Facts About the Alder Tree

Every tree has its own magic, but few give more back to the forest community than the alder (Alnus spp.). This remarkable tree provides shade and protection for wildlife, stabilizes river banks with its deep roots and also fixes nitrogen in the soil thanks to a symbiotic relationship with a bacterium.

The genus Alnus unites about 30 species of alders ranging from small shrubs to the giants: black alders (Alnus glutinosa) of Europe that can grow to 80 feet or more and equally tall red alders (Alnus rubra) and white alders (Alnus rhombifolia), native to the U.S. Both male and female alder flowers are catkins, appearing on the trees early in springtime before the leaves. In time, the flowers give way to the fruit, woody cones that hang on the tree's branches like small lanterns.

Black alders began their stay on the planet in Europe, but currently also grow in North America in U.S. Department of Agriculture plant hardiness zones 3 through 7. They sport dark brown, warty bark and can grow one or many trunks. Although black alders like wet soils, they are also drought-tolerant and grow quickly in either sun or shade. Black alders propagate so readily that the tree has naturalized along river banks and in swampy areas in the Eastern U.S. The species shades out native trees and is considered invasive.

White alders also prefer to keep their roots in the water and like to grow by rivers or streams. Given the proper location, the tree shoots up some 3 feet a year to reach 80 feet or taller in USDA zones 8 through 11. This alder's straight trunk is a pale, attractive silver, and its branches are slender, dipping at the tips. It is an easy-care tree that generally needs neither fertilizer nor pruning when cultivated.

If you counted every hardwood tree in the Pacific Northwest, you would find that red alder comes in first on the list. This tree -- also known as Oregon alder or western alder -- grows fast when young if it gets sufficient sunshine. This makes it a successful pioneer tree on disturbed land in USDA zones 4 through 8. A seedling can shoot up more than 3 feet in its very first year. Recognize the red alder by its deep mahogany bark.

The most remarkable thing about the alder is its relationship with the nitrogen-fixing bacterium Frankia alni. Living in the tree's roots, the bacterium takes in nitrogen from the air, making it available to the alder and fixing it in the soil; in exchange, it uses the tree's carbon. In this way, an alder tree increases the soil's fertility and provides nutrients to assist all future plant species in the area.