Alders are trees comprising the genus Alnus in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few species extending into Central America, as well as the northern and southern Andes. Description With a few exceptions, alders are deciduous, and the leaves are alternate, simple, and serrated. The flowers are catkins with elongate male catkins on the same plant as shorter female catkins, often before leaves appear; they are mainly wind-pollinated, but also visited by bees to a small extent. These trees differ from the birches (Betula, another genus in the family) in that the female catkins are woody and do not disintegrate at maturity, opening to release the seeds in a similar manner to many conifer cones. The largest species are red alder (A. rubra) on the west coast of North America, and black alder (A. glutinosa), native to most of Europe and widely introduced elsewhere, both reaching over . By contrast, the widespread Alnus alnobetula (green alder) is rarely more than a shrub. Taxonomy Classification The genus is divided into three subgenera: Subgenus Alnus Trees with stalked shoot buds, male and female catkins produced in autumn (fall) but stay closed over winter, pollinating in late winter or early spring, about 15–25 species, including: Alnus acuminata Kunth – Andean alder, aliso. Mexico, Central and South America. subsp. acuminata Kunth subsp. arguta (Schltdl.) Furlow subsp. glabrata (Fernald) Furlow Alnus cordata (Loisel.) Duby – Italian alder. Italy, Corsica. Alnus cremastogyne Burkill – China. Alnus firma Siebold & Zucc. – Kyūshū Island in Japan Alnus glutinosa (L.) Gaertn. – black alder. Europe, Central Asia. subsp. barbata (C.A.Mey.) Yalt. subsp. glutinosa (L.) Gaertn. subsp. incisa (Willd.) Regel subsp. laciniata (Willd.) Regel Alnus hirsuta (Spach) Rupr. – Manchurian alder. Japan, Korea, Manchuria, Siberia, Russian Far East Alnus incana (L.) Moench subsp. incana (L.) Moench – speckled alder or grey alder. Eurasia, North America subsp. kolaensis (Orlova) Á.Löve & D.Löve subsp. rugosa (Du Roi) R.T.Clausen subsp. tenuifolia (Nutt.) Breitung Alnus japonica (Thunb.) Steud. – Japanese alder, Japan, Korea, Taiwan, eastern China, Russian Far East Alnus jorullensis Kunth – Mexican alder. Mexico, Guatemala, Honduras. subsp. lutea Furlow subsp. jorullensis Kunth Alnus lusitanica Vít, Douda, & Mandák - Spain, Portugal, Morocco Alnus matsumurae Callier – Honshū Island in Japan Alnus nepalensis D.Don – Nepalese alder. Himalayas, Tibet,