Alder is the common name of a genus of flowering plants (Alnus) belonging to the birch family (Family Betulaceae). The genus comprises about 30 species of monoecious trees and shrubs, few reaching large size, distributed throughout the North Temperate Zone and in the Americas also along the Andes southwards to Argentina.

Description

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.

The largest species are Red Alder (A. rubra) on the west coast of North America and Black Alder (A. glutinosa), native to most of Europe and widely introduced elsewhere, both reaching over 30 m. By contrast, the widespread Green Alder (A. viridis) is rarely more than a 5 m tall shrub.

Uses

Environmental uses

As soil enrichers

Alders establish symbioses with the nitrogen-fixing Actinobacteria Frankiella alni. This bacteria converts atmospheric nitrogen into soil-soluble nitrates which can be utilized by the alder, and favorably enhances the soil fertility. Alders benefit other plants growing near them by taking nitrogen out of the air and depositing it into the soil in usable form; fallen alder leaves make very rich compost.

As pioneer species

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.

As wildlife fodder

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.

As a shelterbelt

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

Commercial uses

In cabinetry

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.

Alders are also grown in gardens, and are sometimes made into bonsai .

In musical instrumentation

Alder is popular as a material for electric guitar bodies, used by many guitar makers such as Fender and Jackson. 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.

In industry

Alder is a preferred wood for charcoal making, formerly used in the manufacture of gunpowder, or for smelting metal ores.

In fish smoking

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.

As a dye

In the 17th century it is recorded that in Ayrshire that alder bark was collected and used for tanning.

Nitrogen fixation

Alder is particularly noted for its important symbiotic relationship with Frankia alni, actinomycete filamentous nitrogen-fixing bacterium. This bacterium is found in root nodules which may be as large as a human fist, with many small lobes and light brown in appearance. The bacterium absorbs nitrogen from the air and makes it available to the tree. Alder, in turn, provides the bacterium with carbon, which it produces through photosynthesis. As a result of this mutually-beneficial relationship, alder improves the fertility of the soils where it grows, and as a pioneer species, it helps provide additional nitrogen for the successional species which follow.

Word origin

The common name alder is derived from an old Germanic root. Alsofound to be the translation of the Old French "verne" for alder or copse of alders. The botanic name Alnus is the equivalent Latin name. Both the Latin and the Germanic words derive from the Proto-Indo-European root el-, meaning "red" or "brown", which is also a root for the English words elk and another tree: elm, a tree distantly related to the alders.

Edibility and medicinal uses

Alder catkins are edible and high in protein. Although they are reported to have a bitter and unpleasant taste, they are best remembered for survival purposes. Alder wood is also commonly used to smoke various food items.

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.

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 firma — Kyūshū (Japan)
• Alnus glutinosa — Black Alder. Europe.

• Alnus incana — Grey Alder. Eurasia.
  • Alnus hirsuta (A. incana subsp. hirsuta) — Manchurian Alder. Northeastern Asia, and central Asia in mountains.
  • 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 or Mountain Alder. Northwestern North America.
• Alnus japonica — Japanese Alder. Japan.
• Alnus jorullensis — Mexican Alder. Mexico, Guatemala.
• Alnus mandshurica — Russian Far East, China, Korea.
• Alnus matsumurae — Honshū (Japan).
• Alnus nepalensis — Nepalese Alder. Eastern Himalaya, southwest China.
• Alnus orientalis — Oriental Alder. Southern Turkey, northwest Syria, Cyprus.
• Alnus pendula — Japan, Korea.
• 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 sieboldiana — Honshū (Japan).
• Alnus subcordata — Caucasian Alder. Caucasus, Iran.
• Alnus trabeculosa — China, Japan.

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.

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:

• 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.

Weed status

A. glutinosa and A. viridis are classed as environmental weeds in New Zealand.

Cultural references

Alder is illustrated in the coat of arms for the Austrian town of Grossarl.

References

Further reading

• Chen, Zhiduan and Li, Jianhua (2004). Phylogenetics and Biogeography of Alnus (Betulaceae) Inferred from Sequences of Nuclear Ribosomal DNA ITS Region. International Journal of Plant Sciences 165: 325–335.

External links

• Flora Europaea: Alnus
• Flora of Bolivia: Alnus
• Flora of China: Alnus
• Flora of North America: Alnus
• Flora of Pakistan: Alnus