Quercus suber

Quercus suber is a species of the 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.[9]

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.[9]

Together with the Turkey oak (Quercus cerris) and the holm oak (Quercus ilex), the cork oak forms hybrid bastards.[7]

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.[10]

Cork oak distribution

Cork oak forest habitat

The cork oak occupies the area around the western Mediterranean basin. In Portugal, natural and cultivated stands cover an area of 750,000 hectares.[9][11] There are natural populations of the nominate form at altitudes between 150 to 300 m (490 to 980 ft) above sea level, the subspecies occidentalis is found along the Atlantic coast. In Spain the occurrences remain mostly below 600 m (2,000 ft), but rarely reach heights of 1,200 m (3,900 ft). In Spain, cork oaks are common in the southern half of the country, as well as in the western and northeastern areas, but 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 1,000 m (3,300 ft), on the High Atlas up to 2,000 m (6,600 ft).[9] In its native range, cork oak forests cover approximately 22,000 square kilometres (8,500 square miles).[1] Outside of its natural range, the cork oak is cultivated in the Crimea, the Caucasus, India and the Southwestern United States.[12] 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 13 to 17 °C (55 to 63 °F) and can withstand maximum temperatures of up to 40 °C (104 °F). In the area of distribution, the temperature rarely falls below freezing point, but temperatures down to −5 °C (23 °F) without damage and down to −10 °C (14 °F) 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 500 to 700 millimetres (20 to 28 in) is considered optimal, in cooler locations 400 to 450 millimetres (16 to 18 in) 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.[13]

The cork oak is considered a pyrophyte because it recovers quickly after forest fires as it is protected by the cork.

Cork oak savanna

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.[14] 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.[13]

Cork oak forests are home to several rare species, such as the Iberian Peninsula for the threatened Iberian lynx (Lynx pardinus)[15][16] and the endangered Iberian imperial eagle (Aquila adalberti),[17] and a large part of the European crane population winters here. In the northwestern Maghreb, some cork oak forests are habitat to the endangered Barbary macaque (Macaca sylvanus), a species whose habitat is fragmented and whose range was prehistorically much wider.[18] The Berber deer (Cervus elaphus subsp. Barbarus) lives in the cork oak forests of Tunisia.[4]

As a pyrophyte, this tree has a thick, insulating bark that makes it well adapted to forest fires.[1] After a fire, many tree species regenerate from seeds (as, for example, the maritime pine) or resprout from the base of the tree (as, for example, the holm oak). The bark of the cork oak allows it to survive fires and then simply regrow branches to fill out the canopy. The quick regeneration of this oak makes it successful in the fire-adapted ecosystems of the Mediterranean biome.[16]

Symbiosis

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:[8]

• Caesar's mushroom (Amanita caesarea)
• Death cap (Amanita phalloides)
• Panther cap (Amanita pantherina)
• Gilbert's limbed lepidella (Amanita gilbertii)
• Cep (Boletus edulis)
• Russula rubra

Diseases and predators

Gypsy moth caterpillar

Cork 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, and cankers.[19]

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.[19]

The common water mould Phytophthora cinnamomi grows in the roots of the tree and has been known to devastate cork oak woodlands.[19]

Several species of butterflies damage the cork oak, the most important being the 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.[20]

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.[20]

Cork oaks are an important component of the montado/dehesa

Bark storage

Cork harvesting

The 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[11]) and southern Spain, where low trees with large crowns and strong branches are grown in large areas, which provide the highest yield of cork.[21] These mostly extensively managed habitats are called montados in Portugal and dehesas in Spain.[22] They are considered to be extremely valuable from the point of view of biodiversity and cultural heritage.[23]

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 70 cm (28 in), though new techniques (such as better irrigation systems) could shorten it to only 8 to 10 years.[24][1] 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.[24] 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 2.7 to 4 cm (1 to 1+1⁄2 in) 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.[24] 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 3 m (10 ft). Some sources say an oak can provide around 100 to 200 kilograms (220 to 440 pounds) of cork over its lifespan, and one hectare around 200 to 500 kg (440 to 1,100 lb) per year[25] while others suggest a single tree can produce on average 40 to 60 kg (88 to 132 lb) of cork per harvest,[24] a comparatively higher value, as cork oaks can live more than 200 years in good conditions.[16][1]

The cork is mainly used for the production of stoppers and corks, as well as for heat and sound insulation, cork paper, badminton shuttlecocks, cricket balls, handles of fishing rods and hand tools, special devices for the space industry[22] and for other technical applications (including composite materials, shoe soles, floor coverings).[25][26] 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.[25]

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 15 to 30 kg (33 to 66 lb) of acorns per year.[25]

Cork oaks cannot legally be cut down in Portugal, except for forest management felling of old, unproductive trees, and, even in those cases, farmers need special permission from the Ministry of Agriculture.[27][1]

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.[28]

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).

'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.[29]

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.[30]

• 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.

Wikisource has the text of an 1879 American Cyclopædia article about Quercus suber.

Wikimedia Commons has media related to Quercus suber.

Wikispecies has information related to Quercus suber.

• 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).