Alnus rubra (red alder) is a deciduous broadleaf tree native to western North America.
It is the largest species of alder in North America and one of the largest in the world, reaching heights of 20–35 m. The official tallest red alder (1979) stands 32 meters tall in Clatsop County, Oregon (USA). The name derives from the bright rusty red color that develops in bruised or scraped bark. The bark is mottled, ashy-gray and smooth, often draped with moss. The leaves are ovate, 7–15 cm long, with bluntly serrated edges and a distinct point at the end; the leaf margin is revolute, the very edge being curled under, a diagnostic character which distinguishes it from all other alders. The leaves turn yellow in the autumn before falling. 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.
Alnus rubra grows from southeast Alaska south to central coastal California, nearly always within about 200 km of the Pacific coast, except for an extension 600 km inland across northern Washington into northernmost Idaho.
In southern Alaska, western British Columbia and the northwestern Coast Ranges of the USA, red alder grows on cool and moist slopes; inland and at the southern end of its range (California) it grows mostly along the margins of watercourses and wetlands.
Commonly associated trees
Red alder is associated with coast Douglas-fir Pseudotsuga menziesii subsp. menziesii, western hemlock Tsuga heterophylla, grand fir Abies grandis, western redcedar Thuja plicata, and Sitka spruce Picea sitchensis forests.
Along streambanks it is commonly associated with willows Salix spp., red osier dogwood Cornus stolonifera, Oregon ash Fraxinus latifolia and bigleaf maple Acer macrophyllum
In marginal habitat
To the southeast of its range it is replaced by white alder (Alnus rhombifolia), which is closely related but differs in the leaf margins not being rolled under. In the high mountains it is replaced by the smaller Sitka alder (Alnus viridis subsp. sinuata), and east of the Cascade Mountains by thinleaf alder (Alnus incana subsp. tenuifolia).
As pioneer species
In moist forest areas Alnus rubra 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).
Role as wildlife fodder
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.
As soil enricher
Alnus rubra 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.