These trees differ from the birches (Betula, another genus in the family) in that the female catkins are woody and do not disintegrate at maturity, opening to release the seeds in a similar manner to many conifer cones.

The genus is divided into three subgenera: Trees with stalked shoot buds, male and female catkins produced in autumn (fall) but stay closed over winter, pollinating in late winter or early spring, about 15–25 species, including: Trees or shrubs with stalked shoot buds, male and female catkins produced in autumn (fall) and expanding and pollinating then, three species: Shrubs with shoot buds not stalked, male and female catkins produced in late spring (after leaves appear) and expanding and pollinating then, one to four species: The status of the following species is unresolved:[citation needed] The following hybrids have been described:[4][5] The status of the following hybrids is unresolved:[4] The oldest fossil pollen that can be identified as Alnus is from northern Bohemia, dating to the late Paleocene, around 58 million years ago.

This bacterium is found in root nodules, which may be as large as a human fist, with many small lobes, and light brown in colour.

Red alder stands have been found to supply between 130 and 320 kilograms per hectare (120 and 290 pounds per acre) of nitrogen annually to the soil.

In addition, the light weight of alder seeds – numbering 1.5 million per kilogram or 680,000 per pound – allows for easy dispersal by the wind.

Groves of red alder 25 to 50 centimetres (10 to 20 in) in diameter intermingle with young Douglas-fir forests west of the Cascades, attaining a maximum height of 30 to 33 m (100 to 110 ft) in about sixty years and then are afflicted by heart rot.

[14] Some Native American cultures use red alder bark (Alnus rubra) to treat poison oak, insect bites, and skin irritations.

Blackfeet Indians have traditionally used an infusion made from the bark of red alder to treat lymphatic disorders and tuberculosis.

Recent clinical studies have verified that red alder contains betulin and lupeol, compounds shown to be effective against a variety of tumors.

[15] The inner bark of the alder, as well as red osier dogwood, or chokecherry, is used by some Indigenous peoples of the Americas in smoking mixtures, known as kinnikinnick, to improve the taste of the bearberry leaf.

Alder is appreciated for its tone that is claimed to be tight and evenly balanced, especially when compared to mahogany, and has been adopted by many electric guitar manufacturers.