Description from Flora of China
Trees or shrubs deciduous. Buds stalked with 2(or 3) scales or sessile with several overlapping scales. Leaves usually serrate or dentate, rarely incised or entire. Male inflorescence elongate, pendulous, cylindric, with numerous overlapping bracts, each bract subtending (3 or)4(or 5) bracteoles and 3 flowers; calyx 4-lobed; stamens (1-)4; anthers 2-loculed, thecae connate, apex glabrous. Female inflorescence 1, or 2-numerous in a raceme or panicle, ovoid or ellipsoid, conelike; bracts numerous, overlapping, woody, persistent, apex 5-lobulate, each bract subtending 2 flowers. Nutlets 2 in each bract axil, compressed, with membranous or papery wings. Fl. mainly spring, Alnus formosana and A. nepalensis autumn.
Three or more divergent subgroups (subgenera) of Alnus are often treated as separate genera (J. J. Furlow, J. Arnold Arbor. 71: 1-67. 1990).
Various Alnus species are grown to protect dikes, some are valued for timber, and the roots have nodules with nitrogen-fixing bacteria.
About 40 species: Bangladesh, Bhutan, China, India, Japan, Korea, Nepal, Sikkim; Europe, North and South America; ten species (five endemic) in China.