birch

birch, (genus Betula), genus of about 40 species of short-lived ornamental and timber trees and shrubs of the family Betulaceae, distributed throughout cool regions of the Northern Hemisphere.

Physical description

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A birch has smooth, resinous, varicoloured or white bark, marked by horizontal pores (lenticels), which usually peels horizontally in thin sheets, especially on young trees. On older trunks the thick, deeply furrowed bark breaks into irregular plates. Short, slender branches rise to a narrow pyramidal crown on a young tree; they become horizontal, often pendulous, on an older tree. The egg-shaped or triangular, usually pointed leaves have toothed margins; they are alternately arranged on the branchlets. They are usually bright green, turning yellow in autumn. The drooping male catkins flower before the leaves emerge; smaller, upright female catkins on the same tree develop in conelike clusters, which disintegrate at maturity, releasing tiny, one-seeded, winged nutlets.

Major species

Birch species often have two or more common names, and a single common name may be applied to a number of different species. For example, the European white birch is usually called silver birch in England, but the latter name is also sometimes given to paper birch and to yellow birch. For clarity, the taxonomic name should be used in conjunction with the common name.

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Trees of the World

Gray birch (Betula populifolia), paper birch (B. papyrifera), river birch (B. nigra), sweet birch (B. lenta), yellow birch (B. alleghaniensis), and various species of white birch (notably B. pendula and B. pubescens) are the best known.

The Japanese monarch birch (B. maximowicziana) is a valuable timber tree of Japan, especially in the plywood industry. Usually 30 metres (100 feet) high, with flaking gray or orange-gray bark, it has heart-shaped leaves about 15 cm (6 inches) long and is a hardy ornamental. The similar Japanese cherry birch (B. grossa) also produces useful timber.

Water birch (B. occidentalis), a shrubby tree native to moist sites along the western coast of North America, has nonpeeling dark red bark; it grows in clusters, with all stems rising from a common root system. It is sometimes called red birch, black birch, or mountain birch. Swamp birch (B. pumila), a similar but smaller shrub, is found on boggy sites; it may be erect or trailing and matted. Bog birch (B. glandulosa) of North America, also called tundra dwarf birch or resin birch, and dwarf birch, or dwarf Arctic birch (B. nana), native to most far northern areas of the world, are small alpine and tundra shrubs commonly known as ground birch. Both species have almost circular leaves, are food sources for birds and grazing animals, and may be planted as ornamentals. Several Chinese birches and the Asian white birch (B. platyphylla) are sometimes used ornamentally. A few natural hybrids between trees and shrubs of the genus Betula are cultivated as ornamentals in Europe and North America.

Ivory birch (Baloghia inophylla, family Euphorbiaceae) and West Indian birch, also known as gumbo-limbo (Bursera simaruba, family Burseraceae), are not true birches.

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Uses

Birches were among the first trees to become established after the glaciers receded. Hardy, quick growing, and relatively immune to disease and insect attack, they are valuable in reforestation and erosion control and as protective cover, or nurse trees, for development of more permanent plants. Most require moist, sandy, and loamy soil; they are usually propagated by seeding or grafting. Many ornamental varieties are cultivated for their leaf colour, leaf shape, or growth habit.

Pale to red-brown birchwood is used for flooring, furniture, cabinetry, interior finishing, vehicle parts, plywood, pulp, and turnery. The thin water-impervious bark provided roofing, canoes, and shoes for North American Indians and early settlers. Sweet birch (B. lenta) is the source of birch oil and has been used to make birch beer, an effervescent beverage somewhat similar to root beer. Hikers and other outdoor enthusiasts can rely on the ability of yellow and paper birch bark to burn even when wet.