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 10 to 15 metres (33 to 49 feet) or in rare cases up to 25 m and a trunk diameter (DBH) of 50 to 100 centimetres (20 to 39 inches). It forms a dense and asymmetrical crown that starts at a height of 2–3 m (6+1⁄2–10 ft) 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 7 and 15 cm (3 and 6 in) 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 3 to 5 cm (1+1⁄4 to 2 in) 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 1 to 2 m (3+1⁄4 to 6+1⁄2 ft) 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.
Leaves
The leathery leaves are alternate and are 2.5 to 10 cm (1 to 4 in) long and 1.2 to 6.5 cm (1⁄2 to 2+1⁄2 in) 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 6 to 18 millimetres (1⁄4 to 3⁄4 in) long and are also hairy. At the base of the petiole are two narrow, lanceolate, 5 mm (1⁄4 in) 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 flower The cork oak is single sexed (monoecious), with 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 5 to 30 mm (1⁄4 to 1+1⁄4 in) 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, 4 to 7 cm (1+1⁄2 to 2+3⁄4 in) 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 stamens are whitish with yellow, egg-shaped anthers. They are longer than the bracts.
Infructescence, fruit and seed The fruit clusters are 0.5 to 4 cm (1⁄4 to 1+1⁄2 in) long and carry two to eight acorns. About half of the fruits are enclosed in the fruit cup (cupule); the fruit cups are 2 to 2.5 cm (3⁄4 to 1 in) 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 2 to 4.5 cm (3⁄4 to 1+3⁄4 in) and diameters of 1 to 1.8 cm (1⁄2 to 3⁄4 in). The fruit casing (pericarp) is bare, smooth and shiny brownish red. The hilum (the starting point of the seed) is convex and has a diameter of 6 to 8 mm (1⁄4 to 3⁄8 in).
Taxonomy Quercus suber is a species of the section Cerris 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 holm oak (Quercus ilex), the cork oak forms hybrids. 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 is Latin for cork oak, and also cork.
Distribution and habitat
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