Tree Species of the Alnus Genus

Picture of a Alder Tree
Alnus incana

The genus Alnus (Alder) belongs to the birch family (Family Betulaceae). This genus is comprised of about 30 species of monoecious Alder trees (of which only a few reach a large size) and shrubs.

All Alder tree species are deciduous. They have flowers that are called catkins with elongated male catkins on the same plant as the shorter wider female catkins. They are often visible before leaves appear, and they are mainly wind pollinated. They differ from birches (Betula, the most similar genus) 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 Alder species are Red Alder (Alnus rubra) on the west coast of North America and Black Alder (Alnus glutinosa), native to most of Europe and widely introduced elsewhere, both reaching over 30m or 90ft tall. However, the widespread Green Alder (Alnus viridis) is more of a shrub than a tree, rarely reaching 5m or 15ft.

Facts about the Alnus Genus of Trees

• Genus Scientific Name = Alnus
• Genus Common Names = Alder
• Number of Species in the Alnus Genus = 32

List of Alder Trees, Alnus Genus - All known species, taxa on Earth, organized by scientific Latin botanical name first and common names second.