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Quercus the genus.   Click a photo to enlarge it.   back to list

Quercus agrifolia 56 Tree
Ref No: 11909
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Quercus castanifolia5 Tree
Ref No: 11935
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Quercus L. (1753), the oak, contains around 500 species throughout the northern hemisphere, in the family Fagaceae.

Description Trees to 30m or more, or shrubs, with rounded buds. The leaves are deciduous or evergreen, alternate, lobed, holly-like (Ilex), toothed, or with smooth edges. The flowers are unisexual, males and females on the same tree; the males on slender, hanging, usually interrupted catkins, with minute sepals, no petals, and usually 6 stamens surrounding a tuft of hairs. The female flowers are on short stalks, 1 per cupule, with 3 styles. Pollination is by wind. The fruits are solitary acorns, up to 6cm long in Q. lobata Née, set in a cup, which may be smooth, variously scaly, or with soft spines.

Key Recognition Features Acorns in a cup, combined with the male flowers on hanging spikes. In spite of the many exceptions, most species have recognisably oak-shaped leaves.

Evolution and Relationships Within the family Fagaceae, oaks are linked with the unusual and primitive genus Trigonobalanus Forman, which has 2 species in the mountains of northern Thailand to Borneo and the Celebes, and 1 in Colombia. They have primitive, scaly, lobed cups around nuts with 3 or more angles. Fossil Fagaceae are recognised as far back as the middle of the Cretaceous, and Trigonobalanus itself has been found in the Eocene in Europe.

The genus Quercus is divided into subgenera or sections: in subgenus Cyclobalanopsis, sometimes recognised as a separate genus, which includes Q. acuta Thunb., the leaves are evergreen and not lobed, and the cup scales are in concentric rings; all are native of eastern Asia. In section Lobatae, the American red and black oaks, the leaves are varied in shape, but if lobed, the lobes end in long points. The acorn cups mostly have narrow, overlapping scales, and most have acorns that take 2 years to ripen. All are native from North America to Colombia, and include Q. rubra L., Q. phellos L., the willow oak, Q. imbricaria Mich., the shingle oak, and Q. agrifolia Née, the coast live oak. Section Protobalanus, the golden-cup oaks, contains 5 species found from Oregon through California to Arizona and Mexico; they have evergreen leaves and acorns maturing in 2 years, with the scales of the cup embedded in golden or glandular hairs. Quercus chrysolepis Liebm. is the most common of this group. Section Quercus, the white oaks, contains around 200 species throughout the northern hemisphere, the leaves mostly deciduous with rounded lobes, but sometimes evergreen and either holly-like or smooth-edged. The acorn scales are varied, but are not glandular. Quercus dentata from China and Japan, the rare chestnut-leaved Q. pontica, and the English species Q. robur L. and Q. petraea (Matt.) Liebl. belong to this group.

Two further sections are confined to Europe and Asia. In the Turkey oaks, section Cerris Loudon, the buds often have thread-like scales between them; leaf-shapes vary, and the acorns, which take 2 years to mature, usually have bristly cups. Quercus suber L., the cork oak, Q. cerris L., the Turkey oak, Q. castaneifolia C.A. Mey., and Q. libani Oliv. belong to this section. Section Ilex Loudon includes evergreen species in which the fruit matures in 1 year or 2, and the cup scales lie flat. Quercus ilex L., the holm oak, and Q. coccifera L. belong here.

Ecology and Geography In various habitats from Mediterranean semi-desert to subtropical rainforest, but mainly in deciduous or evergreen forest with summer rainfall. From Colombia in Central America to Canada, and in Asia as far south as Malaysia. Around
90 species are recognised in the United States and Canada, and even more in Mexico, but only 27 in Europe.

Comment Oaks have a large variety of uses. The timber, especially that of Q. robur, the English oak, is hard and strong, and is used for shipbuilding, rafters, wine barrels, and furniture; Irish bog oak, which has been pickled for thousands of years in acid peat, is used today for furniture and guitar fingerboards. Quercus alba L. and Q. macrocarpa Mich. previously provided similar timber in North America, but these species have now been replaced as commercial crops by the faster-growing red oaks, such as Q. rubra. Oak bark was much used in tanning, as were the huge cups of the Valonia oak, Q. macrolepis Kotschy. Bark from Q. suber L. is the source of cork; it is grown mainly in western Spain and Portugal, being peeled off the trunks and lower branches about every 8 years. Acorns were eaten in the past in times of famine, and as a substitute for coffee; some species are more edible than others but most are very bitter. Pigs were commonly put out to forage in the woods for them in autumn, a practice called pannage, which is still important in Corsica and parts of Spain. Quercus coccifera, the kermes oak, forms low, holly-like bushes in the Mediterranean garrigue. It produces a red gall, formed by the insect Kermes vermilio, the blood of which was the source of a vermilion dye used by the Spartans to dye their battledress scarlet so that blood stains would not show. A yellow dye was extracted from the American Q. velutina Lam., called quercitron bark. Galls from Q. infectoria Oliv. from Turkey were used to make ink. Many species of oak are cultivated as ornamentals, notably the American red oaks Q. rubra and Q. coccinea Münchh. for their fast growth, distinctive leaves and good autumn colour, the evergreen holm oak Q. ilex as a tough hedge or large tree, and Q. macrolepis for its particularly large, well-shaped leaves. There is a good variegated form of Q. cerris and several upright forms of Q. robur, as well as a purple-leaved form and the golden-leaved ‘Concordia’.

Quercus agrifolia
Quercus castanifolia, acorns and leaves, September 20th

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