a survey of 5 blue oak sites in Sequoia National Park, interior live oak was most frequent (15%) on a site that burned 5 years previously. The other 4 sites had not burned for about 40 years, and interior live oak frequency ranged from 5% to 10% on those sites [44]. Minnich [140] noted that interior live oak and other spouting species dominated early postfire succession in Coulter pine-canyon live oak woodlands on the eastern Transverse Ranges. Vegetation from <1-year-old to 37-year-old burns was surveyed. Interior live oak was described as a dominant in early postfire succession. Interior live oak and other sprouting woody vegetation provided up to 9% cover in postfire years 0 to 9; 85% cover in postfire years 10 to 19; 75% cover in postfire years 20 to 29; and 77% cover in postfire years 30 to 37 (Minnich 1978 field data cited in [140]).
Surveys in southern California show that interior live oak chaparral remains stable over time. On a site that burned in a 1919 wildfire on the San Dimas Experimental Forest, Angeles National Forest, crown cover of interior live oak had not changed from that recorded in a survey conducted in postfire year 14 (1933) and in a survey conducted in postfire year 34 (1950). Interior live oak and toyon were the 2 most common species in the mixed chaparral community. Interior live oak showed minimal gains in crown cover on a similar site that had gone 55 years without fire prior to wildfires in 1933 and 1936 [114].
Surveys conducted by Griffin [81] in the Santa Lucia Mountains suggest that fire-return intervals that are longer than those that occurred historically favor interior live oak and other evergreen oaks over valley oak in high-elevation (4,575 feet (1, 525 m)) savannas. He noted that interior live oak, canyon live oak, and tanoak were replacing valley oak successionally on high-elevation sites, while coast live oak was replacing valley oak on lower-elevation sites. He suggested that this successional replacement may be occurring because in the past, frequent, low-severity surface fires favored valley oak over the evergreen oaks [81].
The chaparral belts in which interior live oak grows contain highly flammable vegetation [105,134]. This, coupled with the hot, dry conditions that occur during the fire season (see Climate and moisture regime), makes chaparral sites easily ignitable [23]. When vegetation is dense, the often interlocking chaparral crowns ensure fire spread due to highly flammable and continuous fuels [161], especially with high winds [23]. In interior live oak chaparral, vegetation may be so dense that it is impenetrable except during the first 5 to 10 years after a fire. Mature interior live oak chaparral stands reach about 12 feet (4 m) tall and are usually denser than adjacent, mature chamise stands [120]. Mixed chaparral stands in Santa Cruz County formed an "almost impenetrable growth" of interior live oak, California coffeeberry, and other sclerophyllous species. Overstory shrubs ranged from 4 to 12 feet (1-4 m) high, with a 0.5- to 3.0-inch (1.3-7.6 cm) litter layer. The author deemed the community "a high fire hazard" [105].
Compared to many sclerophyllous species, however, interior live oak foliage [138] and litter are relatively nonflammable. One comparison of the flammability of chaparral vegetation listed interior live oak as low in flammability relative to manzanita and ceanothus species, tanoak, and California black oak [209]. Interior live oaks did not ignite during a 3 August prescribed fire in wedgeleaf ceanothus chaparral in Kern County. Interior live oaks on the site had a rounded form, with branches extending to the ground. However, the author observed that the fire "failed to affect this species" because fuels beneath interior live oak trees were scant and did not carry the fire [122].
Interior live oak's sclerophyllous leaves may be slow to decay. Latting [120] described the litter layer of interior live oak stands at the ponderosa pine-oak woodland ecotone as "slippery piles of leathery oak leaves that defy decomposition". The interior live oaks were small, with little understory beneath their crowded crowns [120].
Litter accumulation beneath interior live oak can vary depending, in part, on time since the last fire. Plumb and Gomez [164] report that the litter layer of interior live oak is typically thick. In southern California, Halsey [89] found barberry-leaved scrub oak-interior live oak-Muller's scrub oak (Q. cornelius-mulleri) chaparral had a "moderate" leaf litter layer (~7 inches (18 cm) thick). These communities typically occur on north-facing slopes below 3,000 feet (900 m) and on all aspects above that elevation. Overstory oaks are 4 to 12 feet (1-4 m) tall [89]. An interior live oak-valley oak community in Tehama County had a mean litter depth of 0.5 inch (1.3 cm) in September; dried annual grasses comprised a far larger proportion of the ground layer (26.3%) than did evergreen leaves (0.6%). The canopy averaged 13.5 feet (4.1 m) tall with 25.2% closure; tree basal area averaged 7.8 m²/ha [196]. After a fire in chaparral or oak woodlands with interior live oak, the ground layer may accumulate interior live oak debris until the decay rate equals or exceeds the rate of biomass accumulation. In burned, mixed-chaparral sites on the San Dimas Experimental Forest, biomass of interior live oak litter and woody debris increased linearly from postfire years 1 to 11 at an average rate of 0.082 ton/acre/year but then decreased without further fire [114].
From 1991 to 1994, the Forest Inventory and Analysis Program found that the greatest volume of live trees and coarse woody debris (CWD) of interior live oak was in the southern Sierra Nevada region (336.3 million feet³ live trees, 69.0 million feet³ CWD), and the least volume was in the North Coast Ranges (17.1 million feet³ live trees, 7.1 million feet³ CWD) (n=3,316 transects on 495 plots). Interior live oaks were considered tree-size when ≥5 inches (13 cm) DBH [197].
Pillsbury and Kirkley [162] provide equations to estimate total aboveground volume, wood volume, and saw-log volume of interior live oak and other California hardwoods.
With fire exclusion, interior live oak may become a ladder fuel in blue oak, valley oak, and other communities that historically burned less often than interior live oak-dominated communities. In oak woodland/annual grassland, dry herbaceous vegetation is the main fuel that carries fire [28]; however, ingrowth of understory interior live oak and ponderosa pine can increase fuel loads in and flammability of blue oak woodlands [82,154].
Interior live oak acorns require 2 years of development on the tree to complete maturation [45,68,96].
Fresh interior live oak acorns are not dormant [159], so when there is enough moisture, they may germinate soon after dispersal. Fully mature, fresh acorns have germinated in the laboratory a few days after collection (review by [47]), and interior live oak seedlings may begin germinating in late fall in the field. Momen and others [146] suggest that for germination and seedling establishment, interior live oak and other evergreen oaks are adapted to use soil moisture from late-fall rains, when deciduous species are dormant. Interior live oak showed 75% mean germination after 30 to 60 days of cold stratification in the laboratory. Increased rates of interior live oak germination after cold stratification in the laboratory (review of Bonner's [34] laboratory studies) suggest that winter temperatures enhance its germination rates in the field.
Fire top-kills most interior live oaks [78,83,163]. Low-severity fire causes little mortality for age classes, although it may kill some seedlings. Moderate-severity fire may kill small trees [87], and severe fire may kill even large interior live oaks [88]. Fire-scarred interior live oak trees were common on 7 sites across interior live oak's range in California [184].
Interior live oak's thin bark makes young trees susceptible to fire kill. Although the bark of mature trees is still relatively thin and has a high live tissue:dead outer bark ratio [164], mature trees may survive fire without top-kill [88,164]. Plumb and Gomez [164] observed that mature interior live oaks with heavily charred bark suffered no scarring and lost little bark to sloughing. They reported that surface fires rarely burned through to the wood, and repeated fires resulted in a hard, fire-cured bark surface [164]. Haggerty [88], however, reports that fire scars large interior live oaks easily.
Fuel mastication in oak-knobcone pine or other communities may result in fires that are more lethal than fires in communities with unmanipulated fuels. In a California black oak-knobcone pine community in Whiskeytown National Recreation Area, sites where fuels were masticated prior to spring burning had higher flame lengths, higher fire temperatures in the litter layer, and greater mortality of overstory and pole-sized oaks—including California black, interior live, and canyon live oaks—than sites where fuels were not manipulated. Mastication was done in November, and the study sites were burned under prescription in April. Interior live oak and canyon live oak were overstory associates [42].
Use as rangeland: Oak communities with interior live oak are important rangelands [15] for wild and domestic ungulates. Blue oak-interior live oak-foothills pine woodland/annual grasslands of the Sacramento and San Joaquin valleys are particularly prized as productive rangelands [22]. Mule deer [13,20,54] and feral hogs [13] use oak woodlands with interior live oak as their primary habitats. In the Sacramento Valley, mule deer used oak woodlands as often as expected, and wedgeleaf ceanothus chaparral more than expected, based on availability. Feral hogs used interior live oak woodlands more than expected [13].
Interior live oak is an important deer food. In Lake County, mule deer browsed interior live oak year-round, with heaviest use in spring and summer [20]. Use may also be high in winter, when deciduous species have shed their leaves, and in spring, when new shoots are available [24]. A study on the Tehama Winter Deer Range found acorns and dry oak leaves were the primary components (65% of total) of the mule deer diet in October and November. Mule deer used interior live oak as much as expected based on its availability [123].
Oak/annual grassland types are California's primary livestock grazing lands [3,23,63,196]. Cattle [13] and domestic sheep [20] forage in oak woodlands on low foothills. Cattle use flat, open woodlands, while mule deer generally prefer more closed sites with rockier terrain [13]. In Lake County, domestic sheep browsed interior live oak mostly in late spring and summer [20].
Many wildlife species consume interior live oak acorns, including bears [89,189], mule deer [9,24], squirrels [9,81], other rodents [81], acorn woodpeckers [9,116], scrub jays [9], and band-tailed pigeons. Acorns, including those of interior live oak, are a winter staple for band-tailed pigeons [150]. American black bears in the Transverse Ranges consumed large volumes of acorns (canyon live oak and interior live oak, 13%-19% of total diet); behind garbage, acorns were their primary food source [189]. Historically, the California grizzly bear, the largest race of grizzly bears [89], also consumed acorns [81]. Chaparral was a preferred habitat of California grizzly bears [89].
Acorns can be important cattle feed; however, acorns are low in protein and become available after annual herbs have died, so cattle consuming large amounts of acorns require a protein supplement [207].
Habitat use: Oak woodlands, including those with interior live oak, are tremendously important wildlife habitat [183]. A study on the Central Coast Ranges found mule deer generally preferred a mixed-oak woodland habitat over chamise chaparral, but they preferred a chamise community after a prescribed fire. Mule deer used the chamise chaparral burn as primary habitat from about postfire year 2.0 to 2.5, then resumed using the mixed-oak woodland as their primary habitat [115]. On the Sierra Foothill Range Field Station, a 3-year study found wildlife species diversity was directly related to diversity of the mixed-oak woodland. Hutton's vireo, orange-crowned warblers, and Wilson's warblers were positively associated with interior live oak. Over 60 bird species bred and resided year-round in the oak woodland, and many others used the area as winter habitat. Several rodent and herptile species, such as brush mice and western fence lizards, were positively associated with the oak woodlands (P<0.1 for all variables). See Block and Morrison [30] for a list of these wildlife species. In a Kern County study, salamanders were positively associated with interior live oak-foothill pine woodlands on north-facing slopes. Except for the ground layer, vegetation cover was higher in salamander habitats than on sites without salamanders (P<0.05). Ensatina was the most commonly captured amphibian [31]. Black-bellied, California slender, and yellow-blotched salamanders are also positively associated with interior live oaks [32].
On 2 sites in the Sierra Nevada and 1 in the Tehachapi Mountains, Nuttall's woodpeckers foraged heavily in interior live oak-gray pine woodlands outside the breeding season, but they used blue oak woodlands during the breeding season. Interior live oaks selected for foraging were larger than average, but acorn woodpeckers typically selected large gray pines over large interior live oaks for foraging [29]. Surveys across California's oak woodlands found Nuttall's woodpeckers used live oaks, including interior, canyon, and coast live oaks, for foraging about 19% of the time. They used blue oak (51% use) more than the evergreen oaks but less than other deciduous oaks or gray pine [147].
See these sources for lists of birds using oak woodlands with interior live oak as habitat: [167,172,205].
Interior live oak woodlands are high-quality dusky-footed woodrat habitats [121]; in part, because they provide important food. On the San Dimas Experimental Forest in the San Gabriel Mountains, acorns of scrub interior live oaks were the primary food stored in dusky-footed woodrat nests at high elevations (>4,500 feet (1,400 m)), even though canyon live oak acorns were more plentiful and larger [99].
Many insects use interior live oaks as habitat. Interior live oak hosts Cynipidae gall wasps [52]. The pan-like depressions that are created by scar tissue around branch breaks collect water in spring; these depressions are habitat to maturing insects including mosquitoes, midges, syrphid flies, and moth-flies [215].
Palatability and nutritional value: New spring growth and sprouts arising after fire or other top-killing events are highly palatable to mule deer [24]. Livestock also find interior live oak palatable, and they utilize it increasingly as annual grasses dry and lose nutritional value [129].
Overall nutritive value of interior live oak appears low. In a laboratory experiment using captive mule deer and domestic sheep, total digestible nutrient content of interior live oak was less than that of alfalfa (Medicago sativa) or chamise. The authors concluded that interior live oak was of little to no value as a source of protein but overall, it was a fair source of total digestible nutrients [20]. However, interior live oak provides a little protein in late fall and winter months, when deciduous browse species have shed their leaves. Bissell and Strong [19] found interior live oak protein content peaked in June at 8% and was least in December and February at 1%. See these sources for further details on the nutritional value of interior live oak browse: [19,20,176].
Browse of interior live oak and other evergreen oaks is generally less palatable than that of deciduous oaks due to higher concentrations of tannins and lignins in the leaves [155]. However, domestic goats usually find interior live oak moderately to highly palatable [79]. In the Sierra Nevada, they ate interior live oak stems "avidly" (observations by [79]). In mixed chaparral in southern California, domestic goats ate 5-year-old, postfire scrub interior live oak about as much as expected, preferring sprouts of birchleaf mountain-mahogany, redberry buckthorn (Rhamnus crocea), and barberry-leaved scrub oak over sprouts of interior live oak [79].
Cover value: Oak woodlands provide vitally important cover for wildlife. Squirrels and cavity-nesting birds often prefer cavities in oak branches or boles for nesting, while rodents, skunks, and foxes dig and den in the roots or in downed interior live oak logs [9].
Many wildlife species may prefer interior live oak and other evergreen oaks as cover in late fall and winter, when deciduous trees lack foliage. Feral hogs in the Sierra Nevada used interior live oak woodlands as bedding and forage sites. Their use increased in winter, when associated blue oaks had lost their leaves and provided less cover [13]. In urban Sacramento, yellow-billed magpies selected interior live oaks as communal roosts over all other tree species during the December through May study period. Evergreen species in general were selected over deciduous species [53].
In a blue oak woodland on the San Joaquin Experimental Range, understory interior live oaks apparently helped protect California towhee nests from predation. On cattle-grazed sites, California towhees preferred interior live oaks for nesting (25% frequency vs. 8% frequency for all other nest-trees), and nesting success was greater in interior live oaks than in other nest-trees. For cover near the actual nest-tree, successful nests were built on sites with more understory interior live oak cover than occurred on nest-predated sites (P=0.003). Western scrub-jays were responsible for most nest predation. On ungrazed sites, California towhees preferred to nest in wedgeleaf ceanothus (18%, 4%, and 12% use for wedgeleaf ceanothus, interior live oak, and other nest-trees, respectively). Nest failure was significantly higher on ungrazed than on grazed sites (P=0.008) [168].
See Plumb and MacDonald [165] for a guidebook on managing California's oaks.
Interior live oak is apparently resistant to sudden oak death disease. As of 2003, it was the only red oak in California in which the disease had not been detected in the field [60].
Possible impacts of climate change on interior live oak are uncertain. Models of McBride and Mossadegh [133] suggest the distributions of most California's oak species, including interior live oak, will not shift with climate change. However, paleobotanical investigations by Davis [55] revealed distributions of California's oak species have shifted in the past with climate change, and he predicts that the distributions of California's oaks will shift with new changes in climate. Large-scale vegetation monitoring (>17,000 plots) across California suggests that the elevational range of interior live oak is extending upslope [195].
Although interior live oak's value for wildlife and livestock is now appreciated, it has been disparaged in the past. In the 1950s and 1960s, some management plans called for removing oaks
in general and interior live oak in particular from California's foothills in order to increase herbaceous livestock forage and water yields [21,43,64,94].These efforts greatly increased rates of soil erosion on steep slopes [43,65] and had inconsistent results regarding herbaceous forage yield production after oak removal [183]. Studies have shown decreases [72], no clear trends [169], or increases in forage production [73] after interior live oak removal. In general, oak removal did little to increase water yields on foothill slopes [25,65], although some studies showed increased water yields on valley bottoms after oaks were cut [25].
On the San Joaquin Experimental Range, forage production was greater beneath interior live oak canopies than in the open during 2 drought years. The 1st year of the drought, herbaceous forage biomass peaked in May, at about 700 kg/ha more under interior live oak canopies than in the open. The 2nd year, forage production peaked in May at about 1,000 kg/ha more under interior live oaks than in the open. Herbaceous production early in the growing season (November-January) was similar under interior live oaks and in the open, but it was significantly greater under interior live oaks from March through May (P=0.05) [73]. In general, late-successional annual grasses such as wild oat and ripgut brome were more common under interior live oak than in open areas. Filaree (Erodium spp.), clover (Trifolium spp.), sixweeks grass (Vulpia spp.), and other early-successional species were most common in open areas (review by [183]).
Interior live oak produces good-quality firewood [164,173]. Much interior live oak was cut for cordwood around the turn of the 20th century [173]. The wood has little value as lumber [155].
Acorns of interior live oak and other oaks were a staple of California Indians [8,130]. In order to produce new sprouts for basketry, Indian women used fire regularly to top-kill interior live oaks. They preferred 1-year-old sprouts for making baskets [7].
Interior live oak's growing season peaks in early spring; in the Sierra Nevada, most vegetative occurs in March [175]. Interior live oak flowers [68] and sheds pollen in late spring. Photoperiod evidently regulates release of interior live oak pollen [204]. Acorns ripen from mid-August [160] to October [144,207]. The leaves are retained for 2 years [137,159]. Acorns germinate slowly over fall and winter [131,132].
In the Santa Lucia Mountains, time of germination initiation varied with elevation but regardless of elevation, interior live oak germination took several months to complete. Acorns began germinating in November at low elevations (76 feet (23 m)); they began germinating in December at high elevations (4,460 feet (1,360 m)). Germination was complete for acorns at low and midelevations (1,840 feet (560 m)) by February, while acorns at high elevations finished germination by March [132].
Wind disperses interior live oak pollen [57,58].
Interior live oak is monoecious [34]. Dodd and Kashani [60] suggest that past population fragmentation has resulted in a metapopulation structure for interior live oak. Pollen-mediated gene flow is relatively free among interior live oak populations, and introgression with other red oaks contributes to interior live oak's genetic diversity [57,58,59] (see Hybridization). For successful pollination between interior live oak and other red oaks, genetic studies show that climate compatibilities of interior live oak and the other parent are more important than distance from the pollen source [59].
Interior live oak is a hardy oak that can regenerate from acorns or by sprouting. Sprouting is apparently the most common method of interior live oak regeneration.
Interior live oak is well adapted to regenerating after fire or cutting. The Hopland Research Field Station was nearly de-wooded from 1959 to 1965 in the belief that removing trees would provide more livestock forage and increase water yields (see Other Management Considerations for a discussion of this practice). After almost complete clearcutting except for a few large trees left for shade and a prescribed fire in 1965, a different management practice was started: Trees were allowed to regenerate. Despite the cutting and burning, oak regeneration on slopes ranging from 0° to 40° was significantly higher in 1996 compared to pretreatment levels in 1952 (P>0.05). Among tree species, interior live oak had gained greatest cover (28.4%) by 1996. This was attributed mainly to sprouting after cutting and burning [43].
Little information was available as of 2011 on rates of interior live oak seedling establishment. Interior live oak showed widely different degrees of establishment on 4 sites. In Eastwood manzanita-interior live oak chaparral on Mt Tamalpais, interior live oak seedlings and saplings had an average density of 26,980 plants/ha, while interior live oak was absent from plots in Eastwood manzanita-interior live oak chaparral at Northridge. Neither site had burned for at least 56 years [109,110]. For acorns planted in interior live oak's natural elevational ranges, interior live oak showed 18% mortality at seedling emergence on the Santa Lucia Range and 2% to 5% mortality at seedling emergence in the Sierra Nevada [132].
Limited information suggests that interior live oak is reproducing at rates adequate to maintain its populations ([148], review by [182]). Some data suggest that interior live oak is maintaining the expected age-class distributions of more seedlings than saplings and more saplings than mature trees [14], but a few studies suggest rates of interior live oak regeneration may be lower than historical rates. Urban encroachment into oak woodlands poses a serious threat to interior live oak regeneration [74]. Forest Inventory and Analysis data from 2001 to 2005 showed that across California's forestlands, interior live oak numbered about 275 million seedlings (diameter class of 1.0-2.9 inches (2.5-7.5 cm)); 125 million saplings (3.0-4.9 inches (7.6-22.9 cm)), and about 2 million relatively large trees (9.0-10.9 inches (23-27.7 cm)). Compared to California black oak, interior live oak showed higher rates of regeneration but also had higher rates of mortality [14]. Bartolome and others [17,149] reported widespread presence of interior live oak saplings in the late 1980s, but saplings did not outnumber mature trees. Ratios of saplings:mature plants were ≤1:1 in the North Coast Ranges and Klamath-Siskiyou regions and from 1:1 to 1:2 in the Central Coast Ranges and Sierra Nevada [149]. In manzanita chaparral in northern California, scrub interior live oak regeneration averaged ≤1.2 seedlings/m². Most were between 0 and 20 inches (8 cm) tall (Parker unpublished data cited in [153]). Some interior live oaks had apparently grown into the canopy since the last fire [153].
There is evidence that in general, many oak species in the blue oak woodland belt are failing in the pole stage [186], but as of this writing (2011), information of interior live oak in particular was sparse.
On 192 plots in Madera, Fresno, Tulare, and Kern counties, 75% of plots had interior live oak seedlings and 48% had saplings. Interior live oak regeneration was not significantly associated with grazing or elevation. Solar radiation, however, was positively associated with interior live oak seedling presence (P=0.1). The authors predicted that because sclerophyllous interior live oak is more drought-tolerant than deciduous blue oak, it might regenerate more successfully and dominate on drier sites than blue oak [185].
Interior live oak is reported as slow-growing [159]. This is may be due to the dry habitats it typically occupies, but studies exploring interior live oak growth rates on moist vs. dry sites were not available of as 2011.
Heavy mule deer [54] or other browsing can reduce or eliminate interior live oak regeneration. One year following a stand-replacement wildfire on Quail Ridge Reserve near Lake Berryessa, mule deer had browsed 95% of new interior live oak sprouts. The authors suggested that mule deer's preferential selection of interior live oak and blue oak sprouts was hindering postfire regeneration of the oaks [10]. After domestic sheep were removed from Sequoia National Park in the 1890s, there was a flush of oak (Quercus spp.) seedling establishment. The authors claim that unlike fire exclusion, which can favor shrubs over trees, density of woody species has increased since cessation of livestock grazing, but this has not resulted in a shift in species composition towards shrubs [174].
Site characteristics: Interior live oak mostly grows on harsh sites that other oaks cannot tolerate.
Climate and moisture regime: Interior live oak grows strictly in a mediterrean climate, which is characterized by mild, wet winters and hot, dry summers [18,22,23]. It is adapted to dry sites [102]; among California's red oaks, interior live oak has the highest tolerance for xeric conditions [60,179]. Mean annual precipitation across interior live oak's distribution in California ranges from 15 to 50 inches (380-1,300 mm) [155]. Except for the deserts, the oak (Quercus spp.) woodland/grassland regions of the Sierra Nevada are driest areas in California [202], typically receiving <25 inches (625 mm) of precipitation annually. During the fire season, maximum summer temperatures in interior live oak foothill communities sometimes reach 105° F (41° C), with ≤5% relative humidity [23].
Interior live oak's evergreen leaves help protect it from desiccation, but it is not well adapted to snowy or cold sites. The branches do not hold snow loads well, and the evergreen leaves freeze easily. California black oak, which is better adapted to snow and cold, usually replaces interior live oak on upper foothills [155].
Interior live oak sometimes grows in riparian and other wetland areas. It may be frequent to dominant in riparian zones, especially in southern California [174,214]. In the East Bay Hills, it is a component of coast live oak communities on hillside springs [4].
Elevation and topography: A major vegetation survey (>17,000 plots) across California's oak communities found interior live oak had the greatest elevational range among California's 5 most frequently dominant oaks: blue oak (Q. douglasii), California black oak, canyon live oak (Q. chrysolepis), interior live oak, and valley oak (Q. lobata). Survey data suggested that interior live oak was becoming more common in montane regions compared to its 1930s distribution [195].
Interior live oak grows from 1,000 to 6,200 feet (300-1,900 m) elevation across its range [68]. It tends to occur at lower elevations in northern than in southern California. Mixed-oak woodlands with interior live oak, valley oak, and/or blue oak occur from 3,000 to 4,000 feet (914-1,218 m) along the entire west slope of the Sacramento River valley [171]. Interior live oak chaparral may occur in scattered clumps at the highest elevations (>5,500 feet (1,700 m)) of foothills in southern California [136]. Scrub interior live oak grows at elevations from 1,000 to 6,600 feet (300-2,000 m) across its range [96], occurring at elevations up to 2,000 feet (600 m) in northern California [137] and usually from 3,500 to 6,200 feet (1,200-1,900 m) in southern California [68,99].
Landforms with interior live oak include dry valleys, canyons, and foothill slopes [68,96]. Interior live oak prefers north-facing or other relatively mesic slopes within these dry habitats [120,190]. A 1932 publication noted that on the basalt table mountains above San Joaquin Valley, interior live oak was dominant on north-facing slopes and had a scattered presence on south- and west-facing slopes. All slopes had mostly shallow soils and ephemeral streams, so they were dry for most of the year [76].
Soils: Interior live oak tends to occur on shallow soils in chaparral and on deeper soils in oak woodlands. Chaparral soils are nearly always dry and shallow [98]. On sites with minimal soil development, interior live oak roots may force their way through fractured rock to reach groundwater [48,124]. The soils of California's oak woodlands are typically deep and productive [21,23]; hence, the frequent management of oak woodlands as rangelands. Interior live oak woodlands may occur on shallow to deep soils, but they generally occupy shallower soils than those of other oak series. In the San Bernardino Mountains, canyon live oak stands grade into interior live oak stands on shallow soils and ridgetops [51]. However, interior live oak and other oak chaparral communities usually occur on relatively more productive and deeper soils than soils supporting chamise (Adenostoma fasciculatum) or manzanita (Arctostaphylos) chaparral [151].
Interior live oak typically grows in soil of igneous [24,128] or granitic [213] origin. Interior live oak communities in Tehama County have formed over volcanic breccia. Soils are 2.5 to 5 feet (0.8-1.5 m) deep and slightly acid [24]. In the San Luis Obispo Valley, scrub interior live oak grows in siliceous sandstone [210]. Interior live oak is rarely associated with serpentine soils [155]. It does not grow with gray pine (Pinus sabiniana) on serpentine sites, but it is commonly associated with gray pine on nonserpentine sites [93,98]. Interior live oak does, however, grow in serpentine and other ultramafic soils in knobcone pine (P. attenuata) communities of the Klamath Mountains and the North Coast Ranges [98].
Interior live oak grows in soils of all textures. Interior live oak-blue oak communities in Sutter County occur on gravelly loams and shallow to moderately deep (<41 inches (100 cm)), well-drained sandy loams. One blue oak-interior live oak series had a claypan layer from 15 to 30 inches (38-76 cm) deep. Wood production of interior live oak and blue oak was greatest on sites with moderately deep soils without claypans [128].
Plant communities:Interior live oak communities on Table Mountain and in Coal Canyon, Butte County.
Interior live oak occurs in chaparral, oak woodland, and conifer-oak woodland [96] communities. Typically, communities dominated by nonnative annual grasses [27] and/or chaparral shrubs [21] bound or form a mosaic with oak woodlands at low elevations, and oak woodlands meld into ponderosa pine (Pinus ponderosa) communities on upper foothills [27]. Interior live oak scrub chaparral merges into interior live oak woodlands on some sites; a more frequent fire-return interval and/or drier soils apparently helps maintain the scrub type [98]. Two interior live oak vegetation types were identified on the San Bernardino National Forest: chaparral and forest. Interior live oak chaparral occurred on steep (x = 45°), dry slopes, and associated vegetation was mostly sprouting, sun-tolerant chaparral species including chaparral whitethorn (Ceanothus leucodermis) and chamise. Interior live oak forest occurred on more moderate (x = 20°), mesic slopes with a sparse, mixed understory of "obligate seeders" (that is, species that are killed by fire and establish afterwards from seed) and shade-tolerant sprouting shrubs such as Pacific poison-oak (Toxicodendron diversilobum). These types were not discrete on most sites; instead, the 2 types formed a blended continuum [211].
Gray pine and California buckeye (Aesculus californica) commonly associate with interior live oak across the ranges of all 3 species [15,18,155]. Pacific poison-oak is widespread in most woodlands with interior live oak (for example, [2,42,90,212]). As well as dominating California's annual grasslands, nonnative annual grasses comprise most of the groundlayer vegetation in California's chaparral [6] and oak woodlands [3]. These annuals also dominate the groundlayer of chaparral ecosystems in Baja California [143]. Wild oat (Avena fatua), ripgut brome (Bromus diandrus), soft chess (B. hordeaceus), and hare barley (Hordeum murinum subsp. leporinum ) are typical annual grass dominants [87,183,196]. Composition of the groundlayer prior to European settlement is unknown [3]. Interior live oak may finger into annual grasslands on valley floors. For example, interior live oak is an occasional species in annual grasslands of El Dorado County [213].
Chaparral: "Chaparra" translates from Spanish to "scrub oak" in English. Scrub oak chaparral, in which scrub interior live oak is often a primary component, comprises about 15% of the chaparral landscape of California. Codominant and associated species in scrub oak chaparral are mostly shrubs such as chamise and deer brush (C. integerrimus) [33]. The associated shrubs are often a mix of species that sprout after fire, such as chamise, and obligate seeders [56] such as wedgeleaf ceanothus (C. cuneatus) [108].
Interior live oak usually dominates the "scrub" or "live oak" chaparral vegetation types in the Inner Coast Ranges and the Sierra Nevada [23,98,106,120]. About 25% of interior live oak's total population lies within chaparral ecosystems [195]. Sawyer and others [178] place a plant community in the interior live oak scrub series if >60% of the overstory is shrubby interior live oak. If cover of shrubby interior live oak is less, the series is classified as mixed chaparral [178]. Interior live oak-dominated chaparral typically occurs on slopes; soils may be alluvial or derived from bedrock, and they are often rocky. Chamise, wedgeleaf ceanothus and other Ceanothus, and barberry-leaved scrub oak (Q. berberidifolia) often codominate with interior live oak in chaparral communities [27].
Northern California: In interior northern California, interior live oak is typically the dominant evergreen in scrub oak communities [49]. Interior live oak scrub communities are most common on north-facing slopes [120]. Chamise, manzanita, wedgeleaf ceanothus [23], and whitethorn ceanothus (C. cordulatus) [178] are common codominants or associates. Interior live oak occurs in and sometimes dominates montane chaparral in the Sierra Nevada [120]. Van Wagtendonk [201] describes the montane chaparral-woodlands of Yosemite National Park as overstories of interior live oak, canyon live oak, and gray pine with whiteleaf manzanita (A. viscida), deer brush, birchleaf mountain-mahogany (Cercocarpus montanus var. glaber), and other chaparral shrubs in the midstories. A foothill mixed-chaparral type is described along the Kaweah River in Sequoia National Park. Interior live oak, California buckeye, and canyon live oak codominate the mix. Tree cover is around 40% to 60%, shrub cover from 30% to 60%, and cover of annual herbs around 50% to 75%. There has been some influx of forest conifers that is attributed to fire exclusion [203].
Interior live oak is a minor to important associate in scrub oak communities dominated by other oaks, usually coast live oak [66] or canyon live oak [194]. Interior live oak is rare in barberry-leaved scrub oak communities of Sonoma County [40].
Interior live oak is a characteristic to dominant species in mixed chaparral of northern California; chamise, and sometimes barberry-leaved scrub oak, are usually codominant [98,140]. In the Outer North Coast Ranges of Santa Cruz County, interior live oak is "quite common" in the chaparral belt [105]. In mixed chaparral near Lakeport, interior live oak and Eastwood manzanita (A. glandulosa) tend to dominate on north- and west-facing facing slopes, while chamise tends to dominate on south- and east-facing slopes [190].
Southern California: Interior live oak scrub communities of southern California are likely maintained by frequent fire [178]. Coast live oak, canyon live oak [106], barberry-leaved scrub oak, and/or coastal sage scrub oak (Q. dumosa) [98] often codominate. Generally, interior live oak or coastal sage scrub oak dominate oak scrub of the Inner Southern Coast Ranges, while barberry-leaved scrub oak dominates oak scrub of the Outer Southern Coast Ranges [111]. The interior live oak scrub vegetation type is common on xeric slopes, often sandwiched between mixed chaparral at low and conifer forests at high elevations. Shrubby interior live oaks may spread into mixed chaparral in intermittent stream draws [157]. In the San Bernardino Mountains, interior live oak may dominate the upper reaches of barberry-leaved scrub oak and coastal sage scrub oak types [100]. Interior live oak is the primary dominant in some oak scrub series in the western Transverse Mountains, where it codominates with canyon live oak, barberry-leaved scrub oak, birchleaf mountain-mahogany, chamise, and/or chaparral whitethorn. It is occasional in riparian coast live oak and other riparian oak woodlands [41].
Mexico: Interior live oak was rare in barberry-leaved scrub oak chaparral of the Sierra de San Pedro Mártir in Baja California. It was found on west-facing slopes near 5,200 feet (1,600 m) elevation [143].
Oak woodlands and forests: Interior live oak-dominated woodlands and occasional forests are most common in northern California, occupying west slopes of the Southern Cascade Range and the Sierra Nevada. In 1844, the explorer John Fremont made the first recorded observation of interior live oak when descending into the Sacramento Valley near the American River from upper slopes of the Sierra Nevada: "At every step the country improved in beauty; the pines were rapidly disappearing and oaks became the principal trees of the forest. Among these the prevailing tree was the evergreen live oak" [155]. Interior live oak gains dominance with elevation in the foothills; interior live oak-gray pine woodland/annual grasslands extend from about 1,000 to 2,500 feet (300-800 m) elevation in the Sierra Nevada [178].
The interior live oak series is placed in the mixed broadleaved, evergreen-cold deciduous woodland formation. The series often grades in from lower-elevation interior live oak scrub. Woodlands and occasional forests dominated by tree-sized interior live oaks occur on valleys, slopes, and ridgetops; these landforms often have moderately to excessively drained, shallow soils [178]. On foothills surrounding the Sacramento and San Joaquin valleys, interior live oak tends to dominate the drier slopes of the Sierra Nevada, while coast live oak tends to dominate the relatively wetter slopes of the Coast Ranges [45]. Shrubs are typically chaparral types such as toyon (Heteromeles arbutifolia), wedgeleaf ceanothus, and whiteleaf manzanita. In the Sierra Nevada, interior live oak woodlands ranged from a low of 1,144 feet (249 m) for the interior live oak-gray pine/whiteleaf manzanita subseries to 2,120 feet (646 m) for the interior live oak/yerba santa (Eriodictyon californicum)/annual grass subseries [2]. Interior live oak woodlands are rare in Pinnacles National Monument, and they are the only oak woodlands in the Monument. Sprouting shrubs, including toyon, creeping snowberry (Symphoricarpos mollis), and Pacific poison-oak are common in the type [90]. In the San Bernardino Mountains, interior live oak may dominate upper reaches of canyon live oak woodlands [158].
Interior live oak is frequent to codominant in many blue oak woodlands [11,16]. Interior live oak-blue oak-gray pine communities lie just beneath the ponderosa pine belt [117]. Blue oak-interior live oak/annual grass woodlands typically occupy the lowest foothills, with gray pine often codominating [1,2,5,98]. They average about 1,550 feet (500 m) elevation [2]. Near Clear Lake, blue oak-interior live oak communities tend to occupy north-facing slopes, while chamise or mixed manzanita (Arctostaphylos)-chamise chaparral occupies south-facing slopes [26]. Interior live oak is common, but rarely dominant, in blue oak communities in the low foothills of Sequoia National Park [11]. A blue oak-interior live oak/whickerbrush (Leptosiphon ciliatus) community occurs on fine loamy soils in northern Santa Barbara County [35].
Many mixed-oak woodland communities contain interior live oak as an associated or codominant species. Codominant oaks may include coast live oak, blue oak, valley oak, and/or Oregon white oak (Q. garryana) in the northern portion of interior live oak's distribution and Engelmann oak (Q. engelmannii) [15,18], barberry-leaved scrub oak, and/or coastal sage scrub oak [27] in the south. Interior live oak is a characteristic species in some Oregon oak woodlands of the North Coast Ranges [50,98] and the Klamath Mountains [98]. On the Hopland Research Station in Mendocino County, interior live oak codominates with coast live oak, blue oak, and California black oak [43]. Latting [120] describes a northern oak woodland type that occurs inland from redwood (Sequoia sempervirens) forests north of the Bay Area. These woodlands are composed of Oregon white oak, California black oak, canyon live oak, interior live oak, and other broadleaved species. They range from 3,000 to 5,000 feet (900-2,00 m) elevation in the North Coast Ranges and the Yolla Bolly Mountains [120].
Interior live oak is incidental to dominant in riparian oak or other hardwood riparian communities of northern California [174], and it may be frequent in riparian zones of otherwise dry slopes in southern California [214]. In riparian areas, interior live oak cover is sometimes dense enough to form a closed-canopy forest (see the photo of Coal Canyon Creek area). Interior live oak riparian communities occur below about 3,000 feet (900 m) in northern California and above about 6,000 feet (2,000 m) in southern California [98]. In Sequoia National Park, riparian interior live oak-blue oak-California buckeye communities occur at low elevations (1,300-3,300 feet (390-1,000 m)), with denser stands than those of upland blue oak-interior live oak communities [174]. The typical variety of interior live oak is occasional in riparian woodlands in the San Gabriel Mountains [120].
Conifer-oak: Interior live oak is a component of many pine-oak and other conifer-oak communities. It may finger into [120], and sometimes codominate in, ponderosa pine communities. In Monterey County, ponderosa pine-interior live oak-canyon live oak communities occur around 3,000 feet (900 m) elevation [86]. Scrub interior live oak associates with knobcone pine in the North Coast Ranges [5,12]. Interior live oak is an associated species in Coulter pine (P. coulteri) communities in the Machesna Mountain Wilderness [37] and other locations on the Los Padres National Forest [38]. It codominates with Coulter pine at high elevations 4,890 to 4,920 feet (1,490-1,500 m) of the Santa Lucia Range [84]. Interior live oak associates with bishop pine (P. muricata) on Santa Cruz Island [5].
Mixed-evergreen and mixed-conifer zones may support interior live oaks, with interior live oaks becoming increasingly scattered with increasing elevation. The interior live oak-Pacific madrone (Arbutus menziesii)/Pacific poison-oak series occurs on mesic foothills at around 1,500 feet (450 m) in the North Coast Ranges and the Sierra Nevada [1]. Interior live oak is a minor [103] to characteristic [179] associate in Douglas-fir-tanoak (Pseudotsuga menziesii-Lithocarpus densiflorus), Douglas-fir-Pacific madrone, and other mixed-evergreen forests. In Santa Cruz County, it was noted in a redwood-mixed evergreen-hardwood forest in Big Basin Redwoods State Park [101]. Interior live oak was rare in redwood forests of southern Monterey County [39]. In the Sierra Nevada, it is sometimes associated in the mixed-conifer overstory with ponderosa pine, Douglas-fir, white fir (Abies concolor), sugar pine (Pinus lambertiana), Jeffrey pine (Pinus jeffreyi), and/or red fir (A. magnifica) [67,145,166]. In mixed-evergreen forests of the Santa Lucia Range, interior live oak codominates with bristlecone fir (A. bracteata), coast live oak, and canyon live oak [191]. On the eastern Transverse Ranges, it fingers into bigcone Douglas-fir (Pseudotsuga macrocarpa) communities from lower-elevation (~780 feet (230 m)) chamise chaparral [139]. In the San Gabriel Mountains, interior live oak is confined to north-facing slopes and draws; bigcone Douglas-fir and canyon live oak are commonly associated species [97]. Scrub interior live oak sprouts are often prominent in early postfire, seral bigcone Douglas-fir woodlands [5].
See the Fire Regime Table for a list of plant communities in which interior live oak may occur and information on the FIRE REGIMES associated with those communities.Interior live oak is more frequent in open or early-seral communities than in late-seral communities. It is moderately shade tolerant; young plants are more tolerant than mature individuals [164]. In the North Coast Ranges of Mendocino County, interior live oak saplings were found in the understory of a mixed-evergreen forest, but they rarely grew over 0.3 foot (1 m) tall [104].
Interior live oak may replace valley oak successionally on valley-foothill interfaces [81]. (See the discussion of Griffin's study [81] in Plant response to fire for more information.) Conversely, Douglas-fir may replace interior live oak on favorable sites in mixed-evergreen communities of Mendocino County [104]. Chaparral and oak woodlands usually remain distinct, with little conversion of one type to another [120].
Fire is important in maintaining interior live oak chaparral and woodlands. Some consider relatively high-elevation interior live oak scrub a fire-maintained community, with ponderosa pine and other conifers replacing interior live oak without frequent fire [98]. See Postfire successional patterns for further information on interior live oak succession.The scientific name of interior live oak is Quercus wislizeni A. DC. (Fagaceae) [75,107,126,192,198]. It is in the red or black oak subgenus (Lobatae) [34,58]. There are 2 varieties of interior live oak [75,192,198]:
Quercus wislizeni A. DC. var. wislizeni, typical variety of interior live oak
Quercus wislizeni A. DC. var. frutescens Englem., scrub interior live oak
Most information on interior live oak is written at the species level. In this review, "interior live oak" refers to the species as a whole, and the varieties are referred as "the typical variety" or "scrub interior live oak".
Hybridization:
Facile hybridization among red oaks makes the separation of species within that subgenus a taxonomic challenge. Among California's red oaks, interior live oak hybridizes frequently with coast live oak (Q. agrifolia) [45,46,59,61,126,198,204], Santa Cruz Island oak (Q. parvula) [59], California black oak (Q. kelloggii) [59,198,199], and oracle oak (Q. × moreha Kell.) [126]. Oracle oak is a stable California black oak × interior live oak hybrid [198].
In California, all red oak species show some degree of introgression
with other red oaks. Interior live oak populations in northern California show genetic evidence of considerable introgression with coast live oak and Shreve oak (Q. parvula var. shrevei); all 3 taxa are evergreen. Interior live oak populations show less introgression with California black oak, which is deciduous [57,58]. Backcrossing and hybrid swarms
are most common between interior live oak and coast live oak [61], which genetic tests show are the most closely related of California's red oaks [58,61]. Dodd and others [62] suggest that coastal populations of interior live oak, which have high amounts of introgression overall, should be reclassified as Santa Cruz Island oak, with gene flow from interior live oak to coast live oak, then to Santa Cruz Island oak, making separation of the 3 species difficult in coastal locations. Interior live oak and Santa Cruz Island oak are sometimes treated as synonyms [62], but are treated as distinct species in this review.
Interior live oak sprouts after top-kill by fire [87,98], cutting [127], or herbicide use [94]. Field experiments in the Santa Lucia Range and the Sierra Nevada showed that damaged interior live oaks may sprout in low numbers (2%-13%) even during stages of epicotyl emergence [131]. Large trees may produce epicormic sprouts after fire [87] or other injury to the bole.
A study in Mendocino County suggests that some interior live oaks may sprout after top-killing disturbances in most seasons. Sprouting responses of cut interior live oak and other oaks were compared throughout the year at the Hopland Field Station. In general, more interior live oaks sprouted after cutting compared to blue oaks; a similar number of interior live oaks and California black oaks sprouted; and fewer interior live oaks sprouted compared to barberry-leaved scrub oaks. Sprouting response of interior live oak was strongest from February through April, with 100% of cut interior live oaks sprouting during that time. Sprouting response was least in July (20%) but increased to 50% in September. Sprouts originated from both the base and the sides of interior live oak stumps. The author concluded that interior live oak was relatively insensitive to season of cutting [127]. This study did not explore sprouting response in late fall. Biswell and Gilman [24] observed that interior live oaks top-killed by fire in late fall sprouted the next spring.
Quercus wislizeni, known by the common name interior live oak,[4] is an evergreen oak, highly variable and often shrubby, found in many areas of California[5] in the United States continuing south into northern Baja California in Mexico. It generally occurs in foothills, being most abundant in the lower elevations of the Sierra Nevada, but also widespread in the Pacific Coast Ranges—where since 1980 it has been known as a separate species Quercus parvula[6][7]—and the San Gabriel Mountains. It was named for its collector, Friedrich Adolph Wislizenus (1810–1889).[4]
It is a large shrub or tree[8] growing to 22 metres (72 feet) tall, although where it is common in the low-elevation Sierra Nevada foothills it seldom exceeds 10 m (33 ft). The dark-green leaves—appearing grayish from a distance—are usually small, 2–5 centimetres (1–2 inches) long, thick, and often spiny-toothed at higher elevations, particularly on young trees. The male flowers are on catkins, the female flowers in leaf axils. The acorns are 1–2 cm (1⁄2–1 in) long, and mature the second season (about 18 months) after flowering.[8]
Although originally published by Alphonse Pyramus de Candolle as "wislizeni",[2] some sources, e.g., Jensen in Flora of North America,[8] mistakenly spelled the specific epithet "wislizenii." Correct spelling is with one "i," per ICN article 60C.2.[9] Wislizenus' specimen was thought by de Candolle to have been collected in Chihuahua, Mexico. However, German-born American botanist Georg Engelmann later corrected the location to the American fork of the Sacramento River near Auburn, California.
California physician and botanist (and one of the founding fathers of the California Academy of Sciences) Albert Kellogg described an oak in an 1855 publication as Quercus arcoglandis (spur acorn oak),[10] apparently the same species as Q. wislizeni. This clearly predates French-Swiss botanist de Candolle's 1864 name, and if confirmed to be this same taxon would have priority. More investigation is needed to resolve this taxonomic conflict.
Currently there are two recognized varieties of interior live oak:[11]
The interior live oak is a red oak (section Lobatae) in the California Floristic Province (series Agrifoliae). Q. wislizeni hybridizes with California black oak (Q. kelloggii) (= Quercus × morehus, Abram's oak). All California red oaks show evidence of introgression and/or hybridization with one another.
A common alliant tree is gray pine (Pinus sabiniana).[13]
Deer browse the tree's foliage.[14]
Humans use the wood as a fuel source.[14]
Quercus wislizeni, known by the common name interior live oak, is an evergreen oak, highly variable and often shrubby, found in many areas of California in the United States continuing south into northern Baja California in Mexico. It generally occurs in foothills, being most abundant in the lower elevations of the Sierra Nevada, but also widespread in the Pacific Coast Ranges—where since 1980 it has been known as a separate species Quercus parvula—and the San Gabriel Mountains. It was named for its collector, Friedrich Adolph Wislizenus (1810–1889).
