Alnus rubra

Red adler


The Basics

Taxonomy: Kingdom - Plantae (plants). Subkingdom - Tracheobionta (vascular plants). Superdivision - Spermatophyta (seed plants). Division - Magnoliophyta (flowering plants). Class - Magnoliopsida (dicotyledons). Subclass - Hamamelididae. Order - Fagales. Family - Betulaceae (birch). Genus -Alnus Mill. Species - Alnus rubra Bong.

Ecology: Red alder is the largest American alder. It is a rapidly growing, short-lived, medium-sized, deciduous tree, generally with one straight distinct trunk. Red alder is an early, nitrogen fixing, seral species. It quickly invades forest openings, such as those created from fires, logging, wind throws, or road cuts, and it also pioneers volcanic mud flows. Historical evidence suggests that the distribution of red alder was much more restricted than it is today; it occurred chiefly along streams and in other wet areas. Continual disturbance over the past 100 years, primarily from logging, has created an abundance of open areas with bare mineral soil which red alder has colonized, thus increasing its acreage dramatically. This is especially true of uplands, where it was previously infrequent.


Trees , to 28 m; trunks often several, crowns narrow or pyramidal. Bark gray, smooth, darkening and breaking into shallow rectangular plates in age; lenticels inconspicuous. Winter buds stipitate, ellipsoid, 6--10 mm, apex rounded, long; stalks 2--8 mm; scales 2--3, outer 2 equal and valvate, usually heavily resin-coated. Leaf blade ovate to elliptic, 6--16 3--11 cm, leathery, base broadly cuneate to rounded, margins strongly revolute, deeply doubly serrate or crenate, with distinctly larger secondary teeth, apex acute to obtuse; surfaces abaxially glabrous to sparsely pubescent. Inflorescences formed season before flowering and exposed during winter; staminate catkins in 1 or more clusters of 2--6, 3.5--14 cm; sistillate catkins in 1 or more clusters of 3--8. Flowering before new growth in spring. Infructescences ovoid to nearly globose, 1--3.5 0.6--1.5 cm; peduncles 1--10 mm. Samaras ovate or elliptic, wings much narrower than body, irregularly elliptic to obovate, leathery. 2 n = 28.


In the WFDP: Red alder is rare in the WFDP, with only ten trees above 1 cm at DBH that are tagged. Suppression is the likely cause of mortality of red alder in this energy-limited system. Red alder is classed as intolerant of shade; young seedlings can withstand partial shade for a few years but will grow very little.

Fire effects: Red alder's bark, although thin, is sufficiently fire resistant to protect trees from light surface fires. The foliage and leaf litter do not carry fires well. Red alder stands often lack flammable understory debris and are often on moist sites which burn infrequently. Red alder is an early seral species which quickly invades burned areas. Off-site plants inhabiting fire resistant draws and streambeds provide an abundance of seed, which reportedly can travel several hundred yards via wind. Thus red alder quickly colonizes soils exposed after forest fires.

Pests and pathogens: Red alder is fairly free from most insect and disease problems, especially when young (age 40 or 50) and uninjured. Phellinus igniarius, a white heart rot, is probably the major cause of cull in older trees.


Flowering and Fruiting - Red alder reaches sexual maturity at age 3 to 4 years for individual trees. It is generally monoecious, with separate male and female catkins developing on the previous year's twigs. Staminate catkins occur in pendulous clumps. In late winter they elongate, changing from green to reddish brown and from 2 to 3 cm long to about 7 or 8 cm. Pistillate catkins also occur in clumps but are borne upright (resembling tiny pine cones). They are 5 to 8 cm long and reddish green when receptive. Flowering occurs in late winter or early spring. Most alder seed is probably the result of outcrossing, but some selfpollination does occur.

Seed Production and Dissemination - Red alder is a prolific and consistent producer of seed. Moderate seed crops are produced almost annually and bumper crops occur every 3 to 5 years.

Seedling Development - Red alder germinates and grows well on moist mineral soil with full sunlight. The species is an aggressive pioneer on avalanche paths, road cuts, log landings, skid trails, or other areas where mineral soil has been freshly exposed to seed fall. Height growth of red alder seedlings is exceptionally rapid. On favorable sites, seedlings can grow 1 m or more the first year.

Vegetative Reproduction - Red alder sprouts vigorously from the stump when young. Older trees rarely sprout and coppice regeneration cannot be expected after polesize or saw-log-size material is harvested.

Species Distribution


Silvics of North America
Burns, R.M., and B.H. Honkala. 1990. Silvics of North America (Volume 1: Conifers, Volume 2: Hardwoods). USDA Forest Service Agricultural Handbook 654.

USDA Plants Database
USDA, NRCS. 2016. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.

USFS Plant Database
Habeck, R. J. 1992. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

Flora of North America
Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 19+ vols. New York and Oxford.

Burke Museum of Natural History and Culture
Burke Museum. 2016 [Online]. University of Washington.
Photo credit: Ben Legler 2004
Photo credit: M. R. Smith