An oak is a hardwood tree or shrub in the genus Quercus of the Fagaceae. 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; it includes 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 , 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 . Oaks support more than 950 species of caterpillar, many kinds of gall wasp which form distinctive (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 . Individual oak trees of cultural significance include the Royal Oak in Britain, the Charter Oak in the United States, and the Gernikako Arbola in the Basque Country.
Etymology
The
genus 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 village"),
which in turn is from Proto-Germanic
*aiks, "oak".
Description
Oaks are
hardwood (
) trees,
deciduous or
evergreen, with spirally arranged leaves, often with
Dentate leaf; 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
Monoecy male and female flowers. The
staminate (male) flowers are arranged in
, while the small
pistillate (female) flowers
produce an
acorn (a kind of nut) 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 often large and slow-growing; Q. alba can reach an age of 600 years, a diameter of and a height of .
File:Brockwell Oak (9).jpg| Quercus robur habit
File:Quercus lusitanica - Köhler–s Medizinal-Pflanzen-253.jpg|Illustration of Q. lusitanica showing staminate (left) and pistillate flowers (right)
File:2021-04-22 18 54 32 Male flowers (catkins) on a White Oak within a wooded area in the Franklin Farm section of Oak Hill, Fairfax County, Virginia.jpg|Catkins of Quercus alba containing the staminate or 'male' flowers
File:Летен дъб - цъфтеж, нови листа.JPG|New leaves and reddish pistillate or 'female' flowers of Quercus robur
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.
File:Quercus hispanica 20170317.jpg| Quercus × hispanica leaf. Miocene, Lleida, Spain
File:Quercus hiholensis acorn UWBM 56470-3 Pigg & Wehr 2002 Plt2 fig16.png| Quercus hiholensis acorn, Langhian age (Middle Miocene), Washington State, US (c. 15 mya)
File:Quercus kobatakei leaf (cropped).jpg| Quercus kobatakei leaf. Early Oligocene, Japan
File:Early Oligocene oak acorn from Bridge Creek Flora.jpg|Early Oligocene acorn, Oregon, US (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:
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
DNA sequencing,
revealing an array of
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
Quercus cerris,
Q. coccifera,
Quercus ilex,
Quercus smilax, and
Quercus suber.
He chose
Quercus 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 (or high-latitude clade), mostly native to North America
-
Section Lobatae Loudon – North American red oaks
-
Section Protobalanus (Trelease) 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 (or mid-latitude 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
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.
Subgenus Quercus
-
Sect. Lobatae (synonym Erythrobalanus), the Erythrobalanus 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 Protobalanus, 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 , 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 (synonyms Lepidobalanus and Leucobalanus), the Lepidobalanus 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
The type species is
Quercus 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 , 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 (Lauraceae).
[
]
-
Sect. Cerris, the Cerris 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
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.
[Hogan, C. Michael (2012) "Oak" . ed. Arthur Dawson. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC]
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 .
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 (north of the Sahara) 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.
Ecology
Oaks are
keystone 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 the
Ericaceae in oak–heath forests.
[ The Natural Communities of Virginia Classification of Ecological Community Groups (Version 2.3), Virginia Department of Conservation and Recreation, 2010 . Dcr.virginia.gov. Retrieved 2011-12-10.][Schafale, M. P. and A. S. Weakley. 1990. Classification of the natural communities of North Carolina: third approximation. North Carolina Natural Heritage Program, North Carolina Division of Parks and Recreation.] Several kinds of
, including two well-known varieties – black Périgord truffle
and the white Piedmont truffle
– have symbiotic relationships with oak trees. Similarly, many other fungi, such as
Ramaria flavosaponaria, associate with oaks.
Oaks support more than 950 species of caterpillars, an important food source for many birds.
Animals including
Hybridisation
Interspecific hybridization is quite common among oaks, but usually between species within the same section only,
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.
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.
The Fagaceae, or beech family, to which the oaks belong, is a slowly-evolving clade compared to other ,
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.
This diversity includes 106
Macrolepidoptera, 83
Microlepidoptera, 67
, 53
wasps, 38
bugs, 21
bugs, 17
sawfly, and 15
.
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
, and finally by gall wasps such as
Neuroterus.
Several
affect oak species. In Europe, the species
Erysiphe alphitoides is the most common.
It reduces the ability of leaves to photosynthesize, and infected leaves are shed early.
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.
A little-understood disease of mature oaks, acute oak decline, has affected the UK since 2009.
In California, goldspotted oak borer (
Agrilus auroguttatus) has destroyed many oak trees,
while sudden oak death, caused by the
oomycete pathogen
Phytophthora ramorum, has devastated oaks in California and Oregon, and is present in Europe.
Japanese oak wilt, caused by the fungus
Raffaelea quercivora, has rapidly killed trees across Japan.
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
—primarily from the order
Chalcidoidea—which lay their larvae inside the gall wasps, ultimately leading to the hosts' demise. Additionally,
live
Commensalism within the galls without harming the gall wasps.
File:Gallrazorback.jpg|Oak apple gall on Quercus garryana
File:Oak apples on oak leaf and in cross section.JPG|Oak apples on oak leaf
File:Biorhiza pallida male.jpg| Biorhiza pallida male, the cause of oak apple galls
Conservation
An estimated 31% of the world's oak species are threatened with extinction, while 41% of oak species are considered to be of conservation 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 are
climate change and
Invasive species in the US, and
deforestation and
urbanization in Asia.
In the Himalayan region of India, oak forests are being invaded by
pine trees due to global warming. The associated pine forest species may cross frontiers and integrate into the oak forests.
[Kala, C.P. (2012). Biodiversity, communities and climate change. Teri Publications, New Delhi, .]
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.
[Kappelle, M. (2006). "Neotropical montane oak forests: overview and outlook", pp 449–467 in: Kappelle, M. (ed.). Ecology and conservation of neotropical montane oak forests. Ecological Studies No. 185. Springer-Verlag, Berlin, .] 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.
[Lorimer, C.G. (2003) Editorial: The decline of oak forests . American Institute of Biological Sciences.] However, disturbance-tolerant oaks may have benefited from grazers like
American bison, and suffered when the bison were removed following European colonization.
Toxicity
The leaves and acorns of oaks are poisonous to
livestock, including
cattle and
, if eaten in large amounts, due to the toxin
tannic acid, which causes kidney damage and
gastroenteritis.
An exception is the
domestic pig, which, under the right conditions, may be fed entirely on acorns,
[Cappai, Maria Grazia, et al. "Pigs use endogenous proline to cope with acorn ( Quercus pubescens Willd.) combined diets high in hydrolysable tannins." Livestock Science 155.2-3 (2013): 316–322.] and has traditionally been pastured in oak woodlands (such as the Spanish
dehesa[Joffre, R; Rambal, S.; Ratte, J.P. (1999). "The dehesa system of southern Spain and Portugal as a natural ecosystem mimic," Journal of Agroforestry 45(1-3): 57-79.] and the English system of
pannage).
Humans can eat acorns after leaching out the tannins in water.
Uses
Timber
Oak timber is a strong and hard wood with many uses, such as for
furniture, floors, building frames, and
Wood veneer.
The wood of
Quercus cerris (the Turkey oak) has better mechanical properties than those of the white oaks
Quercus petraea and
Quercus robur; the heartwood and sapwood have similar mechanical properties.
[Merela, Maks, and Katarina Čufar. " Density and mechanical properties of oak sapwood versus heartwood in three different oak species". Drvna industrija 64.4 (2013): 323–334.] Of the North American red oaks, the northern red oak,
Quercus rubra, is highly prized for lumber.
[Aldrich, Preston R., et al. "Whole-tree silvic identifications and the microsatellite genetic structure of a red oak species complex in an Indiana old-growth forest." Canadian Journal of Forest Research 33.11 (2003): 2228–2237.]
The wood is resistant to insect and fungal attack.
[Thaler, Nejc; Humar, Miha. " Performance of oak, beech and spruce beams after more than 100 years in service". International Biodeterioration & Biodegradation 85 (2013): 305–310.]
Wood from Q. robur and Q. petraea was used in Europe for shipbuilding, especially of naval man-of-war, until the 19th century.[Kala, C.P. (2004). Studies on the indigenous knowledge, practices, and traditional uses of forest products by human societies in Uttarakhand state of India. GBPIHED, Almora, India][Kala, C.P. (2010). Medicinal Plants of Uttarakhand: Diversity Livelihood and Conservation. BioTech Books, Delhi, .]
Other traditional products
Oak bark, with its high tannin content, was traditionally used in the Old World for tanning
leather.
[Clarkson, L. A. "The English bark trade, 1660–1830" The Agricultural History Review 22.2 (1974): 136–152. ] Oak
were used for centuries as a main ingredient in iron gall ink for manuscripts, harvested at a specific time of year.
In Korea,
sawtooth oak bark is used to make
Roof shingle for traditional roof construction.
The dried bark of the white oak was used in traditional medical preparations; its tannic acid content made it astringent and antiseptic.
[Henkel, Alice. American medicinal barks. No. 139. US Government Printing Office, 1909.] Acorns have been ground to make a flour,
[Szabłowska, Emilia; Tańska, Małgorzata. "Acorn flour properties depending on the production method and laboratory baking test results: A review." Comprehensive Reviews in Food Science and Food Safety 20.1 (2021): 980-1008.] and roasted for acorn coffee.
Culinary
Barrels for
Aging barrel wines,
sherry, and spirits such as
brandy and
Scotch whisky are made from oak, with single barrel malt whiskies fetching a premium.
[Piggott, John R.; Conner, John M. "Whiskies." Fermented beverage production. Boston, Massachusetts: Springer, 2003. 239–262.] 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.
[Pérez-Prieto, Luis J., et al. "Extraction and formation dynamic of oak-related volatile compounds from different volume barrels to wine and their behavior during bottle storage." Journal of Agricultural and Food Chemistry 51.18 (2003): 5444–5449.][Perez‐Prieto, Luis Javier, et al. "Oak‐matured wines: influence of the characteristics of the barrel on wine colour and sensory characteristics." Journal of the Science of Food and Agriculture 83.14 (2003): 1445–1450.]
Oak wood chips are used for smoking foods such as fish, meat, and cheese.
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 cooking 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.
File:Sherry cellar, Solera system, 2003.jpg|Sherry maturing in oak barrels
File:Cork oak trunk section.jpg|A cross section of the trunk of a cork oak, Quercus suber, showing the thick spongy bark used for making wine bottle corks
File:Kashiwa-mochi.jpg|Kashiwa-mochi rice cake wrapped in oak leaf, Japan
Culture
Symbols
The oak is a widely used symbol of strength and
endurance.
It is the
national tree of many countries,
including the US,
,
Croatia,
Cyprus (golden oak),
Estonia,
France,
Germany,
Moldova,
Jordan,
Latvia,
Lithuania,
Poland,
Romania,
Serbia, and
Wales.
Ireland's fifth-largest city,
Derry, is named for the tree, from .
Oak branches are displayed on some German coins, both of the former
Deutsche Mark and the
euro.
Oak leaves symbolize
military 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.
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,
and formerly of the Progressive Democrats in Ireland.
[ Coalition Government 1989 To 1992. progressivedemocrats.ie]
Religion
The prehistoric Indo-European tribes worshipped 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
divination the pronouncements of the god by interpreting the rustling of the oak's leaves.
[Frazer, James George (1922). The Golden Bough. Chapter XV: The Worship of the Oak. ] Mortals who destroyed such trees were said to be punished by the gods since the ancient Greeks believed beings called
inhabited them.
In
Norse mythology and
Baltic mythology, the oak was sacred to the thunder gods
Thor and Perkūnas respectively.
In Celtic polytheism, the name
druid, Celtic priest, is connected to Proto-Indo-European
*deru, meaning oak or tree.
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.
History
- Category: Individual oak trees
Several oak trees hold cultural importance; such as the Royal Oak in Britain,[ Arborecology, containing a photograph of the Millais oak . arborecology.co.uk][Millais, J. G. (1899) Life and Letters of Sir John Everett Millais, vol. 1, p. 166, London : Methuen.]
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".
Notes
Further reading
-
Byfield, Liz (1990) An Oak Tree, Collins Book Bus, London: Collins Educational,
-
Phillips, Roger (1979). Trees of North America and Europe, Random House, New York .
-
Logan, William B. (2005) Oak: The Frame of Civilization, New York; London: W. W. Norton,
-
Paterson, R. T. (1993). Use of Trees by Livestock, 5: Quercus, Chatham: Natural Resources Institute,
-
Royston, Angela (2000). Life Cycle of an Oak Tree, Heinemann first library, Oxford: Heinemann Library,
-
Savage, Stephen (1994). Oak Tree, Observing nature series, Hove: Wayland,
-
Arthur Tansley (1952). Oaks and Oak Woods, Field study books, London: Methuen. .
-
Żukow-Karczewski, Marek (1988). "Dąb – król polskich drzew" ("Oak – the king of the Polish trees"), AURA: A Monthly for the Protection and Shaping of Human Environment, 9, 20–21.
External links
