Oak

Fossil history

Potential records of Quercus have been reported from Late Cretaceous deposits in North America and East Asia. These are not considered definitive, as macrofossils older than the Paleogene, and possibly from before the Eocene are mostly poorly preserved without critical features for certain identification. Amongst the oldest unequivocal records of Quercus are pollen from Austria, dating to the Paleocene-Eocene boundary, around 55 million years ago. The oldest records of Quercus in North America are from Oregon, 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 the Cyclobalanopsis group.^[17]

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  Quercus × hispanica leaf. Miocene, Lleida, Spain
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  Quercus hiholensis acorn, Langhian age (Middle Miocene), Washington State, USA (c. 15 mya)
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  Quercus kobatakei leaf. Early Oligocene, Japan
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  Early Oligocene acorn, Oregon, USA (33 mya)

External phylogeny

Quercus forms part, or rather two parts, of the Quercoideae subfamily of the Fagaceae, the beech family. Modern molecular phylogenetics suggests the following relationships:^[18][19]

Internal phylogeny

Molecular techniques for phylogenetic analysis show that the genus Quercus consisted of Old World and New World clades.^[20][21][22] The entire genome of Quercus robur (the pedunculate oak) has been sequenced,^[23] revealing an array of mutations that may underlie the evolution of longevity and disease resistance in oaks.^[24] In addition, hundreds of oak species have been compared (at RAD-seq loci), allowing a detailed phylogeny to be constructed. However, the high signal of introgressive 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]

Taxonomy

Hybridisation

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 are wind 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 much introgression and the evolution 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]

The Fagaceae, or beech family, to which the oaks belong, is a slowly-evolving clade compared to other angiosperms,^[51][52] and the patterns of hybridization and introgression in Quercus pose a significant challenge to the concept 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 of Quercus would be lumped together according to their geographic and ecological habitat, despite clear distinctions in morphology and genetic data.^[53]

Diseases and pests

Oaks are affected by a large number of pests and diseases. For instance, Q. robur and Q. petraea in Britain host 423 insect species.^[54] This diversity includes 106 macro-moths, 83 micro-moths, 67 beetles, 53 cynipoidean wasps, 38 heteropteran bugs, 21 auchenorrhynchan bugs, 17 sawflies, and 15 aphids.^[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 by leaf miners, and finally by gall wasps such as Neuroterus.^[55] Several powdery mildews affect oak species. In Europe, the species Erysiphe alphitoides is the most common.^[56] It reduces the ability of leaves to photosynthesize, and infected leaves are shed early.^[57] Another significant threat, the oak 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 the oomycete pathogen Phytophthora ramorum, has devastated oaks in California and Oregon, and is present in Europe.^[61] Japanese oak wilt, caused by the fungus Raffaelea quercivora, has rapidly killed trees across Japan.^[62]

Gall communities

Many galls are found on oak leaves, buds, flowers, and roots. Examples are oak artichoke gall, oak marble gall, oak apple gall, knopper gall, and spangle gall. These galls are the handiwork of tiny wasps from the Cynipidae. In a complex ecological relationship, these gall wasps become hosts to parasitoid wasps—primarily from the order Chalcidoidea—which lay their larvae inside the gall wasps, ultimately leading to the hosts' demise. Additionally, inquilines live commensally within the galls without harming the gall wasps.^[63]

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  Oak apple gall on Quercus garryana
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  Oak apples on oak leaf
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  Biorhiza pallida male, the cause of oak apple galls

Timber

Oak timber is a strong and hard wood with many uses, such as for furniture, floors, building frames, and veneers.^[70] The wood of a red oak Quercus cerris (the Turkey oak) has better mechanical properties than those of the white oaks Q. petraea and Q. robur; the heartwood and sapwood have similar mechanical properties.^[71] Of the North American red oaks, the northern red oak, Quercus rubra, is highly prized for lumber.^[72][73] The wood is resistant to insect and fungal attack.^[74]

Wood from Q. robur and Q. petraea was used in Europe for shipbuilding, especially of naval men of war, until the 19th century.^[75] In hill states of India such as Uttarakhand, along with being used for fuelwood and timber, oak wood is used for agricultural implements, while the leaves serve as fodder for livestock during lean periods.^[76][77]

Other traditional products

Oak bark, with its high tannin content, was traditionally used in the Old World for tanning leather.^[78] Oak galls were used for centuries as a main ingredient in iron gall ink for manuscripts, harvested at a specific time of year.^[79] In Korea, sawtooth oak bark is used to make shingles for traditional roof construction.^[80] The dried bark of the white oak was used in traditional medical preparations; its tannic acid content made it astringent and antiseptic.^[81] Acorns have been ground to make a flour,^[82] and roasted for acorn coffee.^[83]

Culinary

Barrels for aging wines, sherry, and spirits such as brandy and Scotch whisky are made from oak, with single barrel malt whiskies fetching a premium.^[84] The use of oak 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 oaky vanillin 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]

Oak wood chips are used for smoking foods such as fish, meat, and cheese.^[87][88] In Japan, Children's Day is celebrated with Kashiwa-mochi rice cakes, filled with a sweet red bean paste, and wrapped in a kashiwa oak leaf.^[89] The bark of the cork oak is used to produce cork stoppers for wine bottles. This species grows around the Mediterranean Sea; Portugal, Spain, Algeria, and Morocco produce most of the world's supply.^[90]

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: the oil content can be as high as 30%. Oaks have also been used as fodder, 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]

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  Sherry maturing in oak barrels
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  A cross section of the trunk of a cork oak, Quercus suber, showing the thick spongy bark used for making wine bottle corks
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  Kashiwa-mochi rice cake wrapped in oak leaf, Japan

Symbols

The oak is a widely used symbol of strength and endurance.^[101] It is the national tree of many countries,^[102] including the US,^[103] Bulgaria, Croatia, Cyprus (golden oak), Estonia, France, Germany, Moldova, Jordan, Latvia, Lithuania, Poland, Romania, Serbia, and Wales.^[102] Ireland's fifth-largest city, Derry, is named for the tree, from Irish: Doire, meaning 'oak'.^[104] Oak branches are displayed on some German coins, both of the former Deutsche Mark and the euro.^[105] Oak leaves symbolize rank in armed forces including those of the United States. Arrangements of oak leaves, acorns, and sprigs indicate different branches of the United States Navy staff corps officers.^[106][107] The oak tree is used as a symbol by several political parties and organisations. It is the symbol of the Conservative Party in the United Kingdom,^[108] and formerly of the Progressive Democrats in Ireland.^[109]

Religion

The prehistoric Indo-European tribes worshiped the oak and connected it with a thunder god, and this tradition descended to many classical cultures. In Greek mythology, the oak is the tree sacred to Zeus, king of the gods. In Zeus's oracle in Dodona, Epirus, the sacred oak was the centerpiece of the precinct, and the priests would divine 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 called hamadryads inhabited them.^[111] In Norse and Baltic mythology, the oak was sacred to the thunder gods Thor and Perkūnas respectively.^[112][113] In Celtic polytheism, the name druid, Celtic priest, is connected to Proto-Indo-European *deru, meaning oak or tree.^[114] Veneration of the oak survives in Serbian Orthodox Church tradition. Christmas celebrations include the badnjak, a branch taken from a young and straight oak ceremonially felled early on Christmas Eve morning, similar to a yule log.^[115]

History

Category: Individual oak trees

Several oak trees hold cultural importance; such as the Royal Oak in Britain,^[116] the Charter Oak in the United States,^[117] and the Guernica oak in the Basque Country.^[118] "The Proscribed Royalist, 1651", a famous painting by John Everett Millais, depicts a Royalist hiding in an oak tree while fleeing from Cromwell's forces.^[119][120]

In the Roman 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 poem The Garden, Andrew Marvell critiqued the desire to be awarded such a leafy crown: "How vainly men themselves amaze / To win the palm, the oak, or bays; And their uncessant labors see / Crowned from some single herb or tree, ..."^[121][122]