ALDER (Alnus), the name given to a group of shrubs and trees of the birch family (Betulaceae), comprising some 20 spe cies distributed through the north Temperate zone and in the New World extending in the Andes to Chile. They are most readily distinguished from their allies the birches (q.v.) by the thick woody scales of the fruit (strobile), which persists on the branches long after the seeds and leaves have fallen.
The black alder (A. glutinosa), the most important Old World species, widespread in wet soils in Europe, Asia and North Africa and naturalized in Eastern North America, is the only alder na tive to Great Britain. It grows to a height of 75 ft., with smooth glossy leaves that are somewhat sticky when young. The soft brown wood, very durable in watet, is used for piles and similar purposes. The speckled or hoary alder (A. incana), usually a shrub 8 ft. to 20 ft. high, found widely in Europe and northern Asia, is the common alder of swamps in the eastern United States and Canada. The green alder (A. viridis), 2 ft. to 10 ft. high, of Europe and Asia, occurs also in Eastern North America. The alders of the Rocky Mountain region and the Pacific coast often become large trees. Among these are the red alder (A. oregona), the Sitka alder (A. sinuata), the white alder (A. rhombifolia). The red alder, which attains a height of 8o ft., with a trunk 31- ft. in diameter, is the most valuable; the soft but durable wood is used for furniture. In mountain regions alders are among the first woody plants to spring up in areas denuded by avalanches and in the rubble around the ends of receding glaciers.
Many alders are grown for ornament, especially varieties of the black and the speckled alder and also ,the Japanese alder (A. japonica), one of the largest and most beautiful species.