Alders (Alnus spp.) are fast-growing deciduous trees and shrubs commonly found along rivers, wetlands, and moist forest edges throughout the Northern Hemisphere. Several species occur across North America and Europe, including red alder (Alnus rubra) of the Pacific coast, speckled alder (Alnus incana) of northern regions, and black alder (Alnus glutinosa) of Europe. These trees are easily recognized by their rounded leaves, small woody cone-like fruits, and early spring catkins that release pollen before most other trees have leafed out.

Many alder species thrive in wet or disturbed soils where few other trees grow well. Their roots host nitrogen-fixing bacteria that enrich the surrounding soil, making alder an important pioneer species in forest regeneration. Because of this ability, alders often colonize landslides, streambanks, and recently cleared areas.

The wood is relatively soft but valued for carving, furniture, and smoking foods. Medicinally, the bark has long been used for its astringent and cleansing properties, particularly for skin and digestive conditions. With their ecological importance and ability to improve soil fertility, alders play a key role in restoring and supporting healthy forest ecosystems.