alder, name for deciduous trees and shrubs of the genus Alnus of the family Betulaceae (birch family), widely distributed, especially in mountainous and moist areas of the north temperate zone and in the Andes. The black alder (A. glutinosa) is an Old World species now naturalized in E North America. Its bark, still used for dyes and tanning, was formerly considered medicinal; its wood is useful chiefly as charcoal. A. rugosa, the speckled alder, forms extensive swamp thickets in Eurasia and North America. The red alder (A. rubra), the largest tree of the genus, is the most important hardwood timber tree in its native region, the Pacific coast of North America. Alder trees are classified in the division Magnoliophyta, class Magnoliopsida, order Fagales, family Betulaceae.