Oaks are trees and shrubs in the family Fagaceae, which family also includes beeches, chestnuts, and chinquapins. The genus of oaks, with the scientific name Quercus, is broadly distributed in the Northern Hemisphere, and has a pronounced diversity and endemism in North America, particularly in the country of Mexico. Oaks are wind pollinated, and may occur as evergreen, winter deciduous, or drought deciduous morphologies. Many oaks occur as top level canopy species, but an equally large number are shrubs or sub-canopy level associates.
There is a strong association between human civilizations and oaks, beginning at least 30,000 years before present (b.p.) in Europe and Asia, and 14,000 years b.p. in North America. Oaks have been a significant source of building materials as well as foodstocks for humans since prehistory, and remain a significant resource of many modern economies.
Oaks are usually divided into two subgenera: Quercus and Cyclobalanopsis; however, some taxonomies consider these two divisions are distinct genera. Taxa within Cyclobalnopsis occur in China and Southeast Asia and consist of about 150 different species.
Subgenus Quercus is subdivided into the following sections:
- Cerris (Turkey oaks) occur in Eurasia. Styles long; acorns taste very bitter. Interior of the acorn shell is hairless. Leaves typically have sharp lobe tips, with bristles at the lobe tips.
- Lobatae (red oaks) occur in North and Central America as well as northern South America. Most species have biennial acorn maturation. Styles long, acorns mature in 18 months and taste very bitter. The inside of the acorn shell is woolly. The nut is encased in a thin, clinging, papery skin. Leaves typically have sharp lobe tips, with spiny bristles at the lobe.
- Mesobalanus (Hungarian oaks) occur in Eurasia. Acorns have a bitter taste and their shell interiors are without hairs. Styles are long.
- Quercus (white oaks) occur in Eurasia and North America. Leaf lobes are typically rounded and generally lack bristles on lobe tips. Interior of acorn shells are without hairs, and acorns have a notable taste, which may be bitter or sweet.
- Protobalanus (Canyon live oaks) occur in the southwest USA and northwest Mexico. Acorn maturation is biennial. Styles short, acorns and taste very bitter.The inside of the acorn shell appears woolly. Leaves typically have sharp lobe tips, with bristles at the lobe tip.
Biogeography and Lineage
Fagaceae are woody trees and shrubs whose nut is enclosed in a shell-like casing. The Fagaceae family originated in Asia, first appearing in the fossil record during the Early Cretaceous, more than 100 million years ago. Subsequent radiation toward Europe and North America produced geographic dispersion as well as divergence of genera. The chromosomal composition across the entire family is remarkably similar (2n=24), but there are isolated triploid Quercus species. The extinct genus Dryophyllum, one of the earliest known Fagaceae, is believed to be the ancestor of modern oaks.
Based on molecular genetics analysis, the genus Quercus is estimated to have separated from Castanea about 60 million years ago. Oaks first appear in the fossil record in North America during the Paleogene between 55 to 50 million years b.p. Most interspecific separations occurred within the Quercus species between 22 and three million years ago. During this period, oaks became the most dominant tree type in the Fagaceae family.
Depending on the classification scheme, there are somewhere between 450 and 600 oak species; one of the chief points of confusion is the taxonomic status of many hybrid oaks. The geographical center of oak diversity is clearly in the Northern Hemisphere, particularly North America. At least 220 species occur on this continent, chiefly in Mexico and the USA. Mexico alone has 160 different oak species, 90 of which are endemic. Only one oak species occurs in South America, Quercus humboldtii, which is found in Columbia. After the Isthmus of Panama connected North and South America several million years ago, oaks propagated to the Andes, dating as early as 186,000 years b.p., based upon pollen core records.
China has 104 different oak species, 35 of which are of the sub-genus quercus, which is in turn divided into several sections as discussed above under taxonomy
The genus of oaks is one of the most significant cladistic groups of woody angiosperms in the Northern Hemisphere, based upon diversity as well as ecological dominance. Species of oaks are prominent members of many plant communities including: oak woodlands, temperate deciduous forests, matorral, pine-oak forests, chaparral-oak associations, tropical and subtropical savannas, tropical montane and premontane forests.
In addition oaks are often present at the fringes of entirely distinct plant communities such as: deserts, prairies, tropical grasslands, tropical dry forests and semi-desert scrublands.
Ancient Quercus robur pygmy forest caused by highly mineralized soil
Mull, Scotland. @ C,Michael Hogan
In mesic environments oaks are frequently canopy dominants, but in dry, rocky, high elevation or edaphically extreme settings, oaks are often shrub forms or even dwarfed. Edaphic extremes may include ultramafic soils, sandy barrens or inundated lands. The greatest concentrations of oaks are betweeen subtropical and middle-temperate climate regimes. Further north than this, conifer species typically become dominant; further south, oaks cannot successfully compete with taller trees of tropical rainforests with respect to sunlight, and in some cases due to intolerance to high rainfall combined with high temperatures.
Some oak species are restricted to highly specific soil types. For example the endangered Georgia oak is found only in a small area of the southeastern USA that consists of granitic outcrops. The Leather oak is strongly associated with California serpentine soils.
There are a number of occurrences of oaks that are notable due to cultural, historic, edaphic or conservation issues.
California Floristic Province
Leather oak acorn and leaves. Source: Mark S.Brunell The California Floristic Province is dominated throughout much of its forested extent by oak species. These include: Coast live oak (Quercus agrifolia), Blue oak (Quercus douglasii), Valley Oak (Quercus lobata), Oregon Oak (Quercus garryana), Black oak (Quercus kelloggii), Interior live oak (Quercus wislizenii), Leather oak (Quercus durata) and the Vulnerable classified Engelmann Oak (Quercus Engelmannii) and MacDonald oak (Quercus macdonaldii). The latter is restricted to occurrences on Santa Rosa, Santa Catalina and Santa Cruz Islands . The oak woodlands, mixed oak-chaparral and mixed oak-conifer forests provide much higher biodiversity than simple conifer forests within this Province.
Loss and fragmentation of California's oak woodlands began in the late 18th century with the development of livestock grazing operations, which directly and indirectly affect oaks through trampling and soil compaction, and the cutting of oaks for firewood and for use as a raw material for manufacturing. In the mid-20th century, policies of the U.S. Government and State of California compounded these losses by providing livestock subsidies that encouraged grazing. Only in the past several decades have private non-profit land trusts begun to reverse the trend of oak woodland loss by advancing protection of these habitats.
Madrean Pine-oak Forests Ecoregion
This ecoregion is a center of biodiversity of Quercus, with occurrence of 135 distinct oak speces. The ecoregion chiefly spans the higher elevations of Mexico, but also includes numerous mountain ranges in southern Arizona and New Mexico. Principal high elevation occurrences of oaks in Mexico are the pine-oak woodlands in the Sierra Madre Occidental, Sierra Madre Oriental, Sierra Madre del Sur, Sierra Madre de Oaxaca,Trans-Mexican volcanic belt, and the Baja Peninsular Ranges of Baja California. In the extreme south of Arizona and New Mexico, these pine-oak habitats are found on over two dozen isolated peaks known as the Madrean Sky Islands.
Important oaks in this region are the Mexican oak (Quercus carmenensis) and Q. deliquescens. The latter is classified as a Vulnerable species occurring only from one population in the valley of Río Concho and north along the Río Grande within the state of Chihuahua in Mexico.
Ozark Mountains, Arkansas
A number of oak species are found in this portion of the Arkansas River Basin, some of which range south onto the coastal plain, and many spill over into the Missouri portion of the Ozarks. The oaks are usually found in pine-oak forest plant communities. Chief oak species found here are Quercus stellata, Q. arkansana, Q. enchinata, Q. alba, Q. falcata, Q. velutina, Q. rubra, Q. marilandica and Q. muehlenbergii. Locations such as deep gulches and ravines of the Boston Mountains are important relict habitat for oaks and other native species; such areas serve as refugia for biota whose natural environment has been compromised. Other specialized environments for oaks of the Ozarks include limestone soil regimes. The central part of the Ozarks existed as an island amidst Paleozoic seas, and their height was amplified by subsequent uplift induced by collision of the South American Plate.
The Ozarks are a key transition between the western-most reach of oak-hickory forest and the Cherokee Prairie dominated by Big bluestem and Indiangrass with associated flora of Bluestem, Tall dropseed as well as composite forbs. This community is termed Oak-hickory-tallgrass savanna, and includes Quercus marylandica, Q. velutina and Q. rubra in addition to Texas hickory (Carya texana) and Mockernut hickory (C. tomentosa). There is also an understory of dogwood (Cornus florida) and redbud (Cercis canadensis).
There are several plant communities within the Ozarks that support oaks. One example is the Ozark Chert Glade, a type of barren in the Ozark Plateau are zones of rock outcrop or shallow, rocky soil. These communities are typically between the climax of the western extension of the Eastern Deciduous Forest and the Tallgrass Prairie Region. The chert glade associations usually occur on a southern or western sunlight exposure. Such chert glades are often dominated by scrub forms of Quercus marilandica and Q. stellata.
Mediterranean oak woodlands
The Mediterranean Basin presents challenges for all large broadleaf woody plants, given the modest rainfall and seasonal drought, which makes water least available when it is most needed during the warmest months. Oaks and other woody broadleaf species have developed adaptations in this region that include: (a) setting roots deeply; (b) spacing trees in an open canopy fashion to ensure each oak does note compete unnecessarily with others for water; (c) deciduous adaptations for some species; and (d) ability to regulate stomatal openings to minimize water loss.
These open canopy forests favor a well-developed understory of woody shrubs or grasses. Such grasslands or rangelands have provided an attractive opportunity for livestock grazing, an activity which has greatly diminished the standing oak timber by reducing seedling recruitment. Policies begun in the Middle Ages favored extensive farming and grazing operations, threatening oak woodlands; even recently many policies of the European Union subsidzed marginal livestock uses, enlarging the ecological damage. Resulting overgrazing has led to widespread habitat loss and habitat fragmentation.
The Cork oak (Quercus suber) of Portugal, economically quite valuable for cork production, is a notable oak of the region. Boz pirmal oak (Quercus aucheri) is a near-threatened species found on some of the Greek islands and in Anatolia in Turkey.
Quercus aliena, native to China, Japan and Korea. Source: Ken Pei
Remarkably, even though the genus Quercus originated in East Asia, there are no more than 35 oak species endemic to China. There is a paucity of information on the conservation status of most Chinese endemic oaks. Examples of Chinese endemics include: Xiao ye li (Quercus chenii), found only in mixed mesophytic forests below 600 meters elevation in a handful of Chinese provinces; Bai li (Quercus fabri), found in mixed mesophytic forests between 100 to 1900 meters in fourteen Chinese provinces; Mao dou li (Quercus guyavifolia), occurring chiefly within montane forests or subalpine scrub at altitudes from 2500 to 4000 meters in Guizhou, Sichuan and Yunnan Provinces; Yan li (Quercus acrodonta), in valleys and mountains from 300 to 2300 meters in the provinces of Gansu, Guizhou, Henan, Hubei, Hunan, Shaanxi, Sichuan and Yunnan; and Tan li (Quercus utilis), found within open or dense woods on rocky hills between 1000 to 1500 meters in southwest Guangxi, south Guizhou and southeast Yunnan provinces. Outside of mainland China, there is the narrowly restricted endemic Tai lu ge li (Quercus tarokoensis), found in eastern Taiwan. This species occurs only on steep limestone slopes and ridges.
Oaks in China are under pressure due to harvesting for timber, charcoal, dyes and medicinal substances. Oak species in China are also noted for their importance in stabilizing steep slopes due to the strength and extent of their root structures.
The following oaks are classified as endangered species or critically endangered species by the International Union cor Conservation of Nature; the following list does not include all of the oak taxa known to be endangered, nor does it address hybrids or species classified as vulnerable or threatened:
- Quercus acerifolia (Maple-leaved oak), endemic to Arkansas Ozark Mountains
- Quercus argentata (Palan ugu), restricted to Peninsular Malaysia, Borneo, Java, Sumatra
- Quercus boyntonii (Boynton oak), endemic to southern USA, Chiefly Alabama
- Quercus brandegeei (Brandegee oak), endemic to Baja California in the Sierra Lazaro foothills
- Quercus crenata (Lucombe oak), Eastern Europe
- Quercus dumosa (Coastal sage scrub oak), endemic to California and Baja California
- Quercus georgiana (Georgia oak), southeastern USA, e.g. Georgia, Alabama, South Carolina
- Quercus graciliformis (Slender oak), Critically endangered, only in Chisos Mountains, Texas
- Quercus hinckleyi (Hinckley's oak), Critically endangered, Texas endemic (mainly Big Bend)
- Quercus hintonii, (Encino of Hinton), Critically endangered, endemic to the state of Mexico
- Quercus miquihuanensis, endemic to Peña Nevada and Miquihuana, Mexico
- Quercus miyagii, endemic to the Ryuku Islands of Japan
- Quercus Oglethorpensis, (Oglethorpe's oak), endemic to Piedmont area of Georgia, neighboring S. Carolina, disjunctive population in Mississippi
- Quercus vulcanica, (Kasnak Oak), endemic to Turkey
Quercus lusitanica, native to Morocco, Portugal and Spain.
F.E.Koehler, 1897 Quercus species are either evergreen or winter-deciduous trees or shrubs (Blue oaks are sometimes summer deciduous as a way of coping with drought). Leaf blades present a leather-like or thin appearance, with margins entire, toothed or awn-toothed. Secondary veins are either unbranched or generally parallel, most of the time extending to margins. Leaf stipules are deciduous and inconspicuous. Terminal buds exhibit as spheric to ovoid, terete or angled, with all of the scales imbricate.
Inflorescences are unisexual, in the axils of leaves or bud scales, but normally clustered at base of new growth. Staminate inflorescences are lax, although pistillate inflorescences are typically rigid, with a terminal cupule and occasionally one or more sessile lateral cupules. Staminate flowers have connate sepals, with a circumferential tuft of silken hairs. Pistillate flowers manifest as one per cupule, with connate sepals. Fruits may mature in the first or second year. Acorn cups come in a variety of shapes (cup, saucer, bowl or goblet geometry), with no manifestation of valves; this cup (or cap) encases the nut base (only rarely the entire acorn); acorn caps are composed of scales that are imbricate or reduced to tubercles, but never hook-shaped.
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- IUCN. 2010. Red List of Endangered Species.
- Thomas Foti et al. A Classification System for the Natural Vegetation of Arkansas. Arkansas Natural Heritage Commission