Red Alder (Alnus rubra) is a deciduous tree and is the world's largest species of alder, reaching heights of 20 - 35 m. The name derives from the bright rusty red color that develops in bruised or scraped bark.
Red Alder has ovate leaves 7 - 15 cm long, with bluntly serrated edges and a distinct point at the end. The leaves turn yellow in the autumn before falling. The bark is mottled, ashy-gray and smooth, often draped with moss. The male flowers are dangling reddish catkins 10 - 15 cm long in early spring, and female flowers are erect catkins which develop into small, woody, superficially cone-like oval dry fruit 2 - 3 cm long. The seeds develop between the woody bracts of the 'cones' and are shed in the autumn and winter.
In moist forest areas Red Alder will rapidly cover a former burn or clearcut, temporarily preventing the growth of conifers but also improving soil fertility for future growth of conifers. It is a prolific seed producer, but the seeds require an open area of mineral soil to germinate, and so skid trails and other areas disturbed by logging or fire are ideal seedbeds. Such areas may host several hundred thousand to several million seedlings per hectare in the first year after landscape disturbance (Zavitkovski & Stevens 1972).
Twigs and buds of alder are only fair browse for wildlife, though deer and elk do browse the twigs in fall and twigs and buds in the winter and spring. Beavers eat the bark. Several finches eat alder seeds, notably Common Redpoll and Pine Siskin, and as do deer mice.
Red Alder is also very valuable for playing host to the nitrogen fixing actinomycete Frankia. It is this ability which allows alder to grow in nitrate-poor soils.
Uses and cultivation
Red Alder is an important forestry tree. Its rapid growth makes it useful in covering disturbed land, such as mine spoils. Alder leaves, shed in autumn, decay readily to form a nitrogen-enriched humus.