Birch

Birch, common name for a family of woody trees or shrubs that produce separate, abundant, tiny male and female flowers in dense clusters. The family (see Beech) is widely distributed in temperate and arctic regions of the northern hemisphere, reaching the southern hemisphere only in South America. The family contains 6 genera and about 150 species.

Birch trees of the family's representative genus produce close-grained wood of uniform texture that is used in furniture, flooring, plywood, and veneers. Birch beer, once popular, is derived from the sap. The bark of paper birch was used by Native Americans to build canoes. Oil of wintergreen, once derived from the bark of sweet birch, is now manufactured synthetically. Several species are useful ornamentals, for example, the European white birch, which has many horticultural varieties. One genus is the source of hazelnuts and filberts, which have been used as food since ancient times.

The family also contains alder, hornbeam, ironwood or hop hornbeam, among others. Most are shrubs or short-lived trees that are too small when mature to provide useful lumber. Red alder of the Pacific Northwest is an important exception, providing wood for inexpensive furniture.

Members of the birch family exhibit several adaptations to fertilization by wind pollination, which is common throughout the group. Separate clusters of male and female flowers are produced on the same plant, developing in the fall and maturing in the early spring. Sepals and petals are very small or entirely absent, exposing the pollen-bearing stamens and pollen-collecting pistils to the wind. Enormous quantities of small, easily wind-dispersed pollen grains are shed in the spring by male flowers. The female flowers, however, are not fully mature when the pollen is shed; the styles and pollen-receiving stigmas have formed, but the ovules, which must be fertilized to mature into seeds, have not formed. The pollen remains lodged in the tissue of the styles for several weeks while the ovules mature. Fertilization and seed production follow this process.

Scientific classification: Birches make up the family Betulaceae, of the order Fagales. Paper birch is classified as Betula papyrifera, sweet birch as Betula lenta, and European white birch as Betula pendula. The hazelnut and filbert are classified in the genus Corylus. Red alder is classified as Alnus rubra.