Little, Elbert L., Jr. U.S. Department of Agriculture, Washington DC.
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Any of the deciduous shrubs and trees of the genus Alnus in the birch family (Betulaceae). There are approximately 30 species of alder, with 10 in the United States. Alders are widespread in cool north temperate regions and are also found southward in the Andes of South America. They have a smooth gray bark; elliptical or ovate saw-toothed leaves in three rows; male flowers in long catkins, mostly in early spring; and clusters of several dry, hard, ellipsoid blackish fruits that are 0.5–1 in. (1.25–2.5 cm) in length. The fruits are conelike and present throughout the year. Alders are common in wet soils, such as stream borders. They often form thickets even beyond treelines and are pioneers on bare areas (including landslides and roadsides) and after fire or logging. As in legumes, the roots bear swellings or nodules containing nitrogen-fixing bacteria, which enrich the soil. See also: Fagales; Nitrogen fixation; Nitrogen-fixing trees
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