Red Alder: a Forest Pioneer

Published by Lisa Siddons on 02/05/2025 02/05/2025

Red alder (Alnus rubra) is a deciduous tree that grows abundantly in the Pacific Northwest. A tree native to the west coast, their range extends from California to Alaska, and they grow particularly well in British Columbia’s climate. In the past this species was thought of as being over abundant, but recently people have begun to realize their value.

Benefits of Red Alder

Red alders grow in wetter sites, and are adapted to grow in areas with nutrient-poor soils that other species would not be able to tolerate. They are considered a “pioneer” species because of their ability to grow in recently disturbed sites. Pioneer species, sometimes referred to as “early seral stage” species, typically grow and reproduce quickly, thriving in full sun and loose soils. Sites that have been cleared, either by natural processes such as flooding, landslide, or fire, or by human disturbances such as logging, are excellent habitat for red alder.

Red alder improves and builds soils, preparing the way for other native species. They are nitrogen fixers, meaning they can convert nitrogen from the atmosphere into a form available to plants. They can’t do this on their own, but have developed a mutually beneficial partnership with a type of bacteria that does it for them in exchange for sugar, which the alder makes through photosynthesis. Because red alders are fast growing, their canopies shoot up quickly. This leafy canopy then provides shelter from extreme conditions for young slower growing conifers, and prevents sun-loving introduced species like Himalayan blackberry from gaining a foothold.

Alder trees also provide habitat for many species of moss and lichen. Red alder bark is brown, reddish, or greyish, but it often appears to be whitish and patchy due to the presence of lichens growing on the bark as epiphytes (a plant that grows on another plant but is not parasitic).

Life Cycle

In February in our area, red alder “flowers” appear as early harbingers of spring. The flowers are tiny and arranged in elongated clusters called catkins. Male catkins are yellow and dangly, and release pollen. Female catkins are green and oval-shaped. After fertilization, seeds develop in the female catkin, which becomes a small woody cone that stays on the tree over the winter.

As trees go, red alder is a relatively short-lived species, surviving 40-60 years. As the trees die and begin rotting, they provide excellent nesting habitat for cavity nesting birds. When eventually they fall and decompose on the forest floor, they contribute organic material and nitrogen to the soil.

Use in Restoration

The Conservancy often uses red alder in restoration projects because its fast growth and nitrogen fixing abilities help restore areas disturbed by clearing back to healthy native forests.

Interested in joining our habitat restoration team? Learn how to plant young alders and other native plants, remove invasive species, and more! Tools and treats provided. Volunteer here (long-term commitment not required – come when you can).

References

1. Green Timbers Heritage Society. (n.d.). Red Alder – Fagales Betulaceae Alnus rubra. Retrieved from https://www.greentimbers.ca/vegetation-wildlife/vegetation/deciduous-trees/red-alder/
2. Forest Stewardship Notes. (2018). Retrieved from https://foreststewardshipnotes.wordpress.com/2018/10/15/tree-profile-red-alder/