Sitka alder is a deciduous shrub or, rarely, a small tree. Male and female flowers occur on the same plant in catkins. Male and female flowers of Sitka alder occur in catkins on the same plant. The fruit is a small, single-seeded nutlet with wide lateral wings. The wind-dispersed seeds colonize bare soil created by disturbances such as fire, avalanches, soil slump, and retreating glaciers.
Seeds require a moist mineral soil for germination, which normally takes place in the spring.
Sitka alder plants can sprout from the root collar or stump when damaged. Sitka alder is generally found at middle to high elevations in the mountains of northwestern North America. These sites are often subject to deep winter snow accumulations and recurrent avalanches. The soils exposed by avalanches provide an ideal seedbed for Sitka alder seeds. Once established, its resilient wood, bowed growth form, and ability to resprout if stems are broken allow Sitka alder to withstand repeated avalanche destruction, which would normally kill other plants.