Red Alder (Alnus rubra)

KINGDOM Plantae
PHYLUM Anthophyta
CLASS Eudicotyledonae
ORDER Fagales
FAMILY Betulaceae


Alders are among the few higher plants that have the special ability to fix nitrogen, so they can take atmospheric N2 and convert it to ammonia (NH3), which then is available to be used in nucleotides and amino acids, basic building blocks of life. Thus these plants can grow on newly created soils that lack the nitrogenous compounds that act as natural sources of nitrogen for most plants. Of course, this nitrogen fixation would not be possible without symbiotic bacteria of the genus Frankia located in nodules on the alder roots.

Probably because of this ability, Red Alder is one of the most common trees in the forests of the Pacific Northwest, distributed from southeast Alaska to central California near the coast. It grows on most soils as an early-succession tree, although it is best developed in relatively moist areas, as along rivers. Because their seeds disperse readily, alders colonize newly exposed soil and can form monocultures.

Red Alders are the largest species of their genus, growing to 30 m tall. The trunk of one of these giants may be a meter in diameter. Alder bark is thin and gray, and although smooth, often has an abundant growth of lichens of several genera. When cut, the inner bark turns rusty red, thus giving the tree its name. The bark has been used to make a red dye, and it has been long valued for its medicinal properties, as it has antibiotic properties.

The leaves, up to 15 cm long, are bluntly serrated and with revolute (turned-under) edges. The leaves develop no bright autumn color but just get more and more brownish, then drop by midwinter, later than most of their associates. The alders are easily recognizable by these late-falling leaves and by the reddish catkins that cover the trees in spring before the leaves grow out again.

The flowers look nothing like our idea of flowers but are catkins, cylindrical spikes that hang down. The sexes are separate on the tree, the male catkins up to 12 cm long, the female about 2 cm. Male catkins shed pollen into the air in great abundance in spring, and female catkins receive the wind-borne pollen and mature into cone-like structures, not to be confused with conifer cones. The tiny winged seeds develop within these cones, which remain on the tree all winter and shed their seeds into the air in spring. Wind-blown like the pollen, or carried by water, the seeds colonize the surrounding countryside.

Alder wood is soft and even-grained and is used especially for firewood, but Native Americans have used it for a variety of small items such as bowls and rattles. The trees are not loved by foresters, as they grow more rapidly than the conifers planted in tree plantations and can outcompete them for sunlight and other resources. Herbicides directed toward dicotyledonous plants have been used to take care of this problem, but more recently the herbicides have been banned in some areas, and alders have been controlled by cutting them down at 6 to 10 years of age.