red alder (Trees & Shrubs of the Sunol Region

red alder

Alnus rubra

Summary ⁷

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

Description ⁸

General: Birch family (Betulaceae). Red alder is a deciduous tree native to the Pacific Northwest of North America. The trees are medium-sized, reaching various heights from 15 to 30 meters tall when mature. These fast-growing trees often grow 1 meter per year until 20 years of age. The trees can live to 100 years of age with trunks from 36 to 46 cm in diameter. A shrub form occurs when the trees grow in open exposed areas. The branches are slender and spreading. The thin bark is generally smooth, ashy gray to grayish-brown, and is usually covered with white lichens as it ages. The inner bark is reddish brown.

The alternately arranged leaves are dark green, simple and broadly ovate. The leaves are 6 to 15 cm long with a pointed tip. The leaf edges are serrated or softly lobed and slightly rolled under, giving a dark-green edging effect from the underside of the leaf. The undersides of the leaves are rusty colored and covered with fine soft hairs.

The trees are monoecious, bearing both female flowers and male catkins. The tassel-like catkins grow in cluster of two to four. The catkins are greenish-yellow and 10 to 16 cm long. The flowers appear in spring either before or with the leaves. The flowers develop into small-scaled cones (fruits) that are 2 to 2.5cm long and 1 to 1.5 cm across. Each cone contains from 50 to 100 seeds that are tiny flat nutlets. The abundant seeds are wind dispersed from May to winter months.

Red alder trees invade clearings or burned-over areas and forms temporary forests (Grimm 1967). Over time, red alders build up the soil with their copious litter, and enriched it with nitrogen compounds formed by symbiotic bacteria that live in little nodules on their roots. Red alder stands are eventually succeeded by Douglas fir, western hemlock, and sitka spruce.

Distribution: Red alder is most often observed in moist areas within 200 kilometers of the Pacific Coast of North America from Alaska to Southern California at elevations below 762 meters (Uchytil 1989). It also occurs along streams and lakes from the Yukon Territory and British Columbia south through the Rocky Mountain region to Colorado and New Mexico, and along Sierra Nevada to Lower California (Britton 1908). Red alder has spread to upland areas since European contact because of increased disturbance, such as logging, which opens up sites for colonization.

For current distribution, please consult the Plant Profile page for this species on the PLANTS Web site.

Habitat: Red alder trees are often associated with mixed evergreen forests and redwood forests in coastal areas. The trees grow in riparian forests along streams, in swamps and in marshy areas.