Alnus Mill. (1754), the Alder, consists of around 25 species around the northern hemisphere and southwards to Argentina, in the Betulaceae.

Description Trees to 30m, and shrubs, with often dark brown twigs and bark; winter buds usually with short stalks. The leaves are deciduous, alternate, often rounded to obovate, and usually toothed. The flowers are unisexual, with males and females on the same tree. The males are catkins, formed in autumn, and remaining exposed through the winter, opening in early spring or, in a few species, in autumn; there are 3 flowers per scale, each with 4 stamens, often reddish anthers, and undivided filaments. Female flowers are usually 2 per bract. Pollination is by wind. The fruits are in cone-like heads, with woody, scale-like bracts and small, narrowly winged seeds, dispersed by wind and water.

Key Recognition Features The often rounded leaves, the exposed winter catkins, and the cone-like fruits.

Evolution and Relationships Wind specialisation is taken a stage further in alders than in the larger-fruited genera of the family, putting them closest to Betula. Alnus is a diverse genus, some species, such as the Japanese A. firma Sieb. & Zucc., having hornbeam-like leaves. Two groups flower in autumn: the subtropical subgenus Cremastogyne, and subgenus Clethropsis, which include the common Himalayan species A. nepalensis D. Don and the rare North American relict species A. maritima (Marsh.) Muhlenb. ex Nutt.

Ecology and Geography Usually near water, on river banks, in wet woods, and on lake shores. Eight species are found in North America, 5 in Europe, and 10 in Japan. One species, A. acuminata Kunth, is found through the Andes into South America.

Comment Alders are successful in open, bare areas such as were left after the retreating glaciers, on gravel banks in rivers, or on any bare soil close to water. All species can fix atmospheric nitrogen through actinomycete-filled cells in their roots, which may appear red and swollen, and these also help them to colonise wet, infertile ground. Alder wood was used in the past for piles, as it is resistant to rotting underwater, and is found both in old Amsterdam and in the Rialto in Venice. The wood was also used for violins and for carving. Alnus cordata Desf., the Italian alder, is fast-growing and much planted for shelter belts, especially since it keeps its leaves into early winter.