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The Trees and Shrubs Online Oak Consortium
Allen Coombes & Roderick Cameron (2021)
Coombes, A. & Cameron, R. (2021), 'Quercus suber' from the website Trees and Shrubs Online (treesandshrubsonline.
Tree to 10–22 m, trunk around 1 m dbh, up to 2 m in old specimens, evergreen (semi-evergreen on the Atlantic coast), crown dense, conical at first rounded, later dome-shaped, broad, irregular, spreading. Bark thick (to 5 cm), corky, cracked longitudinally, deeply fissured, dark, after removal of cork reddish brown, later turning darker. Young branchlets covered with a dense greyish tomentum, retained in the second year, slender, some branches can be pendulous; buds ovoid, apex obtuse or rounded, bearing reddish felted scales, up to 3 mm long. Leaves coriaceous, rigid, oval, oval-oblong or oval-lanceolate, persistent two or three years, often convex above and concave beneath, 3–7 cm long × 1.5–4 cm wide, acute or rounded at the apex, base asymmetrically rounded or slightly heart-shaped; with 4–7 pairs of lateral nerves, prominent below, all but the lowest one or two pairs ending in a minute mucronate tooth; leaves emerge slightly felted above, more densely below, upper surface becomes dark green, shiny, glabrous but for the midrib, lower surface covered with a dense grey tomentum; petiole tomentose, 0.6–1.8 cm long. Acorn cups campanulate, obconical, or almost hemispherical, enclosing at least half of the nut, base often attenuate, covered with grey tomentose scales, reddish at the tips, ovate and short in the basal ranks, long, linear, and mostly erect in the upper half of the cupule; acorn variable in shape, ovoid, cylindrical, or ellipsoidal, reddish brown, shiny, 1.5–4.5 cm long × 1.4–1.8 cm wide, glabrous, with a tomentose apical umbo, solitary or more often in pairs on a short grey tomentose peduncle, 0.5–4 cm long, annual maturation, though flowers produced late in the year may ripen their acorns the following year. (le Hardÿ de Beaulieu & Lamant 2010; Elwes & Henry 1906–1913)
Distribution Algeria North France including Corsica Italy including Sardegna and Sicily Morocco Portugal Spain Tunisia
USDA Hardiness Zone 7
RHS Hardiness Rating H6
Conservation status Least concern (LC)
Cork Oak is found in the Western Mediterranean basin, extending eastwards as far as Sicily and Italy in the north and Tunisia in the south. It is also found on the Atlantic seaboard in Portugal, Spain, Morocco, and France. Trees from this population were described by Jacques Gay (1856) as a separate species, Quercus occidentalis, based primarily on their apparent biennial maturation and semipersistant foliage. Subsequently the name was demoted to a variety (Laguna 1883) or subspecies (Bonnier & Layens 1894), then Aimée Camus (1936) found it to be merely a physiological type, and finally it faded into syonymy. According to Francisco Vazquez (pers. comm. 2020), Q. suber has a complex reproductive system. While fruiting is normally annual with acorns ripening in autumn from spring flowers, some trees may sometimes produce acorns that take 18 months to ripen as well as those that ripen at the normal time. Small acorns may ripen the same year from June flowers. The production of biennial acorns may be related to weather, cork cutting and tree age. Acorns from this longer cycle tend to be significantly larger than those that mature in one season (Vázquez Pardo 2015).
What sets this oak apart, though, is its bark. It is the source of cork, a product of multiple uses, but best known in modern times as a traditional stopper for wine bottles. This, however, is a relatively recent use, at least on a large scale. Glass wine bottles were only introduced in the early 19th century, but the Romans were already harvesting and using cork for various purposes. Pliny (Naturalis Historia, lib. XVI, cap. xiii) mentions that the bark of Cork Oak was straightened out into planks ten-feet square and the substance used as a buoy for the ropes of ships’s anchors and fishing nets, as the bungs of casks, and as a material for women’s winter shoes (Bostock 1855). It continued to be used as a shoe liner through the centuries and is still used as a cushioning sole to this day (latterly cork is being used as an vegan alternative to leather (ethicalelephant.com 2021), so perhaps an old word for cork in German, pantoffelholz, ‘slipper-wood’, may come back into vogue!). Evelyn (1664) mentions that cork was used in Spain to insulate stone houses and planks of it were laid by bedsides to tread on. Cervantes also includes cork in Don Quijote’s ‘Golden Age speech’, praising Cork Oak trees for courteously delivering their bark so that people could protect themselves from the inclemencies of the climate (Cameron 2017). The Industrial Age found a host of new uses for cork, especially following the invention of composition cork (a thin layer or cork granules held together by adhesives). It became a key input for industries ranging from bottle caps to automobiles, flooring, various forms of insulation, and bomber airplanes. By the outbreak of World War II the United States depended so heavily on the importation of cork from Portugal and Spain, that when Nazi Germany blockaded all Altantic trade and cut off imports, the shortage was a matter of national security and a nationwide tree-planting campaign was launched to grow Cork Oak for domestic production (Taylor 2019).
Cork’s importance as a sealant and insulator is due to its remarkable properties: low density, buoyancy, very low permeability, low thermal coefficients, elasticity, and the ability to withstand large deformation without fracture under compression. These properties are the combined result of its cellular structure and cell wall composition. They key lies in suberin, an inert waxy substance that lines the cell walls of cork, conferring impermeability to water and gases, and contributing to its compressibility (Leite & Pereira 2017). Suberin, named after Cork Oak’s epithet, is not limited to Quercus suber. It is found in all corky barks, but Cork Oak is unique in the thickness of its phellem, the layer of dead protective tissue between the bark and cambium, and the fact that its cork can be harvested sustainably. Cork is not completely impervious to gases, allowing oxygen to penetrate through wine stoppers and contribute to the maturation of wine as it ages. Too much or too little access to oxygen may have an adverse effect, so natural cork, despite potential variability in its properties, is an ideal and unique material to use for this purpose (Tarko et al. 2020).
Cork is produced in all the countries where the species is native, but principally in Spain and Portugal, the latter accounting for more than half of the world’s production. The practice of removing bark from Cork Oaks dates back to ancient times: Theophrastus in his Enquiry into Plants (c. 350 BC – c. 287 BC), mentions that this species is exceptional in that it does not die when the bark is stripped off, and is in fact all the stronger for it. It takes considerable time for a Cork Oak to produce quality cork. The trunk has to reach about 23 cm dbh before its bark can be removed for the first time, usually at an age of 25 years. The first harvest produces cork of irregular structure, known as virgin cork, that can only be used for industrial applications (flooring, insulation, etc.). A second harvest nine years later produces cork of better quality but still not good enough for wine stoppers; it is known as secondary cork. Only the third and subsequent harvests produce what is called reproduction cork, of regular structure and smooth surfaces, which can be used for quality wine corks. The trees will be harvested every nine years for around a century and a half, yielding about 15 bark harvests in their lifetime. Cork production in Portugal takes places in the montados, a savannah-like landscape consisting of areas of pasture sparsely populated with trees, Cork Oak being the predominant species. It is the typical landscape of the Alentejo region of south and central Portugal (APCOR 2021).
Loudon (1838) claims Q. suber was introducted to England in or before 1699, by the Duchess of Beaufort; Elwes and Henry (1906–1913) cast doubt on the claim, stating that no evidence of the introduction was found in the family archives by the Duchess’s descendants. They judge that planting Cork Oaks in England must have been quite common in the early 18th century. The current champions for the UK and Ireland are a tree at Powderham Castle, Devon (girth champion at 1.62 m dbh) and one at Tregrehan, Cornwall (height champion at 22.5 m) (Tree Register 2021). Another tree at Tregrehan, with a dbh of 1.3 m, has the widest trunk with a clearly defined stem measured at 1.5 m; trees with larger trunks had features such as a low limb that increase the measurement at 1.5 m. These girths are dwarfed by the centuries-old giants in native habitat, such as the sobrerio de Pai Anes in Portugal (18 m × 2.3 m dbh) and the alcornoque de la Corte del Romero in Spain (17 m × 2.4 m dbh) (Cameron 2013). Large cultivated specimens are found in Australia, where the species was widely planted in the 19th century: a tree in Melbourne, Victoria is the tallest recorded Australian Cork Oak – 22.5 m × 1.6 m dbh in 2010 – while another at Tenterfield, New South Wales has a similar dbh and an impressive crown spread of 32 m (National Register of Big Trees 2021). New Zealand trumps these champions with a specimen at the Te Awamutu Cemetery in Waikato: 26 m × 2.2 m dbh in 2000 (New Zealand Notable Trees Trust 2021). A remarkable specimen grows in Parque Anchorena, Uruguay: 23.2 m × 1.8 m dbh in 2013, with an average crown spread of 37 m, which may make it the largest Cork Oak canopy currently recorded (Cameron 2013)
Cork Oak has been grown for centuries in the US. John Bartram observed a tree near Charleston, South Carolina in 1765, and Thomas Jefferson is said to have started plantings of Cork Oak. According to Taylor (2019), Jefferson’s attempts and those of others on the East Coast failed, but through no fault of the species: acorn viability was lost on the long journey crossing the Atlantic (it is now grown in several arboreta in the eastern USA). Many old specimens can be found in California; some of the earliest plantings stem from a shipment of acorns arranged by the US Patent Office in 1858, by which time faster ships had overcome the viability problem. More acorns were obtained in 1880 and distributed to southern states, Arizona, and especially California, considered to have the ideal climate for the species. Importation of seed continued in the following decades and especially during World War II, when planting was encouraged as part of a campaign to mitigate reliance on imports, subject to enemy blockades (Brooks 1997). Impressive specimens currently found on the campus of the University of California, Davis, and in the city’s streets, originate from these plantings. The earliest date to 1925, while a grove is known to have been planted in 1941 as part of the campaign to produce domestic cork (S. Mezger, pers. comm.). The largest Cork Oak in the US is thought to be one growing in the grounds of Napa State Hospital, Napa, California, planted in 1873 (Taylor 2019).
Though it prefers warmer climates, the species does survive in colder conditions. In the United Kingdom, it has done well in areas with cooler summers and mild winters. A fine old specimen grows at Brodick Castle on the Isle of Arran off the west coast of Scotland, measuring 11 m × 1 m dbh in 2011. In Northern Ireland, Cork Oaks of similar dimensions grow in Mount Stewart Gardens and Tollymore Forest Park (The Tree Register 2021). In Europe, it has survived harsher winter conditions in northern Germany (monumentaltrees.com 2021) and Luxembourg, though it remains to be seen whether trees there will recover from damage caused by –25 °C temperatures in the winter of 2021 (E. Jablonski, pers. comm.). In North America, they prosper on the West Coast as far north as Seattle, where five trees grow at the Washington Park Arboretum, the oldest planted in 1958 (Daniel 2019). In Vancouver, however, the wetter climate and cold soils, coupled with lack of early summer warmth, do not favour the species: though trees survive there, they grow poorly (D. Justice, pers. comm.) A cultivar from Hungary, ‘Sopron’, is said to be particularly winter hardy (see below).
Quercus suber hybridizes in the wild with other section Cerris oaks, including Q. afares (Q. × numidica) and Q. cerris (Q. × crenata). The latter hybrid has also often been raised in cultivation and several cultivars have been selected and widely propagated. In gardens in colder climates it is more likely to encounter Cork Oak features in specimens of this hybrid than as the less hardy Q. suber. Cork Oak is also known to hybridize with oaks in other sections, e.g. Q. ilex (Q. × morisii Borzi), Q. robur (Q. × pastorae M.A. Pineda, F.M.Vázquez & Sánchez Gullon), Q. rotundifolia (Q. × avellaniformis Colmeiro & Boutelou) and Q. canariensis (no published hybrid epithet).
Described by Linnaeus in 1753, the epithet is the name of this tree in Latin. The etymology of the name is unclear, but it may derive from the same root at as a word in Old High German, swigen (‘to be silent’), possibly a reference to cork being stripped without harming the tree (Wiktionary 2021).
A selection from Hungary, characterized by greater winter hardiness and a squat habit. Leaves somewhat smaller, glossy on the upper surface. The distinctively corky bark forms at a relatively early age. Named after the village of Sopron, near the Austrian border, which also gives its name to the wine region that surrounds it (Jablonski 2006).