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 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.

The common name alder is derived from an old Germanic root. The botanic name Alnus is the original Latin name. Both the Latin and the Germanic derive from the proto-Indo-European root el-, meaning 'red, brown,' which is also the ultimate root for the English words elk and elm --a tree more distantly related to the alders.

Classification
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:

• Alnus acuminata - Andean Alder. Andes Mountains, South America.
• Alnus cordata - Italian Alder. Italy.
• Alnus cremastogyne
• Alnus glutinosa - Black Alder. Europe.
• Alnus incana - Grey Alder. Europe & Asia.
• Alnus oblongifolia (A. incana subsp. oblongifolia) - Arizona Alder. Southwestern North America.
• Alnus rugosa (A. incana subsp. rugosa) - Speckled Alder. Northeastern North America.
• Alnus tenuifolia (A. incana subsp. tenuifolia) - Thinleaf Alder or Mountain Alder. Northwestern North America.
• Alnus japonica - Japanese Alder. Japan.
• Alnus jorullensis - Mexican Alder. Mexico, Guatemala.
• Alnus nepalensis - Nepalese Alder. Eastern Himalaya, southwest China.
• Alnus orientalis - Oriental Alder. Southern Turkey, northwest Syria, Cyprus.
• Alnus rhombifolia - White Alder. Interior western North America.
• Alnus rubra - Red Alder. West coastal North America.
• Alnus serrulata - Hazel alder, Tag Alder or Smooth alder. Eastern North America.
• Alnus subcordata - Caucasian Alder. Caucasus, Iran.

Subgenus Clethropsis. Trees or shrubs. Shoot buds stalked. Male and female catkins produced in autumn (fall) and expanding and pollinating then. Three species:

• Alnus formosana -Formosan Alder Taiwan
• Alnus maritima - Seaside Alder. East coastal North America, plus disjunct population in Oklahoma.
• Alnus nitida - Himalayan Alder. Western Himalaya.

AlderSubgenus 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:

• Alnus viridis - Green Alder. Widespread:
• Alnus viridis subsp. viridis. Eurasia.
• Alnus viridis subsp. maximowiczii (A. maximowiczii). Japan.
• Alnus viridis subsp. crispa (A. crispa). Northern North America.
• Alnus viridis subsp. sinuata (A. sinuata, Sitka Alder or Slide Alder). Western North America, far northeastern Siberia.

Uses
Alnus serrulata (Tag Alder), female catkins, Johnsonville, South CarolinaAlders 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 species, see list of Lepidoptera which feed on Alders. Alders are also grown in gardens, and are sometimes made into 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.

Alder 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.

Alders are also exeptionally good windbreakers and are planted on the west coast of Scotland to shelter gardens.