The term oak can be used as part of the common name of any of about 400 species of trees and shrubs in the genus Quercus (from Latin "oak tree"), which are listed in the List of Quercus species, and some related genera, notably Lithocarpus. The genus is native to the northern hemisphere, and includes deciduous and evergreen species extending from cold latitudes to tropical Asia and the Americas.
Oaks have spirally arranged leaves, with a lobed margin in many species; some have serrated leaves or entire leaves with a smooth margin. The flowers are catkins, produced in spring. The fruit is a nut called an acorn, borne in a cup-like structure known as a cupule; each acorn contains one seed (rarely two or three) and takes 6–18 months to mature, depending on species. The "live oaks" (oaks with evergreen leaves) are not a distinct group, instead with their members scattered among the sections below.
Classification
Oak trees are flowering plants. The genus is divided into two subgenera and a number of sections:
Subgenus Quercus
The Subgenus Quercus is divided into the following Sections:
•Sect. Quercus (synonyms Lepidobalanus and Leucobalanus), the white oaks of Europe, Asia and North America. Styles are short; acorns mature in 6 months and taste sweet or slightly bitter; the inside of acorn shell is hairless. The leaves mostly lack a bristle on their lobe tips, which are usually rounded.
• Sect. Mesobalanus, Hungarian oak and its relatives of Europe and Asia. Styles long; acorns mature in about 6 months and taste bitter; the inside of acorn shell is hairless. The section Mesobalanus is closely related to section Quercus and sometimes included in it.
•Sect. Cerris, the Turkey oak and its relatives of Europe and Asia. Styles long; acorns mature in 18 months and taste very bitter. The inside of the acorn shell is hairless. Its leaves typically have sharp lobe tips, with bristles at the lobe tip.
•Sect. Protobalanus, the Canyon live oak and its relatives, in southwest United States and northwest Mexico. Styles short, acorns mature in 18 months and taste very bitter. The inside of the acorn shell appears woolly. Leaves typically have sharp lobe tips, with bristles at the lobe tip.
•Sect. Lobatae (synonym Erythrobalanus), the red oaks of North America, Central America and northern South America. Styles long, acorns mature in 18 months and taste very bitter. The inside of the acorn shell appears woolly. The actual nut is encased in a thin, clinging, papery skin. Leaves typically have sharp lobe tips, with spiny bristles at the lobe.
Subgenus Cyclobalanopsis
•The ring-cupped oaks of eastern and southeastern Asia. Evergreen trees growing 10–40 m tall. They are distinct from subgenus Quercus in that they have acorns with distinctive cups bearing concrescent rings of scales; they commonly also have densely clustered acorns, though this does not apply to all of the species. The Flora of China treats Cyclobalanopsis as a distinct genus, but most taxonomists consider it a subgenus of Quercus. It contains about 150 species.
Hybridisation
Interspecific hybridisation is quite common among oaks, but usually only between species within the same section and especially in the white oak group (subgenus Quercus, section Quercus; see List of Quercus species); no verified inter-section hybrids are known, except between species of sections Quercus and Mesobalanus, where several occur. Hybridisation is considered fairly common, if not widespread, in the plant world (certainly much more so than in animal taxa). Yet not all plant groups exhibit hybridisation. Botanists have often described white oaks as having weak internal barriers to hybridisation, that is to say, because they are wind pollinated, oaks often do not discriminate against being pollinated by another species in the same section (Quercus), thus resulting in fertile hybrid offspring. Ecological stresses, especially near habitat margins, can also cause a breakdown of mate recognition as well as a reduction of male function (pollen quantity and quality) in one parent species. Recent systematic studies appear to confirm the high tendency of Quercus species to hybridize as a result of a combination of the aforementioned factors.
Frequent hybridisation has brought about a number of consequences to oak populations around the world. Most notably, hybridization has led to the creation of large populations of hybrids, copious amounts of introgression, and even the evolution of new species. Frequent hybridisation and high levels of introgression have caused different species in the same populations to share up to 50% of their genetic information. As a result of such high rates of hybridisation and introgression, genetic data often does not differentiate between two clearly morphologically distinct species, but rather by different populations. In spite of numerous hypotheses, the way in which oak species are able to remain morphologically and ecologically distinct with such high levels of gene flow remains largely a mystery to botanists.
The consequences of frequent hybridisation can also be seen on a higher level. The Fagaceae, the oak family, is known to be a very slowly evolving clade compared to other angiosperms. More than anything, however, hybridisation patterns in Quercus pose a great challenge to the concept of a species. A species is often defined as a group of “actually or potentially interbreeding populations which are reproductively isolated from other such groups”. By this definition, many species of Quercus would be lumped together according to their geographic and ecological habitat, despite clear distinctions in morphology and, to a large extent, genetic data. Thus, although it may be difficult to place a definition on a species within a genus like Quercus, it is trivial and uninformative to apply the biological species concept to all forms of life.
Uses
Oak wood has a density of about 0.75 g/cm³, great strength and hardness, and is very resistant to insect and fungal attack because of its high tannin content. It also has very attractive grain markings, particularly when quarter-sawn. Wide, quarter-sawn boards of oak have been prized since the Middle Ages for use in interior paneling of prestigious buildings such as the debating chamber of the British House of Commons in London, England, and in the construction of fine furniture. Oak wood, from Quercus robur and Quercus petraea, was used in Europe for the construction of ships, especially naval men of war, until the 19th century, and was the principal timber used in the construction of European timber-framed buildings. Today oakwood is still commonly used for furniture making and flooring, timber frame buildings, and for veneer production. Barrels in which red wines, sherry, brandy and spirits such as Scotch whisky and Bourbon whiskey are aged are made from European and American oak. The use of oak in wine can add many different dimensions to wine based on the type and style of the oak. Oak barrels, which may be charred before use, contribute to the colour, taste, and aroma, of the contents, imparting a desirable oaky vanillin flavour to these drinks. The great dilemma for wine producers is to choose between French and American oakwoods. French oaks (Quercus robur, Q. petraea) give the wine greater refinement and are chosen for best wines since they increase the price compared to those aged in American oak wood. American oak contributes greater texture and resistance to ageing, but produces more violent wine bouquets.Oak wood chips are used for smoking fish, meat, cheeses and other foods.
Sherry maturing in oak barrels
The bark of Quercus suber, or Cork oak, is used to produce wine stoppers (corks). This species grows in the Mediterranean Sea region, with Portugal, Spain, Algeria and Morocco producing most of the world's supply. Of the North American oaks, the Northern red oak Quercus rubra is the most prized of the red oak group for lumber, all of which is marketed as red oak regardless of the species of origin. The standard for the lumber of the white oak group, all of which is marketed as white oak, is the White Oak Quercus alba. White Oak is often used to make wine barrels. The wood of the deciduous Pedunculate Oak Quercus robur and Sessile Oak Quercus petraea account for most of the European oak production, but evergreen species, such as Holm oak Quercus ilex, and Cork oak Quercus suber also produce valuable timber.
The bark of the White Oak is dried and used in medical preparations. Oak bark is also rich in tannin, and is used by tanners for tanning leather. Acorns are used for making flour or roasted for acorn coffee. Oak galls were used for centuries as the main ingredient in manuscript ink, harvested at a specific time of year.
Japanese oak is used in the making of professional drums from manufacturer Yamaha Drums. The rough, hard surface of oak gives the drum a brighter and louder tone compared to traditional drum materials such as maple and birch.