Alders

Alders are deciduous trees and shrubs belonging to the genus Alnus in the family Betulaceae. They are native to cool temperate regions of the Northern Hemisphere and are notable for their relationships with nitrogen-fixing bacteria in their root systems, which enhance soil fertility. This nitrogen fixation occurs in nodules formed with the actinobacteria Frankia, allowing alders to colonize nutrient-poor or disturbed sites.

Alders have simple, alternate leaves with finely serrated margins and a generally rounded to oval shape. They produce distinct catkins in late winter or early spring; male catkins are typically yellowish and feathery, while female catkins develop into small, woody cones containing the seeds. The bark is usually thin and can be smooth on young trees, becoming fissured with age.

Ecology and habitat: Alders commonly grow in moist to wet soils, along riverbanks, lake shores, and wetlands, and they often appear early in ecological succession. Their ability to fix nitrogen helps stabilize soils and supports the establishment of other plant species in riparian and disturbed habitats.

Notable species include Alnus glutinosa (European black alder), Alnus rubra (red alder) of western North America, and Alnus serrulata (tag alder) of eastern North America, among others. These species vary in range and wood characteristics but share the family traits of catkins, cones, and nitrogen-fixing roots.

Uses and management: Alder wood is relatively light and used for furniture, veneers, pallets, and plywood. Bark and leaves contain tannins and have historically been used in leather processing and traditional medicines. Alders are widely employed in restoration and erosion-control projects due to their soil-improving and rapid growth properties.