OregonFlora species comparison

Asia, Europe, North America, South America. ~25 species; 4 species treated in Flora.

Species of Alnus fix nitrogen in root nodules that contain the filamentous bacterium Frankia. This allows these trees to invade habitats where the soil is deficient in nitrogen compounds. Alder bark produces a red dye that has been used in Oregon and many other parts of the world. Alnus rhombifolia and A. rubra are often planted as fast-growing street trees. In addition to our native species, the European black alder, Alnus glutinosa, has been reported to have escaped in Oregon, but is not yet established. It is a small tree; its leaves are very broadly obovate or almost circular, truncate or broadly emarginate, sharply toothed, with very prominent tufts of hair in the axils of the secondary veins. Black alder has been known to escape in the eastern United States and eastern Canada, but this is the first report of this species from western North America.