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 (roundish woody lumps such as the oak apple), 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 (seen in placenames such as Acton, from ac + tun, "oak town/village"), which in turn is from Proto-Germanic *aiks, "oak".
Description Oaks are hardwood (dicotyledonous) 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. Oaks can be grouped into "red oaks" that have soft bristles on their leaves' lobes, and "white oaks" that don't have those bristles. In spring, a single oak tree produces both male and female flowers. The staminate (male) flowers are arranged in catkins, while the small pistillate (female) flowers produce an acorn (a kind of nut) contained in a capped peduncle called a cupule. Each acorn usually contains one seed. 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 4 metres (13 ft) and a height of 44 metres (144 ft). 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 9 metres (30 ft).
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 (the pedunculate oak) 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 (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:
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:
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