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With 2 genera and approximately 340 species of trees and shrubs primarily in cooler regions of the Northern Hemisphere. Intolerant, pioneer, short-lived trees. Leaves are simple, deciduous, alternate, stipulate, and diocecious. Flowers of both sexes appear in amens (catkins). Fruit is a capsule and seeds have white tufts of hair, ripen in the spring and germinate promptly (brief viability).