Alnus rubra

Alnus rubra is a conical tree of the Betulaceae plant family that reaches up to 25m tall. The bark is thin, smooth and largely white in color due to lichen cover. The inner bark is reddish brown. As the bark ages, it breaks in to large plates. Being dioecious, this species has male and female plants producing catikins entirely of male or female flowers. The male catkins are borne in spring and hang 5 to 12cm from the branch tips with bracts subtending the 3 to 6 flowers, composed of a perianth and 3 to 4 stamens. The female catkins are also borne in spring in the form of a 2cm long cone with hardened, persistent scales each associated with two naked flowers and 2 to 3 bracteoles. Clusters of small, winged nutlets are formed in autumn in the female flower. The leaves are broad and elliptic in shape, measuring 5 to 15cm long and doubly-toothed margins that are slightly rolled. As a deciduous species, the alternately arranged leaves fall from the tree in autumn. The lower leaf surface is lighter in color than the upper and has rust colored hairs.

The geographic distribution of A. rubra has a northern boundary in southeastern Alaska and moves southward, staying west of the Cascade mountain range to central California at low elevations. Discrete populations have been observed in northern Idaho along the Clearwater River, where it is the main broad leaf tree on the region. Typically, A. rubra thrives in damp, rich, soils of riparian areas or recently distirbed areas. Often the trees form a pure stand.

A. rubra is a fast growing, short lived species and is the first to regenerate after landslides. The cover provided by A. rubra allow secondary conifers to establish. In addition to protection, A. rubra has an association with a Frankia, a Nitrogen fixing bacteria that invades the root hairs stimulating cell division and the growth of nodule. The Nitrogen fixed in these nodules enriches the soil for secondary colonizers.

The wood is often used for pulp, furniture and firewood. Native people of the Pacific Northwest found the wood useful for many things including smoking fish, making bowls, mask, rattle and dyes. As a food, the inner bark was palatable in the spring time and the bark was regarded as medicinal, treating tuberculosis, respiratory ailments and skin infections or wounds.

The common name of A. rubra is Red Alder, ‘alder’ is from Old English ‘alor’ and high German ‘elo’ or ‘elawer’ for reddish yellow referring to the color of freshly exposed wood.


Hitchcock, C.L. and A. Cronquist (1994) Flora of the Pacific Northwest, University of Washington Press, Seattle and London.

Pojar, J. and A. McKinnon (1994) Plants of the Pacific Northwest Coast. Lone Pine Publishing, Washington, Canada.

Rushforth, K. (2004) A Falcon Guide: The Easy Tree Guide. Falcon CT, MT.

Russel, T., C. Cutler and M. Walters (2006) The New Encyclopedia of American Trees. Hermes House, London.

Recent Posts