Anoak is ahardwood tree or shrub in thegenusQuercus of thebeech family. They have spirally arranged leaves, often with lobed edges, and a nut called anacorn, borne within a cup. The genus is widely distributed in theNorthern Hemisphere; it includes some 500 species, bothdeciduous andevergreen. Fossil oaks date back to the MiddleEocene.Molecular phylogeny shows that the genus is divided intoOld World and New Worldclades, but many oak specieshybridise freely, making the genus's history difficult to resolve.
Ecologically, oaks arekeystone species in habitats from Mediterranean semi-desert to subtropicalrainforest. They live in association with many kinds offungi includingtruffles. Oaks support more than 950 species ofcaterpillar, many kinds ofgall wasp which form distinctivegalls, roundish woody lumps such as theoak apple, and a large number of pests and diseases. Oak leaves and acorns contain enoughtannin to be toxic to cattle, but pigs are able to digest them safely. Oak timber is strong and hard, and has found many uses in construction and furniture-making. The bark was traditionally used fortanning leather.Wine barrels are made of oak; these are used for aging alcoholic beverages such assherry andwhisky, giving them a range of flavours, colours, and aromas. The spongy bark of thecork oak is used to make traditional wine bottle corks. Almost a third of oak species are threatened with extinction due to climate change,invasive pests, andhabitat loss.
In culture, the oak tree is a symbol of strength and serves as thenational tree of many countries. In Indo-European and related religions, the oak is associated withthunder gods. Individual oak trees of cultural significance include theRoyal Oak in Britain, theCharter Oak in the United States, and theGuernica Oak in theBasque Country.
Thegeneric nameQuercus isLatin for "oak", derived fromProto-Indo-European*kwerkwu-, "oak", which is also the origin of the name "fir", another important or sacred tree in Indo-European culture. The word "cork", for the bark of thecork oak, similarly derives fromQuercus.[1] The common name "oak" is fromOld Englishac (seen in placenames such asActon, fromac +tun, "oak village"),[2] which in turn is fromProto-Germanic*aiks, "oak".[3]
Oaks arehardwood (dicotyledonous) trees,deciduous orevergreen, with spirally arranged leaves, often withlobate margins; some haveserrated leaves orentire leaves with smooth margins. Many deciduous species aremarcescent, not dropping dead leaves until spring. In spring, a single oak treeproduces both male and female flowers. Thestaminate (male) flowers are arranged incatkins, while the smallpistillate (female) flowers[4] produce anacorn (a kind ofnut) contained in acupule. Each acorn usually contains one seed and takes 6–18 months to mature, depending on the species. The acorns and leaves containtannic acid,[5] which helps to guard against fungi and insects.[6] There are some 500 extant species of oaks.[7]
Trees in the genus are often large and slow-growing;Q. alba can reach an age of 600 years, a diameter of 13 feet (4.0 m) and a height of 145 feet (44 m).[8] TheGranit oak in Bulgaria, aQ. robur specimen, has an estimated age of 1,637 years, making it the oldest oak in Europe.[9] The Wi'aaSal tree, alive oak in the reservation of thePechanga Band of Indians, California, is at least 1,000 years old, and might be as much as 2,000 years old, which would make it the oldest oak in the US.[10] Among the smallest oaks isQ. acuta, the Japanese evergreen oak. It forms a bush or small tree to a height of some 30 feet (9.1 m).[11]
Global distribution ofQuercus. The New and Old World parts are mostly separateclades.[a] Red: North American.[b] Pink: Central American.[c] Yellow: European.[d] Green: West/Central Asian.[e] Turquoise: Southeast Asian.[f] Blue: East Asian.[g][12] See Phylogeny chapter/tree for sections.[clarification needed]
The genusQuercus is native to the Northern Hemisphere and includesdeciduous andevergreen species extending fromcool temperate totropical latitudes in theAmericas, Asia, Europe, and North Africa. North America has the largest number of oak species, with approximately 160 species in Mexico, of which 109 are endemic, and about 90 in the United States. The second greatest area of oak diversity is China, with approximately 100 species.[13]
In the Americas,Quercus is widespread fromVancouver andNova Scotia in the south of Canada, south toMexico and across the whole of the eastern United States. It is present in a small area of the west ofCuba; inMesoamerica it occurs mainly above 1,000 metres (3,300 ft).[14] The genus crossed theisthmus of Panama when the northern and southern continents came together[15] and is present as one species,Q. humboldtii, above 1,000 metres in Colombia.[14] The oaks of North America are of many sections (Protobalanus,Lobatae,Ponticae,Quercus, andVirentes) along with related genera such asNotholithocarpus.[12]
In the Old World, oaks of sectionQuercus extend across the whole of Europe including European Russia apart from the far north, and north Africa (north of the Sahara) from Morocco to Libya. In Mediterranean Europe, they are joined by oaks of the sectionsCerris andIlex, which extend across Turkey, the Middle East, Iran, Afghanistan and Pakistan, while sectionPonticae is endemic to the western Caucasus inTurkey andGeorgia. Oaks of sectionCyclobalanopsis extend in a narrow belt along the Himalayas to cover mainland and island Southeast Asia as far asSumatra,Java,Borneo, andPalawan.[7][16] Finally, oaks of multiple sections (Cyclobalanopsis,Ilex,Cerris,Quercus and related genera likeLithocarpus andCastanopsis) extend across east Asia including China, Korea, and Japan.[12]
Potential records ofQuercus have been reported fromLate Cretaceous deposits in North America and East Asia. These are not considered definitive, as macrofossils older than thePaleogene, and possibly from before theEocene are mostly poorly preserved without critical features for certain identification. Amongst the oldest unequivocal records ofQuercus are pollen from Austria, dating to thePaleocene-Eocene boundary, around 55 million years ago. The oldest records ofQuercus in North America are fromOregon, dating to the Middle Eocene, around 44 million years ago, with the oldest records in Asia from the Middle Eocene of Japan; both forms have affinities to theCyclobalanopsis group.[17]
Quercus forms part, or rather two parts, of the Quercoideae subfamily of theFagaceae, the beech family. Modernmolecular phylogenetics suggests the following relationships:[18][19]
Molecular techniques for phylogenetic analysis show that the genusQuercus consisted ofOld World andNew World clades.[20][21][22] The entiregenome ofQuercus robur (the pedunculate oak) has beensequenced,[23] revealing an array ofmutations that may underlie the evolution of longevity anddisease resistance in oaks.[24] In addition, hundreds of oak species have been compared (atRAD-seq loci), allowing a detailed phylogeny to be constructed. However, the high signal ofintrogressive hybridization (the transfer of genetic material by repeated backcrossing with hybrid offspring) in the genus has made it difficult to resolve an unambiguous, unitary history of oaks. The phylogeny from Hipp et al. 2019 is:[25]
SectionVirentesLoudon – American southern live oaks
SectionQuercus – white oaks from North America and Eurasia
SubgenusCerrisOerst. – the Old World clade (or mid-latitude clade), exclusively native to Eurasia
SectionCyclobalanopsisOerst. – cycle-cup oaks of East Asia
SectionCerrisDumort. – cerris oaks of subtropical and temperate Eurasia and North Africa
SectionIlexLoudon – ilex oaks of tropical and subtropical Eurasia and North Africa
The subgenus division supports the evolutionary diversification of oaks among two distinct clades: the Old World clade (subgenusCerris), including oaks that diversified in Eurasia; and the New World clade (subgenusQuercus), oaks that diversified mainly in the Americas.[20][30]
Sect.Lobatae (synonymErythrobalanus), thered oaks of North America,Central America and northernSouth America.Styles are long; the 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. The leaves typically have sharp lobe tips, with spiny bristles at the lobe.[29]
Sect.Protobalanus, thecanyon live oak and its relatives, in the southwesternUnited States and northwestMexico. Styles are short; the acorns mature in 18 months and taste very bitter. The inside of the acorn shell appears woolly. The leaves typically have sharp lobe tips, with bristles at the lobe tip.[29]
Sect.Ponticae, a disjunct including just two species. Styles are short, and the acorns mature in 12 months. The leaves have largestipules, high secondary veins, and are highly toothed.[29]
Sect.Virentes, the southern live oaks of the Americas. Styles are short, and the acorns mature in 12 months. The leaves are evergreen or subevergreen.[29]
Sect.Quercus (synonymsLepidobalanus andLeucobalanus), thewhite oaks ofEurope,Asia and North America. Trees or shrubs that produce nuts, specifically acorns, as fruits. Acorns mature in one year for annual trees and two years for biannual trees. Acorn is encapsulated by a spiny cupule as characterized by the family Fagaceae. Flowers in theQuercus genera produce one flower per node, with three or six styles, as well as three or sixovaries, respectively. The leaves mostly lack a bristle on their lobe tips, which are usually rounded. The type species isQuercus robur.[29][31]
Sect.Cyclobalanopsis, thering-cupped oaks of eastern and southeastern Asia. These are evergreen trees growing 10–40 metres (33–131 feet) tall. They are distinct from subgenusQuercus 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. Species ofCyclobalanopsis are common in the evergreen subtropicallaurel forests, which extend from southern Japan, southern Korea, and Taiwan across southern China and northern Indochina to the eastern Himalayas, in association with trees of the genusCastanopsis and the laurel family (Lauraceae).[29]
Sect.Cerris, theTurkey oak and its relatives of Europe and Asia. Styles are long; acorns mature in 18 months and taste very bitter. The inside of the acorn's shell is hairless. Its leaves typically have sharp lobe tips, with bristles at the lobe tip.[29]
Sect.Ilex, the Ilex oak and its relatives of Eurasia and northernAfrica. Styles are medium-long; acorns mature in 12–24 months, appearing hairy on the inside. The leaves are evergreen, with bristle-like extensions on the teeth.[29]
Caterpillars of the North American oak leafroller,Archips semiferanus, can defoliate oak forests.
Oaks arekeystone species in a wide range of habitats from Mediterranean semi-desert to subtropical rainforest. They are important components of hardwood forests; some species grow in associations with members of theEricaceae inoak–heath forests.[32][33] Several kinds oftruffles, including two well-known varieties – blackPérigord truffle[34] and the white Piedmont truffle[35] – have symbiotic relationships with oak trees. Similarly, many other fungi, such asRamaria flavosaponaria, associate with oaks.[36][37]
Oaks support more than 950 species of caterpillars, an important food source for many birds.[38] Mature oak trees shed widely varying numbers of acorns (known collectively as mast) annually, with large quantities inmast years. This may be apredator satiation strategy, increasing the chance that some acorns will survive to germination.[39]
Animals includingsquirrels[40] and jays –Eurasian jays in the Old World,blue jays in North America – feed on acorns, and are important agents ofseed dispersal as they carry the acorns away and bury many of them as food stores.[41][42][43] However, some species of squirrel selectively excise the embryos from the acorns that they store, meaning that the food store lasts longer and that the acorns will never germinate.[40]
Interspecific hybridization is quite common among oaks, but usually between species within the same section only,[44] and most common in the white oak group. White oaks cannot discriminate against pollination by other species in the same section. Because they arewind pollinated and have weak internal barriers to hybridization, hybridization produces functional seeds and 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.[45][46]
Frequent hybridization among oaks has consequences for oak populations around the world; most notably, hybridization has produced large populations of hybrids with muchintrogression and theevolution of new species.[47] Introgression has caused different species in the same populations to share up to 50% of their genetic information.[48] As a result, genetic data often does not differentiate between clearly morphologically distinct species, but instead differentiates populations.[49] The maintenance of particular loci for adaptation to ecological niches may explain the retention of species identity despite significant gene flow.[50]
TheFagaceae, or beech family, to which the oaks belong, is a slowly-evolvingclade compared to otherangiosperms,[51][52] and the patterns of hybridization and introgression inQuercus pose a significant challenge to theconcept of a species as a group of "actually or potentially interbreeding populations which are reproductively isolated from other such groups."[53] By this definition, many species ofQuercus would be lumped together according to their geographic and ecological habitat, despite clear distinctions in morphology and genetic data.[53]
Oaks are affected by a large number of pests and diseases. For instance,Q. robur andQ. petraea in Britain host 423 insect species.[54] This diversity includes 106macro-moths, 83micro-moths, 67beetles, 53cynipoidean wasps, 38heteropteran bugs, 21auchenorrhynchan bugs, 17sawflies, and 15aphids.[54] The insect numbers are seasonal: in spring, chewing insects such as caterpillars become numerous, followed by insects with sucking mouthparts such as aphids, then byleaf miners, and finally by gall wasps such asNeuroterus.[55] Severalpowdery mildews affect oak species. In Europe, the speciesErysiphe alphitoides is the most common.[56] It reduces the ability of leaves to photosynthesize, and infected leaves are shed early.[57]Another significant threat, theoak processionary moth (Thaumetopoea processionea), has emerged in the UK since 2006. The caterpillars of this species defoliate the trees and are hazardous to human health; their bodies are covered with poisonous hairs which can cause rashes and respiratory problems.[58] A little-understood disease of mature oaks,acute oak decline, has affected the UK since 2009.[59] In California, goldspotted oak borer (Agrilus auroguttatus) has destroyed many oak trees,[60] while sudden oak death, caused by theoomycete pathogenPhytophthora ramorum, has devastated oaks in California and Oregon, and is present in Europe.[61] Japanese oak wilt, caused by the fungusRaffaelea quercivora, has rapidly killed trees across Japan.[62]
The leaves and acorns of oaks are poisonous tolivestock, includingcattle andhorses, if eaten in large amounts, due to the toxintannic acid, which causes kidney damage andgastroenteritis.[64][65] An exception is thedomestic pig, which, under the right conditions, may be fed entirely on acorns,[66] and has traditionally been pastured in oak woodlands (such as the Spanishdehesa[67] and the English system ofpannage).[68] Humans can eat acorns afterleaching out the tannins in water.[69]
Oak timber is a strong and hard wood with many uses, such as forfurniture, floors, building frames, andveneers.[70]The wood of a red oakQuercus cerris (the Turkey oak) has better mechanical properties than those of the white oaksQ. petraea andQ. robur; the heartwood and sapwood have similarmechanical properties.[71] Of the North American red oaks, thenorthern red oak,Quercus rubra, is highly prized for lumber.[72][73]The wood is resistant to insect and fungal attack.[74]
Wood fromQ. robur andQ. petraea was used in Europe forshipbuilding, especially of navalmen of war, until the 19th century.[75] In hill states of India such asUttarakhand, along with being used for fuelwood and timber, oak wood is used foragricultural implements, while the leaves serve asfodder for livestock during lean periods.[76][77]
Barrels foraging wines,sherry, and spirits such asbrandy andScotch whisky are made from oak, withsingle barrel malt whiskies fetching a premium.[84] The use ofoak in wine adds a range of flavours. Oak barrels, which may be charred before use, contribute to their contents' colour, taste, and aroma, imparting a desirable oakyvanillin flavour. A dilemma for wine producers is to choose between French and American oakwoods. French oaks (Quercus robur,Q. petraea) give greater refinement and are chosen for the best, most expensive wines. American oak contributes greater texture and resistance to ageing, but produces a more powerful bouquet.[85][86]
Acorns of various oak species have been used as food for millennia, in Asia, Europe, the Middle East, North Africa, and among the native peoples of North America. In North Africa, acorns have been pressed to make acorn oil: theoil content can be as high as 30%. Oaks have also been used asfodder, both leaves and acorns being fed to livestock such as pigs. Given their high tannin content, acorns have often been leached to remove tannins before use as fodder.[91]
An estimated 31% of the world's oak species are threatened with extinction, while 41% of oak species are considered to be ofconservation concern. The countries with the highest numbers of threatened oak species (as of 2020) are China with 36 species, Mexico with 32 species, Vietnam with 20 species, and the US with 16 species. Leading causes areclimate change andinvasive pests in the US, anddeforestation andurbanization in Asia.[92][93][94] In the Himalayan region of India, oak forests are being invaded bypine trees due to global warming. The associated pine forest species may cross frontiers and integrate into the oak forests.[95] Over the past 200 years, large areas of oak forest in the highlands of Mexico, Central America, and the northern Andes have been cleared forcoffee plantations andcattle ranching. There is a continuing threat to these forests from exploitation for timber, fuelwood, andcharcoal.[96] In the US, entire oak ecosystems have declined due to a combination of factors thought to include fire suppression, increased consumption of acorns by growing mammal populations, herbivory of seedlings, and introduced pests.[97] However, disturbance-tolerant oaks may have benefited from grazers likebison, and suffered when the bison were removed followingEuropean colonization.[98][99]
Grīdnieku ancient oak in Rumbas parish,Latvia, girth 8.27 metres (27.1 ft) 2015
The prehistoricIndo-European tribes worshiped the oak and connected it with athunder god, and this tradition descended to many classical cultures. InGreek mythology, the oak is the tree sacred toZeus, king of the gods. In Zeus'soracle inDodona,Epirus, the sacred oak was the centerpiece of the precinct, and the priests woulddivine the pronouncements of the god by interpreting the rustling of the oak's leaves.[110] Mortals who destroyed such trees were said to be punished by the gods since the ancient Greeks believed beings calledhamadryads inhabited them.[111] InNorse andBaltic mythology, the oak was sacred to the thunder godsThor andPerkūnas respectively.[112][113] InCeltic polytheism, the namedruid, Celtic priest, is connected to Proto-Indo-European*deru, meaning oak or tree.[114] Veneration of the oak survives inSerbian Orthodox Church tradition.Christmas celebrations include thebadnjak, a branch taken from a young and straight oak ceremonially felled early on Christmas Eve morning, similar to ayule log.[115]
In theRoman Republic, a crown of oak leaves was given to those who had saved the life of a citizen in battle; it was called the "Civic Crown".[114] In his 17th century poemThe Garden,Andrew Marvell critiqued the desire to be awarded such a leafy crown: "How vainly men themselves amaze / To win thepalm, the oak, orbays; And their uncessant labors see / Crowned from some single herb or tree, ..."[121][122]
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