Oak
An oak is a hardwood tree or shrub in the genus Quercus of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisphere, with some 500 species, both deciduous and evergreen. Fossil oaks date back to the Middle Eocene. Molecular phylogeny shows that the genus is divided into Old World and New World clades, but many oak species hybridise freely, making the genus's history difficult to resolve.
Ecologically, oaks are keystone species in habitats from Mediterranean semi-desert to subtropical rainforest. They live in association with many kinds of fungi including truffles. Oaks support more than 950 species of caterpillar, many kinds of gall wasp which form distinctive galls, and a large number of pests and diseases. Oak leaves and acorns contain enough tannin 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 for tanning leather. Wine barrels are made of oak; these are used for aging alcoholic beverages such as sherry and whisky, giving them a range of flavours, colours, and aromas. The spongy bark of the cork 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, and habitat loss.
In culture, the oak tree is a symbol of strength and serves as the national tree of many countries. In Indo-European and related religions, the oak is associated with thunder gods. Individual oak trees of cultural significance include the Royal Oak in Britain, the Charter Oak in the United States, and the Guernica Oak in the Basque Country.
Etymology
The generic name Quercus is Latin for "oak", derived from Proto-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 the cork oak, similarly derives from Quercus. The common name "oak" is from Old English ac, which in turn is from Proto-Germanic *aiks, "oak".Description
Oaks are hardwood trees, deciduous or evergreen, with spirally arranged leaves, often with lobate margins; some have serrated leaves or entire leaves with smooth margins. Many deciduous species are marcescent, not dropping dead leaves until spring. In spring, a single oak tree produces both male and female flowers. The staminate flowers are arranged in catkins, while the small pistillate flowers produce an acorn contained in a cupule. Each acorn usually contains one seed and takes 6–18 months to mature, depending on the species. The acorns and leaves contain tannic acid, which helps to guard against fungi and insects. There are some 500 extant species of oaks.Trees in the genus are usually large and slow-growing; Q. alba can reach an age of 600 years, a diameter of and a height of. The Granit oak in Bulgaria, a Q. robur specimen, has an estimated age of 1,637 years, making it the oldest oak in Europe. The Wi'aaSal tree, a live oak in the reservation of the Pechanga 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. Among the smallest oaks is Q. acuta, the Japanese evergreen oak. It forms a bush or small tree to a height of some.
Evolution
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.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:Internal phylogeny
Molecular techniques for phylogenetic analysis show that the genus Quercus consisted of Old World and New World clades. The entire genome of Quercus robur has been sequenced, revealing an array of mutations that may underlie the evolution of longevity and disease resistance in oaks. In addition, hundreds of oak species have been compared, allowing a detailed phylogeny to be constructed. However, the high signal of introgressive hybridization in the genus has made it difficult to resolve an unambiguous, unitary history of oaks. The phylogeny from Hipp et al. 2019 is:Taxonomy
Taxonomic history
The genus Quercus was circumscribed by Carl Linnaeus in the first edition of his 1753 Species Plantarum. He described 15 species within the new genus, providing type specimens for 10 of these, and giving names but no types for Q. cerris, Q. coccifera, Q. ilex, Q. smilax, and Q. suber. He chose Q. robur, the pedunculate oak, as the type species for the genus.A 2017 classification of Quercus, based on multiple molecular phylogenetic studies, divided the genus into two subgenera and eight sections:
- Subgenus Quercus – the New World clade, mostly native to North America
- *Section Lobatae Loudon – North American red oaks
- *Section Protobalanus O.Schwarz – North American intermediate oaks
- *Section Ponticae Stef. – with a disjunct distribution between western Eurasia and western North America
- *Section Virentes Loudon – American southern live oaks
- *Section Quercus – white oaks from North America and Eurasia
- Subgenus Cerris Oerst. – the Old World clade, exclusively native to Eurasia
- *Section Cyclobalanopsis Oerst. – cycle-cup oaks of East Asia
- *Section Cerris Dumort. – cerris oaks of subtropical and temperate Eurasia and North Africa
- *Section Ilex Loudon – ilex oaks of tropical and subtropical Eurasia and North Africa
Subgenus ''Quercus''
- Sect. Lobatae, the red oaks of North America, Central America and northern South 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.
- Sect. Protobalanus, the canyon live oak and its relatives, in the southwestern United States and northwest Mexico. 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.
- Sect. Ponticae, a disjunct including just two species. Styles are short, and the acorns mature in 12 months. The leaves have large stipules, high secondary veins, and are highly toothed.
- 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.
- Sect. Quercus, the white oaks of Europe, 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 the Quercus genera produce one flower per node, with three or six styles, as well as three or six ovaries, respectively. The leaves mostly lack a bristle on their lobe tips, which are usually rounded. The type species is Quercus robur.
Subgenus ''Cerris''
- Sect. Cyclobalanopsis, the ring-cupped oaks of eastern and southeastern Asia. These are evergreen trees growing 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. Species of Cyclobalanopsis are common in the evergreen subtropical laurel 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 genus Castanopsis and the laurel family.
- Sect. Cerris, the Turkey 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.
- Sect. Ilex, the Ilex oak and its relatives of Eurasia and northern Africa. 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.
Distribution
In the Americas, Quercus is widespread from Vancouver and Nova Scotia in the south of Canada, south to Mexico and across the whole of the eastern United States. It is present in a small area of the west of Cuba; in Mesoamerica it occurs mainly above. The genus crossed the isthmus of Panama when the northern and southern continents came together and is present as one species, Q. humboldtii, above 1,000 metres in Colombia. The oaks of North America are of many sections along with related genera such as Notholithocarpus.
In the Old World, oaks of section Quercus extend across the whole of Europe including European Russia apart from the far north, and north Africa from Morocco to Libya. In Mediterranean Europe, they are joined by oaks of the sections Cerris and Ilex, which extend across Turkey, the Middle East, Iran, Afghanistan and Pakistan, while section Ponticae is endemic to the western Caucasus in Turkey and Georgia. Oaks of section Cyclobalanopsis extend in a narrow belt along the Himalayas to cover mainland and island Southeast Asia as far as Sumatra, Java, Borneo, and Palawan. Finally, oaks of multiple sections extend across east Asia including China, Korea, and Japan.