thumb|Bark of tree 'wrinkled skin'

Quercus suber, commonly called the cork oak, is a medium-sized, evergreen oak tree in the section Quercus sect. Cerris. It is the primary source of cork for wine bottle stoppers and other uses, such as cork flooring and as the cores of cricket balls. It is native to southwest Europe and northwest Africa. In the Mediterranean basin the tree is an ancient species with fossil remnants dating back to the Tertiary period. It can survive for as long as two centuries. Typically, once it reaches 25 years old, its thick bark can be harvested for cork every 9 to 12 years without causing harm to the tree.

It endures drought and makes little demand on the soil quality and is regarded as a defence against desertification. Cork oak woodlands are home to a multitude of animal and plant species. Since cork for sealing bottles is increasingly being displaced by other materials, these forests are at risk as part of the cultural landscape and as a result animal species such as the Iberian lynx and imperial eagles are threatened with extinction.

Description

General appearance and bark

The cork oak grows as an evergreen tree, reaching an average height of or in rare cases up to 25 m and a trunk diameter (DBH) of . It forms a dense and asymmetrical crown that starts at a height of 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 cork oak develops a taproot that reaches a depth of and from which several meters long, horizontally running side roots extend. The trees can live over 400 years, and harvested specimens can be 150 to 200 years old.

In the species Quercus suber two subspecies are distinguished:

  • Quercus suber subsp. suber: Nominal taxon
  • Quercus suber subsp. occidentalis<small> (Gay) Bonnier & Layens</small>: 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.

Distribution and habitat

thumb|right|Cork oak forest habitat

thumb|Cork oak trees, near Roses, northeastern Spain

The cork oak occupies the area around the western Mediterranean basin. In Portugal, natural and cultivated stands cover an area of 750,000 hectares. The subspecies Quercus suber occidentalis also thrives in mild areas of England.

The species needs a lot of 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 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.

Ecology

thumb|right|Cork oak [[savanna]]

thumb|right|The [[Iberian lynx, a resident of the cork oak forests of the southern Iberian Peninsula]]

The 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 Portuguese oak (Quercus faginea), the Pyrenean oak (Quercus pyrenaica), Mirbeck's oak (Quercus canariensis), 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 Lusitanian oak (Quercus lusitanica) 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.

Symbiosis

thumb|right|The Caesar's mushroom, a symbiotic partner of the cork oak

The 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: Leaf spot can be caused by the fungus Apiognomonia errabunda. Other fungi can cause leaf scorching, powdery mildew, rust, and cankers.

The most virulent cork oak pathogen may be Diplodia corticola, a 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 oak pinhole borer (Platypus cylindrus), a species of weevil.

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.) 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 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 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 (DBH) of , though new techniques (such as better irrigation systems) could shorten it to only 8 to 10 years. while others suggest a single tree can produce on average of cork per harvest, 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. although the Holm Oak (Quercus ilex), is preferred for this due to its sweeter fruits. One cork oak tree can provide of acorns per year.

The European cork industry produces 300,000 tonnes of cork a year, with a value of €1.5 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. 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).

Culture

thumb|'The Whistler Tree', the world's largest cork tree

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 trees

In 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 Guinness World Records lists it as the largest cork tree in the world.

Cork oaks of California

thumb|Cork Oak street sign

While native to the Mediterranean, cork oaks have been planted across several U.S. states with warm, mild climates, particularly in California. One of the oldest recorded cork oaks in the state, was planted in 1858, by Mexican miners, in the town of Campo Seco in the Central Valley. Another historic cork oak is in Napa and stands 89&nbsp;feet tall with a 20&nbsp;ft circumference. There are several streets or lanes in California with the name 'Cork Oak'. A street in Palo Alto, named 'Cork Oak Way' was established in 1960, in an area densely populated with cork oaks. , eleven of these trees still lined this street.

Cork industry during World War II

By 1940, the United States imported nearly half the world’s cork production, for making everything from bottle caps to parts for military equipment. When Nazi Germany blockaded all Atlantic trade and cut off cork imports from Europe, the shortage was deemed a threat to U.S. national security. To address this threat, individuals such as Charles McManus Sr, CEO of Crown Cork and Seal and Woodbridge Metcalf, a Forestry Professor, participated in a national project to map and cultivate the domestic growth of cork oaks. As a result, a military transport plane delivered a load of cork oak acorns from Morocco to the US. Throughout this project, over five hundred cork oaks were planted in the University of California at Davis.

References

Further reading

  • Aronson J., Pereira J. S., Pausas J. G. (eds.). (2009). Cork Oak Woodlands on the Edge: Conservation, Adaptive Management, and Restoration. Washington, D.C.: Island Press, 315 pp.
  • Quercus suber. Plants of the World Online. Kew Science.
  • Cork Oak. World Wildlife Foundation Priority Species.
  • Cork Industry Federation. 2014.
  • PlanetCork.org. Educating primary school children in sustainable development. Cork Industry Federation. 2009.
  • Cork Oak (Quercus suber). European Forest Genetic Resources Programme (EUFORGEN).