Olympic Habitat Development Study


Station 10 -
Red Alder
(Alnus rubra)
Family Betulaceae

Red alder is the most common hardwood tree species in the Pacific Northwest.  Its fast juvenile growth is greater than that of many associated conifers but it also slows down in height growth sooner in its life so the conifers often overtake it in height.  It has a much shorter life span (60-100 years) than its conifer associates.  Alder is unique among our northwest tree species in that it has nodules on its roots which can fix nitrogen from the air into forms that the plant can use.  Because it has a ready source of nitrogen, it doesn’t recycle nitrogen from its leaves and most leaves that drop in the fall are still green.  These leaves are an important fall food source for deer and elk. Alder serves other ecological functions by adding nitrogen to soil via root exudates and decomposition of fine roots, leaves, and branches.

Red alder’s outer bark is relatively plain brown or grayish brown, however, as the tree ages, it develops a mosaic-like pattern of white and grey blotches. These crusty blotches are not the tree bark itself but lichens from different genera: (a genus (plural - genera) is a group of species with related characteristics) Graphis, Arthothelium, Thelotrema, etc.  Alder bark provides an important substrate (growing surface) for many of these species. The presence of these lichens is an important bio-indicator of air quality, since many lichens are highly sensitive to air pollution. Alder trees growing near a pulp mill or coal powered plant will have often have gray-brown bark rather than the more typical lighter gray and white appearance of trees in less polluted areas.

Red alder’s lightweight seeds are primarily dispersed by wind and they can fly long distances. This sun-loving tree can be restricted to wetlands and riparian habitat in areas with primarily older conifer forests, but it can quickly colonize into disturbed landscapes, becoming invasive in some locations.  Its seedlings establish quickly on mineral-poor soils (including recently deposited soil on flood plains or avalanche paths) and after disturbances like fire or clear-cutting logging.

Mountain beaver (Aplodontia rufa) often clip small stems and branches.  Ruffed grouse (Bonasa umbellus)eat catkins and buds, while red-breasted sapsucker (Sphyrapicus ruber) sometimes can cause considerable damage to some individual trees by drilling holes in trees (to eat the sap and insects drawn to the sap).. Stands of red alders in the Pacific Northwest also can suffer from periodic outbreaks of tents caterpillars of the genus Malacosoma.

Commercially speaking, this abundant tree is considered the most important hardwood in the Pacific Northwest.  Its wood is used for furniture, flooring and cabinetry, and is burned for fuel as well as smoking salmon.  The name (red) originates from its inner bark (and wood) that turns bright orange or red when bruised or peeled but the male catkins (which contain the tree pollen) are also red and are quite visible when they elongate in the spring before the leaves come out. The inner bark was traditionally used by Native Americans as a dye for baskets and fishnets.  The bark was also eaten or used in a tonic for medicinal purposes for headaches, digestive issues, poison-oak rashes, skin infections, and tuberculosis.

Alder can be found as a component in conifer stands of all ages but it becomes much less common as stands age and the conifers outgrow it in height and shade out this sun-loving species.  Thinning, especially the creation of gaps will keep alder (and other hardwood species and shrubs) as longer-lived components in the stand.  The presence of hardwoods is often associated with increased biodiversity in conifer ecosystems as many plants and animals depend on the habitat it creates (alder bark, high nitrogen levels) or is associated with (higher light levels under alder leaves than conifers).


Alder seeds are found in small conelets called strobili.
Alder bark is host to many species of lichens. Alder seeds are found in small conelets called strobili. Alder tree growing inside a gap.
Alder seeds are found in small conelets called strobili.