Alnus

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Alnus

Alders
Genus:Alnus
Family:Betulaceae
Pollination:Wind

Alders (Alnus) are trees and shrubs in the birch family (Family Betulaceae). The genus comprises about 20-30 species of monoecious trees and shrubs, few reaching large size, distributed throughout the North Temperate zone, and in the New World also along the Andes southwards to Chile. The common name alder is derived from an old Germanic root. The botanic name Alnus is the original Latin name.

The best-known species in Europe is the Common or Black Alder (A. glutinosa), native to most of Europe and widely introduced elsewhere. The largest species is Red Alder (A. rubra), reaching 35 m (the tallest is 32 m) on the west coast of North America, with Black Alder and Italian Alder (A. cordata) both reaching about 30 m. By contrast, the widespread Green Alder (A. viridis) is rarely more than a 5 m shrub.

Description

The leaves are deciduous, alternate, simple, with serrated margins and pinnate veination. Trunks of most species corded, stems thin, flexible, and triangular in cross-section.

The flowers are catkins with elongate male catkins on the same plant as the shorter female catkins, often before leaves appear; they are mainly wind-pollinated, but also visited by bees as a pollen source. 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. The "cones" are persistent.

Seeds are small winged samaras.

Growing Conditions

Sun to shade, adapted to poor soils, and most species do well in wet soils.

Species

The genus is divided into three subgenera:

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:

Leaves of the Tag Alder

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:

Uses

Alnus serrulata (Tag Alder), female catkins, Johnsonville, South Carolina

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 earliest available sources of pollen for bee species in the spring, especially honeybees, which use it for spring buildup.

Alders are grown for their bark, form, and for the catkins, which are used in dry floral arrangements. Alders can also be grown as bonsai.

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. It is popular as a material for electric guitar bodies, and 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 it is not a particularly dense wood it provides a resonant, well-rounded tone with excellent sustain.

Maintenance

Alders can be pruned either to encourage a tree-like habit, or coppiced as a dense shrub.

Propagation

By cuttings and seed, but cultivars may be budded or grafted.

Seed is collected in autumn as the catkins open, then cold-stratified for spring germination.

Pests and Diseases

Powdery Mildew

Leaf Curl

Flower Distortions (Galls)

Canker (branch dieback)

Canker (trunk)

Rust

Root Rot

Aphids

Scales

Psyllids

Bugs

Beetles

Caterpillars

Sawflies

Mites

References

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