Alder

Alder is the common name for trees and shrubs in the genus Alnus, a group within the birch family, Betulaceae. About 30 species occur in the boreal and temperate regions of the Northern Hemisphere, including Europe, Asia, and North America. Alders are typically found in damp habitats such as riverbanks, wetlands, and lake shores, where they often play a role in soil stabilization and ecosystem recovery.

Alders are usually medium-sized trees or large shrubs with oval to lanceolate leaves that have serrated margins. Leaves are alternately arranged and turn yellow in autumn in many species. The bark is often smooth and grayish in young trees, becoming fissured with age. Alders are monoecious, flowering in early spring with slender catkins. Male catkins are longer and yellowish, while female catkins are shorter and develop into small woody cones that release seeds.

A defining ecological trait of alders is their symbiotic nitrogen fixation. Alder roots host nitrogen-fixing bacteria from the genus Frankia in nodules, allowing alder stands to improve soil fertility and pioneer disturbed or waterlogged sites. Alders can also form ectomycorrhizal associations, enhancing nutrient uptake.

Uses of alder wood include its light weight, workability, and pale color, making it suitable for furniture, veneer, plywood, and pulp for paper. Alder grows are also valued for soil stabilization projects and habitat restoration, particularly along riparian corridors. Conservation status varies by species and region, with some being common and others limited by habitat loss or environmental changes.