oak
Many people think of the oak as a majestic tree of an English forest. In fact, there are 450 oak species. Some of them are large trees and others are shrubs. They are members of the beech family.
Oak plants are distributed throughout the temperate zone of the Northern Hemisphere. This area lies between the Tropic of Cancer and the Arctic Circle. They are also found in the tropics, at great heights above sea level. About 75 oak species are native to the United States. Some oaks grow in very dry habitats under near-desert conditions, while still others may be the dominant trees in swamps in very humid climates. Oaks are also found in grasslands.
Oak trees may take 100 years to develop fully and may then live for another 900 years. The trees have thick trunks and large, wide-spreading branches. Oaks may reach a height of 150 feet (45 meters).
Oak leaves are usually deeply lobed, that is, with a curved or rounded projection or division along the margin of the leaf. But some oak plants have leaves that are almost smooth at the edge. Oaks are easily recognized by their fruit—the acorn, a round nut set in a woody cup.
Oaks can be grown easily from acorns. They grow well in fertile and moderately moist soil or in dry, sandy soil. Many oaks can grow from old stumps also.
Some oaks may have nutlike swellings in their tissues. These are galls or gallnuts. They are formed when insects known as gallflies lay their eggs in the tissues of the trees. The young of the insects develop inside the galls. Some galls are rich in a substance known as tannin.
One of the best-known oak plants in the United States is the white oak. This tree is 70–150 feet (21–45 meters) high. The leaves are large and deeply lobed, light green above and whitish beneath. In autumn the foliage turns deep violet and clings to the tree throughout the winter, falling only just before new leaves appear. This is characteristic of many oaks.
There are many other varieties of oak in the United States as well. The bur, or mossy-cup, oak is the most common oak of the prairies, or temperate grasslands of the United States. The red oak is colorful in all seasons. The pin oak or Spanish oak is a quick-growing medium sized tree that grows in the eastern United States. The live oak is a beautiful southern tree that sometimes reaches 60 feet (18 meters) in height. It is found around the Gulf of Mexico.
The well-known English oak is the largest and most famous of all the world's oak trees. Some fine specimens in England date from around the 12th century. The English oak has sturdy limbs and an extremely stout body. The peculiar zigzag growth of the limbs in older trees gives them a twisted look.
Leaf-eating organisms may injure oak plants. A small organism known as the chestnut wilt fungus may also attack the oak. As a result, the twigs, limbs, and leaves, and eventually, the entire tree die.
In North America several oaks are grown to decorate gardens. These include the pin oak and northern red oak. White oak and bur oak form charming oak groves in the American midwest. The bark of the bur oak is rich in tannin and is used in the preparation of leather. Acorns provide food for small game animals and are used to fatten swine and poultry.
The wood of many types of oak is valued as timber for carpentry and building. The value of oak timber varies with the type of oak. The timber of English oak is tough and hard and was once much used in England for shipbuilding. The wood of red and white oak is used in construction, flooring, and for making furniture.
Many oaks native to the Mediterranean area also have economic value. Commercial cork is obtained from the bark of the cork oak. Galls produced on the twigs of the Aleppo oak are a source of Aleppo tannin, used to make ink.
