Red Alder Female Catkins in Autumn

Alnus rubra, the red alder, is a deciduous broadleaf tree native to western North America.

Alder is the common name of a genus of flowering plants (Alnus) belonging to the birch family (Family Betulaceae). The genus comprises about 30 species of monoecious trees and shrubs, few reaching large size, distributed throughout the North Temperate Zone and in the Americas along the Andes southwards to Argentina.

Betulaceae, or the Birch Family, includes six genera of deciduous nut-bearing trees and shrubs, including the birches, alders, hazels, hornbeams and hop-hornbeams, numbering about 130 species. They are mostly natives of the temperate Northern Hemisphere, with a few species reaching the Southern Hemisphere in the Andes in South America.

In the past, the family was often divided into two families, Betulaceae (Alnus, Betula) and Corylaceae (the rest); however, recent treatments, including the Angiosperm Phylogeny Group, have renamed these two groups as subfamilies within the Betulaceae- Betuloideae and Coryloideae.

The closest relatives of the Betulaceae are believed to be the Casuarinaceae, or the She-Oaks.

The Betulaceae are believed to have originated at the end of the Cretaceous period (c. 70 million years ago) in central China. This region at the time would have had a Mediterranean climate due to the proximity of the Tethys Sea, which covered parts of present-day Tibet and Xinjiang into the early Tertiary period. This point of origin is supported by the fact that all six genera and 52 species are native to this region, many of those being endemic. It is believed that all six modern genera had diverged fully by the Oligocene, with all genera in the family (with the exception of Ostryopsis) having a fossil record stretching back at least 20 million years from the present.