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Alder is the common name of a genus of flowering plants (Alnus) belonging to the birch family Betulaceae. The genus comprises about 30 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few species extending into Central America and the northern Andes.
The common name "alder" evolved from Old English "alor", which in turn is derived from Proto-Germanic root  aliso. The generic 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..
With a few exceptions, alders are deciduous, and the leaves are 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. These trees 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 Alnus viridis (green alder) is rarely more than a 5-m-tall shrub.
Alder leaves and sometimes catkins are used as food by numerous butterflies and moths.
A. glutinosa and A. viridis are classed as environmental weeds in New Zealand.
Alder is particularly noted for its important symbiotic relationship with Frankia alni, an 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 colour. The bacterium absorbs nitrogen from the air and makes it available to the tree. Alder, in turn, provides the bacterium with sugars, which it produces through photosynthesis. As a result of this mutually beneficial relationship, alder improves the fertility of the soil where it grows, and as a pioneer species, it helps provide additional nitrogen for the successional species which follow.
Alder roots are parasitized by northern groundcone.
The catkins of some alder species have a degree of edibility, and may be rich in protein. Reported to have a bitter and unpleasant taste, they are more useful for survival purposes. The wood of certain alder species is often used to smoke various food items, especially salmon and other seafood.
Most of the pilings that form the foundation of Venice were made from alder trees.
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.
The inner bark of the alder, as well as red osier dogwood, or chokecherry, was also used by Native Americans in their smoking mixtures, known as kinnikinnick, to improve the taste of the bearberry leaf.
Alder is illustrated in the coat of arms for the Austrian town of Grossarl.
Electric guitars, most notably the Fender Jazz Bass, Fender Precision Bass, Fender Stratocaster and Fender Telecaster, have been built with alder bodies since the 1950s. Alder is appreciated for its claimed tight and even balanced tone, especially when compared to mahogany, and has been adopted by many electric guitar manufacturers.
As a hardwood, alder is used in making furniture, cabinets, and other woodworking products. For example, in the television series Northern Exposure 1992 season 3 episode "Things Become Extinct", Native American Ira Wingfeather makes duck flutes out of Alder tree branches while Ed Chigliak films.
The genus is divided into three subgenera:
Subgenus Alnus: Trees with stalked shoot buds, male and female catkins produced in autumn (fall) but stay closed over winter, pollinating in late winter or early spring, about 15–25 species, including:
- Alnus acuminata Kunth — Andean alder, Mexico, Central, South America
- Alnus cordata (Loisel.) Duby — Italian alder, Italy, Albania, Corsica; naturalized in Belgium, Spain, Azores, California
- Alnus cremastogyne Burkill - China
- Alnus × elliptica Req. - Italy (A. cordata × A. glutinosa)
- Alnus × fallacina Callier - Ohio, New York State, Vermont, New Hampshire, Maine (A. incana subsp. rugosa × A. serrulata)
- Alnus firma Siebold & Zucc. — Kyūshū Island in Japan
- Alnus glutinosa (L.) Gaertn. — black alder, Europe, Central Asia; naturalized in New Zealand, Chile, Ontario, northeastern United States
- Alnus × hanedae Suyinata - Japan (A. firma × A. sieboldiana)
- Alnus incana (L.) Moench — grey alder, Eurasia, North America
- Alnus hirsuta (Spach) Rupr. (A. incana subsp. hirsuta) — Manchurian alder, Japan, Korea, Manchuria, Siberia, Russian Far East
- Alnus × hosoii Mizush. - Japan (A. maximowiczii × A. pendula)
- Alnus oblongifolia Torr. (A. incana subsp. oblongifolia) — Arizona alder, Arizona, New Mexico, Sonora, Chihuahua
- Alnus rugosa (Du Roi) Spreng. (A. incana subsp. rugosa ) — speckled alder, northeastern North America
- Alnus tenuifolia Nutt. (A. incana subsp. tenuifolia) — thinleaf or mountain alder, northwestern North America
- Alnus japonica (Thunb.) Steud. — Japanese alder, Japan, Korea, Taiwan, eastern China, Russian Far East
- Alnus jorullensis Kunth — Mexican alder, Mexico, Guatemala, Honduras (one of the few evergreen species)
- Alnus mandshurica (Callier) Hand.-Mazz. — Russian Far East, northeastern China, Korea
- Alnus matsumurae Callier — Honshū Island in Japan
- Alnus × mayrii Callier - Russian Far East, Japan (A. hirsuta × A. japonica)
- Alnus nepalensis D.Don — Nepalese alder, Himalayas, Tibet, Yunnan, Nepal, Bhutan, Myanmar, Thailand; naturalized in Hawaii
- Alnus orientalis Decne. — Oriental alder, southern Turkey, northwest Syria, Cyprus, Lebanon, Palestine, Iran
- Alnus × peculiaris Hiyama - Kyūshū Island in Japan (A. firma × A. pendula)
- Alnus pendula Matsum. — Japan, Korea
- Alnus × pubescens Tausch - northern + central Europe (A. glutinosa × A. incana)
- Alnus rhombifolia Nutt. — white alder, California, Nevada, Oregon, Washington, Oregon, Idaho, Montana
- Alnus rubra Bong. — red alder, Alaska, Yukon, British Columbia, California, Oregon, Washington, Oregon, Idaho, Montana
- Alnus serrulata (Aiton) Willd. — hazel alder, tag alder or smooth alder, eastern North America
- Alnus sieboldiana Matsum. — Japan, Ryukyu Islands
- Alnus subcordata C.A.Mey. - Caucasian alder, Caucasus, Iran
- Alnus trabeculosa Hand.-Mazz. — China, Japan
Subgenus Clethropsis. Trees or shrubs with stalked shoot buds, male and female catkins produced in autumn (fall) and expanding and pollinating then, three species:
- Alnus formosana (Burkill) Makino — Formosan alder, Taiwan
- Alnus maritima (Marshall) Muhl. ex Nutt. — seaside alder, United States (Georgia, Delaware, Maryland, Oklahoma)
- Alnus nitida (Spach) Endl. — Himalayan alder, western Himalaya, Pakistan, India, Nepal
Subgenus Alnobetula. Shrubs with 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 alnobetula (Ehrh.) K.Koch (syn. Alnus viridis A. Gray 1848, illegitimate homonym, not (Chaix) DC. 1805) - temperate + subarctic Europe, Asia, North America
- unknown subgenus
- Alnus djavanshirii H.Zare - Iran
- Alnus dolichocarpa H.Zare, Amini & Assadi - Iran
- Alnus fauriei H.Lév. & Vaniot - Honshu Island in Japan
- Alnus ferdinandi-coburgii C.K.Schneid. - southern China
- Alnus glutipes (Jarm. ex Czerpek) Vorosch. - Yakutiya region of Siberia
- Alnus hakkodensis Hayashi - Honshu Island in Japan
- Alnus henryi C.K.Schneid. - Taiwan
- Alnus lanata Duthie ex Bean - Sichuan Province in China
- Alnus mairei H.Lév. - Yunnan Province in China
- Alnus maximowiczii Callier - Japan, Korea, Russian Far East
- Alnus paniculata Nakai - Korea
- Alnus serrulatoides Callier - Japan
- Alnus × suginoi Sugim. - Japan
- Alnus vermicularis Nakai - Korea
- Kew World Checklist of Selected Plant Families
- Clayson, Howell (May 2008). Consolidated list of environmental weeds in New Zealand. Wellington: Department of Conservation. ISBN 978-0-478-14412-3.
- Plants For A Future (Database)
- Kendall, Paul (25 August 2010). "Mythology and Folklore of the Alder". Trees for life. Retrieved 6 August 2011.
- Ewing, Susan. The Great Alaska Nature Factbook. Portland: Alaska Northwest Books, 1996.
- Edible and Medicinal Plants of the West, Gregory L. Tilford, ISBN 0-87842-359-1
- Staff (2009) "Bearberry" Discovering Lewis and Clark The Lewis and Clark Fort Mandan Foundation
- 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.
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