An oak is a tree or shrub in the genus Quercus (/ˈkwɜːrkəs/; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably Lithocarpus (stone oaks), as well as in those of unrelated species such as Grevillea robusta (silky oaks) and the Casuarinaceae (she-oaks). The genus Quercus is native to the Northern Hemisphere and includes deciduous and evergreen species extending from cool temperate to tropical latitudes in the Americas, 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.

Oaks have spirally arranged leaves, with lobate margins in many species; 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 staminate ('male') flowers (in the form of catkins) and small pistillate ('female') flowers, meaning that the trees are monoecious. The fruit is a nut called an acorn or oak nut 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 the species. The acorns and leaves contain tannic acid, which helps to guard against fungi and insects. The live oaks are distinguished for being evergreen, but are not a distinct group and instead are dispersed across the genus.

 

Linnaeus described only five species of oak from eastern North America, based on general leaf form. These were white oak (Quercus alba), chestnut oak (Q. montana), red oak (Q. rubra), willow oak (Q. phellos), and water oak (Q. nigra). Because he was dealing with confusing leaf forms, the Q. montana and Q. rubra specimens included mixed foliage of more than one species.

See also: List of Quercus species

A 2017 classification of Quercus was based on multiple molecular phylogenetic studies and data, mainly originating between 2010 and 2015. The genus was divided into two subgenera and eight sections:

• Subgenus Quercus – the New World clade (or high-latitude clade), mostly native to North America
  • Section Lobatae – North American red oaks
  • Section Protobalanus – North American intermediate oaks
  • Section Ponticae – with a disjoint distribution between western Eurasia and western North America
  • Section Virentes – American southern live oaks
  • Section Quercus – white oaks from North America and Eurasia
• Subgenus Cerris – the Old World clade (or mid-latitude clade), exclusively native to Eurasia
  • Section Cyclobalanopsis – cycle-cup oaks of East Asia
  • Section Cerris – cerris oaks of subtropical and temperate Eurasia and North Africa
  • Section Ilex – 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 (subgenus Cerris), including oaks that diversified in Eurasia; and the New World clade (subgenus Quercus), oaks that diversified mainly in the Americas.

Potential records of Quercus have been reported from Late Cretaceous deposits in North America and East Asia; however, these are not considered definitive. In a survey of the fossil record of Quercus it was concluded that "pre-Paleogene, and perhaps pre-Eocene occurrences of Quercus macroremains are generally represented by poorly preserved fossils that lack critical features needed for certain identification and need to be treated with caution." 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 being from the Middle Eocene of Japan; both forms have affinities to the Cyclobalanopsis group.

Further advances in oak systematics are expected to arise from next-generation sequencing techniques, including a recent project to sequence the entire genome of Quercus robur (the pedunculate oak). The recent completion of that genome has uncovered an array of mutations that may underlie the evolution of longevity and disease resistance in oaks. In addition, the generation of RAD-seq loci for hundreds of oak species has allowed for the construction of the most highly detailed oak phylogeny to date. However, the high signal of introgression (by hybridization) across the tree poses difficulties for deriving an unambiguous, unitary history of oaks. The phylogeny from Hipp et al. 2019 is:

Oaks are keystone species in a wide range of habitats from Mediterranean semi-desert to subtropical rainforest. For example, oak trees are important components of hardwood forests; certain species are particularly known to grow in associations with members of the Ericaceae in oak–heath forests. Several kinds of truffles, including the two well-known varieties, the black Périgord truffle and the white Piedmont truffle, have symbiotic relationships with oak trees. Similarly, many other mushrooms, such as Ramaria flavosaponaria, also associate with oaks. The European pied flycatcher is an example of an animal species that often depends upon oak trees. Oaks also support more than 900 species of caterpillars, which are an important food source for many birds.

Many species of oaks are threatened with extinction in the wild, primarily due to land use changes, livestock grazing, and unsustainable harvesting. For example, over the past 200 years, large areas of oak forest in the highlands of Mexico, Central America, and the northern Andes have been cleared for coffee plantations and cattle ranching. There is a continuing threat to these forests from exploitation for timber, fuelwood, and charcoal. In the US, entire oak ecosystems have declined due to a combination of factors still imperfectly known, but thought to include fire suppression, increased consumption of acorns by growing mammal populations, herbivory of seedlings, and introduced pests. However, it has also been suggested that oaks as generally light-demanding trees with a relatively high tolerance for mechanic disturbances might depend on grazers like bison and the clearances they create in order to regenerate successfully, thus missing them since they were extirpated in most regions following the European colonization.

The mature trees shed varying numbers of acorns annually. Scientists suggest that shedding excess numbers allows the oaks to satiate nut-gathering species, improving germination chances. Certain oak populations will synchronize every four to ten years to produce almost no acorns at all, only to rain them down excessively the following year, known as a mast year. The year preceding the mast year is thought to starve off the mammal populations feeding on the supply, thereby increasing the effectiveness of the overproduction in the following mast year. This is necessary to the survival of any given oak species, as only one in 10,000 acorns results in an eventual tree.

The oak tree is a tree or shrub in the genus Quercus. There are about 500 living species. They are divided into subgenera. The common name "oak (Oaks)" may also appear in the names of species in related genera, such as Lithocarpus.

Oaks have spirally arranged leaves, with rounded edges in many species; some have leaves with jagged edges or entire leaves with smooth margins. Many deciduous species do not drop dead leaves until the next Spring. In Spring, a single oak tree produces both male flowers (as catkins) and small female flowers. The fruit is a nut called an acorn, carried in a cup-like structure. Each acorn has one seed (rarely two or three) and takes 6–18 months to mature, depending on species. The so-called "live oaks" are evergreen. They are not a taxonomic group, just a life style which occurs across the genus.

The oak is a kind of hardwood forest tree. They are well known as a climax vegetation in the temperate zone of the northern hemisphere. That means, left untouched by humans, it would be the dominant tree. Much of England was covered by oak forests before modern farming took over the land. The last extensive oak woodlands were cut down to build ships for the Royal Navy in the 18th century.

Some kinds of oak wood are very hard. That is why people in past centuries cut them down to make ships, furniture and other things. The wood is now scarce and expensive and only used to make a few things anymore. Much cheaper are softwoods like pine.

Oak trees grow slowly and can live up to 1000 years.

A mature oak tree stands about 100 feet tall (~30 metres). It is a home for more animals than any other European tree. 30 species of birds, 45 different bugs and over 200 species of moth have been found on oaks. Beetles burrow under the bark, and some drill holes into the wood. The leaves are eaten by many caterpillars. Many leaves carry strange little bumps on the underside. These are insect galls, caused by many little animals. Midges, moths, worms and tiny wasps lay their eggs in leaves or leaf buds. The leaf reacts by forming a growth around the eggs. Inside the gall, larvae develop. The leaf falls, but the larvae may come out only the next spring. Small galls only have one larva, but larger galls may contain as many as 30 larvae.

Oak trees produce acorns once a year which ripen in autumn. Oak trees may start producing acorns when they are about 20 years old. A mature oak may produce 90,000 acorns a year; this is several millions in its lifetime.