Alder, a tree or shrub belonging to the genus Alnus. There are about 35 species. Alders are related to the birches and are found primarily in the Northern Hemisphere. They range in height from 4 to 100 feet (1.2 to 30 m). Six of the nine species native to North America are trees.

The majority of alders grow best in moist soils and are planted primarily for ornamental purposes. Most alders have oval leaves, about six inches (15 cm) long, with toothed edges. The leaves are shed annually without changing color. The female catkins (elongated flower clusters) develop into woody cones that contain the seeds. The cones are about one-half to one inch (1.3 to 2.5 cm) long. Alders can be raised from seeds or propagated from wood cuttings.

In the United States, the most important alder is the red alder, which grows along the Pacific coast from southeastern Alaska to southern California. The red alder is often mistaken for a birch because, like the birch, it has smooth, light gray or white bark. The red alder is a major source of hardwood on the Pacific coast. The wood is used mainly to make furniture, pallets, and paper.

The red alder is Alnus rubra, or A. oregona. Alders belong to the family Betulaceae.