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Alder, a genus of plants (Alnus) belonging to the birch family Betulaceae. The genus comprises about 30 species of trees and shrubs, few reaching a large size, distributed throughout the North Temperate zone, and in the New World also along the Andes southwards to Chile. This deciduous genus has simple, serrate leaves and is monoecious with elongate male catkins on the same plant as shorter female flowers, often before leaves appear. 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 best-known species is the Common or Black alder (A. glutinosa), native to most of Europe and introduced elsewhere. The largest species is Red alder (A. rubra), known to 40m tall on the west coast of North America, with Black alder and Italian alder (A. cordata) both reaching about 30m. By contrast, the widespread Green alder (A. viridis) is rarely more than a 5m shrub.
Alders establish symbioses with the nitrogen-fixing Actinomycete fungus Frankiella alni. This fungus converts atmospheric nitrogen into soil-soluable nitrates which can be utilised by the alder, and favorably enhances the soil fertility generally.
Classification
The genus is divided into three subgenera:
Subgenus Alnus. Trees. Shoot buds stalked. Male & female catkins produced in autumn (fall) but staying closed over winter, pollinating in late winter or early spring. About 25-30 species, including:
- A. cordata - Italian alder. Italy.
- A. glutinosa - Black alder. Europe.
- A. incana - Grey alder. Europe & Asia.
- A. oblongifolia (A. incana subsp. oblongifolia) - Arizona alder. Southwestern North America.
- A. rugosa (A. incana subsp. rugosa) - Speckled alder. Northeastern North America.
- A. tenuifolia (A. incana subsp. tenuifolia) - Thinleaf alder or Mountain alder. Northwestern North America.
- A. japonica - Japanese alder. Japan.
- A. jorullensis - Mexican alder. Mexico, Guatemala.
- A. orientalis - Oriental alder. Southern Turkey, northwest Syria, Cyprus.
- A. rhombifolia - White alder. Interior western North America.
- A. rubra - Red alder. West coastal North America.
- A. serrulata - Hazel alder, Tag alder or Smooth alder. Eastern North America.
- A. subcordata - Caucasian alder. Caucasus, Iran.
Subgenus Clethropsis. Trees or shrubs. Shoot buds stalked. Male & female catkins produced in autumn (fall) and expanding and pollinating then. Three species:
- A. maritima - Seaside alder. East coastal North America, plus disjunct population in Oklahoma.
- A. nepalensis - Nepalese alder. Eastern Himalaya, southwest China.
- A. nitida - Himalayan alder. Western Himalaya.
Subgenus Alnobetula. Shrubs. Shoot buds not stalked. Male & female catkins produced in late spring (after leaves appear) and expanding and pollinating then. One to four species:
- A. viridis - Green alder. Widespread:
- A. viridis subsp. viridis. Eurasia.
- A. viridis subsp. maximowiczii (A. maximowiczii). Japan.
- A. viridis subsp. crispa (A. crispa). Northern North America.
- A. viridis subsp. sinuata (A. sinuata, Sitka alder). Western North America.
Uses
Alder bloom is one of the first sources of pollen and nectar for bee species, especially honeybees, which use it for spring buildup. 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.
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