Alnus

Alder
Alnus serrulata (Tag Alder blossoms) Male blossom on right, last year's female blossom on left Johnsonville, South Carolina
Scientific classification
Kingdom: Plantae Division: Magnoliophyta Class: Magnoliopsida Order: Fagales Family: Betulaceae Genus: Alnus
Species
Many species, see text.

Alder, a genus of plants (Alnus) belonging to the birch family Betulaceae. The genus comprises about 30 species of shrubs or trees, 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.

Many species (how many?) of alder establish symbioses with nitrogen-fixing bacteria of the genus Frankia. This enhances the availability of nitrogen in the soil, and may favorably affect its fertility.

Classification

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

• Image of an alder

Referenced By