General appearance and barkThe cork oak grows as an evergreen tree, reaching an average height of 10 to 15 meters or in rare cases up to 25 meters and a trunk diameter ( DBH) of . It forms a dense and asymmetrical crown that starts at a height of two to three meters (6 ft 7 in to 9 ft 10 in) and spreads widely in free-standing trees. The crown can be divided into several separate, rounded partial crowns. The young twigs are densely hairy light gray or whitish. Older branches are strong and knotty. Older trees only form short shoots between in length. The thick, longitudinally cracked cork layers of the gray-brown trunk bark are characteristic of the cork oak. The cambium of the smooth bark of young trees forms a cork layer very early on, which can be thick. The light and spongy cork fabric shows vertical cracks and is white on the outside and red to red-brown on the inside. After the cork has been harvested, the trunk appears reddish brown, but later it is significantly darker. The wood is ring-pored, has a brown heartwood and a light reddish sapwood. The cork oak develops a taproot that reaches a depth of and from which several meters long, horizontally running side roots extend. The trees can reach over 400 years old, and harvested specimens can be 150 to 200 years old.
LeavesThe leathery leaves are alternate and are long and wide. The shape varies between round, oval and lanceolate-oval. The leaf blade has five to seven sharp teeth on both edges and a pointed vegetation cone (apex). The midrib stands out clearly on the underside of the leaf, the first-order lateral nerves usually lead to the teeth of the leaf margin. The upper side of the leaf is light green, the underside of the leaf whitish and densely hairy. There is no hair on young trees. The leaf stalks are long and are also hairy. At the base of the petiole are two narrow, lanceolate, long and bright red stipules that fall off in the first year. The new leaves appear in April and May, when older leaves are also shed. They usually stay on the tree for two to three years, less often only one year, the latter especially in severe environmental conditions and on the northern border of the distribution area. Extremely cold winters can also lead to complete defoliation.
Inflorescence and flowerThe cork oak is single sexed ( monoecious), there are both female and male flowers on one specimen. The female flowers form upright inflorescences in the leaf axils of young branches. These are formed from a hairy axis long with two to five separate flowers. The female flowers contain a small, hairy, four- to six-lobed flower envelope and three to four styles. The male catkins also arise on the leaf axils of young branches. They are bright red at the beginning and stand upright, older catkins are yellow and pendulous, long and have a whitish hairy axis. The single flowers are sessile and have a densely hairy flower cover that is colored red when opened. The four to six s are whitish with yellow, egg-shaped anthers. They are longer than the bracts.
Infructescence, fruit and seedThe fruit clusters are long and carry two to eight acorns. About half of the fruits are enclosed in the fruit cup ( cupule); the fruit cups are in diameter. The upper scales of the cupula are gray and hairy, in the subspecies ''Quercus suber occidentalis'' the scales are close together or are fused. The size of the acorns varies between lengths of and diameters of . The fruit casing (pericarp) is bare, smooth and shiny brownish red. The Hilum (biology), hilum (the starting point of the seed) is convex and has a diameter of .
Chromosome numberThe number of chromosomes is 2n = 24.
Distribution and location requirementsThe cork oak occupies the area around the western . In Portugal, natural and cultivated stands cover an area of 750,000 hectares. There are natural populations of the nominate form at altitudes between above sea level, the subspecies ''occidentalis'' is found along the Atlantic coast. In Spain the occurrences mostly remain below , but rarely reach heights of . Cork oaks are rare in central Spain. In Italy one finds natural occurrences along the Tyrrhenian Sea and in eastern Apulia on the Adriatic Sea. Also on the Adriatic is the cork oak on the Dalmatian coast. It is one of the most common forest trees in Sardinia. Natural and man-made occurrences exist in Africa on the Mediterranean coast of Tunisia, Algeria and Morocco and at altitudes up to , on the High Atlas up to . In its native range, cork oak forests cover approximately . Outside of its natural range, the cork oak is cultivated in the Crimea, the Caucasus, India and the Southwestern United States. The subspecies ''Quercus suber occidentalis'' also thrives in mild areas of England. The species needs very little light and cannot survive in dense populations. It loves warmth, grows at annual mean temperatures of and can withstand maximum temperatures of up to . In the area of distribution, the temperature rarely falls below freezing point, but temperatures down to without damage and down to without major damage can be tolerated. The cork oak is not hardy in Central Europe. It endures drought and survives dry periods in summer by reducing its metabolism. An annual rainfall of is considered optimal, in cooler locations can be sufficient with enough humidity. Cork oaks have low soil demands and also grow in poor, dry or rocky locations. They rarely thrive on limestone, calcareous soils, but they are often found on crystalline slates, on gneiss, granite and sands. The acidity of the soil should be between pH 4.5 and 7. The cork oak is considered a pyrophyte because it recovers quickly after forest fires as it is protected by the cork.
EcologyThe cork oak forest is one of the major plant communities of the Mediterranean woodlands and forests ecoregion. In natural populations, the cork oak grows together with the holm oaks (''Quercus ilex'', ''Quercus rotundifolia''), the downy oak (''Quercus pubescens''), the maritime pine (''Pinus pinaster''), the stone pine (''Pinus pinea''), the strawberry tree (''Arbutus unedo'') and the olive tree (''Olea europaea''), in cooler locations also with the sweet chestnut (''Castanea sativa''). In addition to these tree species, the shrub-forming species include the Kermes oak (''Quercus coccifera''), the holly buckthorn (''Rhamnus alaternus''), species of the genus ''Phillyrea'', the myrtle (''Myrtus communis''), the green heather (''Erica scoparia''), the common smilax (''Smilax aspera'') and the Montpellier cistus (''Cistus monspeliensis'') are often found together with the cork oak. Cork oak forests are home to several rare species, such as the Iberian Peninsula for the threatened Iberian lynx (''Lynx pardinus'')Santos Pereira, J., Bugalho, M.N., and Caldeira, M.D. (2008)
SymbiosisThe cork oak enters into a mycorrhizal symbiosis with several types of fungus. The fine root system of the oak is in close contact with the mycelium of the fungus. The oak receives water and nutrient salts from the fungus in exchange for products of photosynthesis. Such a symbiosis exists among others with the following species: *Caesar's mushroom (''Amanita caesarea'') *Death cap (''Amanita phalloides'') *Panther cap (''Amanita pantherina'') *Amanita gilbertii, Gilbert's limbed lepidella (''Amanita gilbertii'') *Cep (''Boletus edulis'') *''Russula rubra''
Diseases and predatorsCork oak is relatively resistant to pathogens, but some diseases occur in the species. Leaf spot can be caused by the fungus ''Apiognomonia errabunda''. Other fungi can cause leaf scorching, powdery mildew, Rust (fungus), rust, and cankers. The most virulent cork oak pathogen may be ''Diplodia corticola'', a Ascomycota, sac fungus which causes sap-bleeding sunken canker wounds in the wood, withering of the leaves, and lesions on the acorns. The fungus ''Biscogniauxia mediterranea'' is becoming more common in cork oak forests. Its fruiting bodies appear as charcoal-black cankers. Both of these fungi are transmitted by the Platypus cylindrus, oak pinhole borer (''Platypus cylindrus''), a species of weevil. The common water mould ''Phytophthora cinnamomi'' grows in the roots of the tree and has been known to devastate cork oak woodlands. Several species of butterflies damage the cork oak, the most important being the Lymantria dispar, gypsy moth (''Lymantria dispar''). The species lays its eggs in the bark of the branches and trunks, and the caterpillars that hatch in spring are distributed in the crown and eat them bare. The bacterial species ''Bacillus thuringiensis'' is used as a biological plant protection agent against the gypsy moth. Another pest is the green oak tortrix (''Tortrix viridana''), whose caterpillars eat flowers and young leaves and roll them up with thread to form typical coils. The lackey moth (''Malacosoma neustria'') also causes damage to the leaves, sticking its eggs to the bark of thin twigs in multiple rows, and also the brown-tail moth (''Euproctis chrysorrhoea''), whose caterpillars skeletonize the leaves and further damage the tree after overwintering in spring. A special cork pest is the jewel beetle ''Coraebus undatus'', which lays its eggs in the cork tissue. Another harmful species of beetle is the great capricorn beetle (''Cerambyx cerdo''), whose larvae eat long corridors in the oak wood. Unfavorable climatic conditions and fungal attack are made responsible for the weakening of trees and for crown damage. Such fungal parasites of weakness are ''Botryosphaeria stevensii'', ''Biscogniauxia mediterranea'', ''Endothiella gyrosa and representatives of the mold genus ''Fusarium''. Drought and parasite infestation are also considered to be the cause of the weakness syndrome in parts of Spain and Portugal.
Systematics and EtymologyQuercus suber is a species of the ''List of Quercus species#Section Cerris, Cerris section to which, for example, the following species also belong: *Valonia oak (''Quercus macrolepis'') *Turkey oak (''Quercus cerris'') *''Quercus × crenata'' *Macedonian oak (''Quercus trojana'') Characteristic for the section are the hairless pericarp and the usually two-year ripening time of the fruits. The cork oak is an exception because the fruits can ripen in both the first and the second year. In the species ''Quercus suber'' two subspecies are distinguished: *''Quercus suber subsp. suber'': Nominal taxon *''Quercus suber subsp. occidentalis''(Gay) Bonnier & Layens: It differs from the nominate form in the shape of the cupula scales, the longer development time of the fruits and the semi-evergreen foliage. The distribution area of the subspecies is the Portuguese Atlantic coast. Together with the Turkey oak (''Quercus cerris'') and the Quercus ilex, holm oak (''Quercus ilex''), the cork oak forms Hybrid (biology), hybrid bastards. The scientific name ''Quercus suber'' is derived from the Latin word ''quercus'', which the Romans used to describe the pedunculate oak (''Quercus robur''). The specific epithet ''suber'' means in Latin cork oak and also cork.
UseThe cork oak is grown for the production of cork in several Mediterranean countries. The centers of cork production are in southern Portugal (accounting for 50% of the total production) and southern Spain, where low trees with large crowns and strong branches are grown in large areas, which provide the highest yield of cork. These mostly extensively managed habitats are called ''dehesa, montados'' in Portugal and ''dehesas'' in Spain. They are considered to be extremely valuable from the point of view of biodiversity and cultural heritage. The cork consists of dead, air-filled, thin-walled cells and contains cellulose and suberin. Cork is heat and sound insulating, the suberine gives it water-repellent properties. The cork layer is replicated by the cork-producing Cork cambium, phellogen and can therefore be harvested repeatedly without damaging the tree too much. The first harvest usually takes place after about 25 years with a trunk diameter of ( DBH), though new techniques (such as better irrigation systems) could shorten it to only 8 to 10 years. The first cork layer is called “male cork” or “virgin cork”, is still not very elastic and cracked and is only used for insulating mats. The second harvested cork (known as ''secundeira''), has a more regular structure and is softer, but is still only used for insulation and in decorative objects. Only the following cork harvests deliver a higher quality cork, the "female cork", which can be used commercially in full. The best quality cork is obtained from the third and fourth harvest. Cork harvesting takes place every nine to twelve years when a layer thickness of is reached. Under favorable (warm) conditions, the harvest can take place every eight years, in North Africa every seven years. A cork oak can be harvested five to seventeen times in total. In order to minimize the damage to the trunk surface, harvesting can be carried out every three years, whereby only a third of the usable surface is removed. An important maintenance measure is pruning, which begins around the age of ten at a height of about . Some sources say an oak can provide around of cork over its lifespan, and one hectare around per year while others suggest a single tree can produce on average of cork per harvest, a comparatively higher value, as cork oaks can live more than 200 years in good conditions. The cork is mainly used for the production of bung, stoppers and wine cork, corks, as well as for heat and sound insulation, cork paper, badminton shuttlecocks, s, handles of fishing rods, special devices for the space industry and for other technical applications (including composite materials, shoe soles, floor coverings). Bottle cork production accounts for around 70% of the added value in cork cultivation. Since natural corks are increasingly being replaced by plastic or sheet metal closures, there could be a significant decline in the cork oak population in southwestern Europe, which endangers the biodiversity in these areas. The bark, which contains around twelve percent extractable tannin, is also used. In addition, the acorns are used as feed in extensive pig fattening (acorn fattening). One tree can provide of acorns per year. Cork oaks cannot legally be cut down in Portugal, except for forestry, forest management felling of old, unproductive trees, and, even in those cases, farmers need special permission from the Ministry of Agriculture. Cork harvesting is done entirely without machinery, being dependent solely on human labour. Usually five people are required to harvest the tree's bark, using a small axe. The process mandates specialized training due to the skill required to harvest bark without inflicting too much damage to the tree. The European cork industry produces 300,000 tonnes of cork a year, with a value of Euro, €1.5 1000000000 (number), billion and employing 30,000 people. Wine corks represent 15% of cork usage by weight but 66% of revenues. Cork oaks are sometimes planted as individual trees, providing a minor income to their owners. The tree is also sometimes cultivated for ornament. Hybrid (biology), Hybrids with Turkey oak (''Quercus cerris'') are not uncommon, both where their ranges overlap in the wild in southwest Europe and in cultivation; the hybrid ''Quercus'' × ''hispanica'' is known as Lucombe oak, for William Lucombe, who first identified it. Some cork is also produced in eastern Asia from the related Chinese cork oak (''Quercus variabilis'')
Cork oak as a symbol*The cork oak is featured in the city arms of several cities in Portugal, such as the city of Reguengos de Monsaraz, which shows a freshly harvested cork tree. *In 2007, a 2 euro commemorative coin with the motif of a cork oak was issued in Portugal in memory of the Portuguese Presidency of the European Union.
Notable treesIn the Portuguese town of Águas de Moura lies the Sobreiro Monumental (''Monumental Cork Oak''), also known as 'The Whistler Tree', a tree 236 years old (planted in 1783/1784), over tall and with a trunk that requires at least three people to embrace it. It has been considered a National Monument since 1988, and the Guinness Book of Records states it as the largest cork tree in the world.
Further reading* Aronson J., Pereira J. S., Pausas J. G. (eds.). (2009).