Alder leaves are deciduous (not evergreen), alternate, simple, and serrated. The flowers are catkins with elongate male catkins on the same plant as shorter female catkins, often before leaves appear; they are mainly wind-pollinated, but also visited by bees to a small extent. They differ from the birches (Betula, the other 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.
Alders establish symbioses with the nitrogen-fixing Actinobacteria Frankiella alni. This bacteria converts atmospheric nitrogen into soil-soluble nitrates which can be utilised by the alder, and favorably enhances the soil fertility generally. Alders benefit other plants growing near them by taking nitrogen out of the air and depositing it in the soil in usable form; fallen alder leaves make very rich compost.
Alders are sturdy and fast-growing, even in acidic and damaged sites such as burned areas and mining sites. Italian Alder is particularly useful on dry, infertile sites. Alders can be used as a producer of simple bio-mass, growing quickly in harsh environments.
Alder catkins are one of the first sources of pollen for bee species, especially honeybees, which use it for spring buildup. Alders are also used as a food plant by some Lepidoptera (butterfly and moth) species, see list of Lepidoptera that feed on alders. Alders are also grown in gardens, and are sometimes made into bonsai.
Alder lumber is an important fast growing renewable resource used in a wide range of applications including furniture, cabinets, panel systems and special products. Alder has a fine close-grained structure and is easily stained and finished. Part of the popularity of Alder lumber is due to the unusually wide range of grades that are available. Each grade is optimized for a functional and/or economic aspect of a product, with visible surfaces, structural components and hidden parts typically using different grades. The grade spectrum, grade photos, technical definitions and an analysis of the features that make up the various grades can be found at the Alder Lumber Gradessites noted in the References and External Links section below. These sites also provide a good look at the process of optimizing Alder Lumber Productionas part of the Sustainable Forestry Initiative (SFI).
Alder is popular as a material for electric guitar bodies. It is used by many guitar makers, notably the Fender Guitar Company, who use it on top quality instruments such as the Stratocaster and Jaguar. Alder provides a brighter tone than other woods (such as mahogany), and as alder is not a particularly dense wood it provides a resonant, well-rounded tone with excellent sustain. Alder is also occasionally used to make harps, although this is a rarity.
Alder is a preferred wood for charcoal making, formerly used in the manufacture of gunpowder, or for smelting metal ores, now used primarily for cooking. The wood is also traditionally used for smoking fish and meat, though this usage has often been replaced by other woods such as oak and hickory.
An exception is the smoked Pacific salmon industry in the Pacific Northwest, where alder smoking is essentially universal. This is partly due to indigenous traditions of food preservation in the area, and partly because oak, hickory, mesquite and other woods favored for smoking elsewhere are not locally available in any large quantities. Species used for Pacific salmon smoking are Red alder A. rubra and to a lesser extent Sitka alder A. viridis ssp. sinuata.
Alders are also exceptionally good windbreakers and are planted on the west coast of Scotland to shelter gardens.
In the 17th century it is recorded that in Ayrshire that alder bark was collected and used for tanning.
Alders have root nodules which contain Frankia sps., actinomycete filamentous nitrogen-fixing bacteria. These nodules are found on the roots and may be as large as a human fist, with many small lobes and light brown in appearance.
Alder bark contains the anti-inflammatory salicin which is metabolized into salicylic acid in the body. Native Americans used Red Alder bark (Alnus rubra) to treat poison oak, insect bites, and skin irritations. Blackfeet Indians 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.
Subgenus Alnus. Trees. Shoot buds stalked. Male and female catkins produced in autumn (fall) but staying closed over winter, pollinating in late winter or early spring. About 15-25 species, including:
Subgenus Clethropsis. Trees or shrubs. Shoot buds stalked. Male and female catkins produced in autumn (fall) and expanding and pollinating then. Three species:
Subgenus Alnobetula. Shrubs. Shoot buds not stalked. Male and female catkins produced in late spring (after leaves appear) and expanding and pollinating then. One to four species: