Alnus is a genus of monoecious trees and shrubs in the birch family Betulaceae, comprising roughly 25 to 35 accepted species depending on the source. Most alders are deciduous, with alternate, simple, serrated leaves and reproductive structures borne as catkins — the male catkins elongate and pendulous, the female catkins shorter and, after pollination, ripening into distinctive small, woody, cone-like infructescences whose persistent five-lobed scales remain on the branches long after the tiny winged seeds have been shed. It is this woody infructescence that most reliably separates Alnus from its close relative Betula, the birches, whose fruiting scales are thin, three-lobed, and deciduous.
The genus is distributed across the north temperate zone, with outliers reaching south into Central America and the northern and southern Andes. Habit ranges from prostrate alpine shrubs through multi-stemmed thicket-formers to single-trunked trees that reach 30 m or more; the SEINet treatment describes Alnus broadly as "trees or shrubs, to 35 m," with light grey to dark brown bark and nearly white wood that turns reddish on exposure to air. Three subgenera are usually recognised within the genus: subgenus Alnus (the largest, with roughly 15–25 species), subgenus Clethropsis (about three species), and subgenus Alnobetula (about four species, including the widespread shrubby green alders).
Alders are best known ecologically for their symbiosis with Frankia alni, an actinomycete bacterium that colonises root nodules and fixes atmospheric nitrogen. This makes alders pioneer species on disturbed, glacial, alluvial, and otherwise nitrogen-poor soils; published field figures range from around 60 kg of nitrogen per hectare per year added by Sitka alder on glacial terrain up to 130–320 kg per hectare per year under red alder. Many species concentrate along streams, rivers, lakeshores, and wetlands, stabilising banks and enriching riparian soils. The seeds themselves are tiny samaras — roughly 1.5 million to the kilogram — well adapted to wind and water dispersal along these corridors.
A handful of species dominate the genus's economic and cultural footprint. Red alder (Alnus rubra) of western North America and black alder (Alnus glutinosa) of Europe both exceed 30 m and are the principal timber alders, while green alder (Alnus alnobetula) typifies the shrubby alpine and boreal species rarely topping 5 m. Alder wood is exceptionally stable when permanently submerged — Venice is partly founded on alder piles — and is also valued for smoking food, for guitar bodies (notably by Fender from the 1950s onward), for charcoal, matchsticks, and as a source of red and yellow dyes and bark tannins. Several alders are also cornerstone agroforestry trees, intercropped to lift soil fertility, and the genus is a frequent pioneer in restoration plantings.
Etymology
The English name "alder" descends from Old English "alor", itself from a Proto-Germanic root reconstructed as aliso, while the Latin genus name Alnus survives in modern Romance languages, giving French aulne and Spanish álamo. The genus was formally circumscribed by Philip Miller and is cited as Alnus Mill. in modern checklists, including the GBIF Backbone.
Distribution
Alnus is centred on the north temperate zone of Eurasia and North America, with several species extending south through montane Central America and along the northern and southern Andes. In Europe, regional floras such as Info Flora (Switzerland) recognise four native or established species — A. glutinosa, A. incana, A. viridis, and the southern European A. cordata — together with the hybrid A. glutinosa × incana, ranging from lowland wetlands up into subalpine shrubland. In the southwestern United States, SEINet records species including A. oblongifolia and A. jorullensis, the latter representing the genus's penetration into highland tropical Mexico and Central America.
Ecology
Alders are defining nitrogen-fixers of cool-temperate riparian and pioneer habitats. Every species in the genus forms a symbiosis with the actinomycete bacterium Frankia (most often Frankia alni), which colonises specialised root nodules and converts atmospheric nitrogen into a plant-usable form. Documented inputs vary widely with species and site: Sitka alder adds on the order of 60 kg of nitrogen per hectare per year on freshly deglaciated terrain, while red alder stands can fix between 130 and 320 kg per hectare per year. Because of this trait, alders dominate early successional stands along streams, rivers, glacial outwashes, and disturbed slopes, and they are routinely used as nurse trees for less hardy species. The tiny winged seeds — about 1.5 million to the kilogram — disperse readily by wind and water along these same riparian corridors.
Cultivation
Alders are generally undemanding given moisture: they tolerate poor and seasonally waterlogged soils that defeat most other timber genera, in part because of their own nitrogen supply. Useful Tropical Plants notes for the Andean Alnus acuminata that it grows fast — to roughly 25 m in ten years — prefers well-drained ground, and grows in nutritionally poor soils, illustrating the general genus pattern. Temperate species are similarly forgiving but typically need full sun and a reliable water table, particularly along streambanks and on reclaimed or eroded ground where they are most often planted.
Propagation
Alder seed of many species requires a brief cold stratification before sowing — the Useful Tropical Plants treatment of A. acuminata reports 10–20 days at about 5 °C as a typical pretreatment, after which seedlings reach plantable size in roughly 4–6 weeks. Across the genus, propagation from fresh seed is the standard nursery method; the tiny samaras are sown shallowly on a moist, well-drained medium.
Cultural uses
Alder wood's most famous property is its stability under permanent submersion, which historically made it the timber of choice for piers, mill races, and waterlogged foundations — the pilings beneath much of Venice include alder. The wood is also burned to smoke salmon, coffee, and other foods, and has been used since the 1950s by Fender for electric-guitar bodies because of its tonal characteristics. Bark and inner bark yield red and yellow dyes and tannins long used in leather working, and the bark contains a salicylic-acid precursor exploited in folk medicine. Beyond temperate uses, the Andean A. acuminata is a major agroforestry tree: bark and leaves are used to treat muscular and joint pain, rheumatism, and skin infections; the wood is worked into furniture, charcoal, fuel, and matchsticks (a Colombian match company evaluating 20 species reportedly chose this one as best); and intercropping with maize and corn has been reported to cut planting costs by up to 60%. Leaves of agroforestry alders also serve as green manure and fodder.
History
Alder's cultural footprint reaches well beyond timber. Ermanno Olmi's 1978 film "The Tree of Wooden Clogs" takes its name from the alder traditionally used to make clogs in northern Italy, and alder pilings continue to support the foundations of Venice — a use of the wood that dates back centuries and depends on its remarkable rot resistance under permanent submersion.
Taxonomy notes
The GBIF Backbone accepts the genus as Alnus Mill., placed in Betulaceae within the order Fagales, with 172 descendant records logged. Three subgenera are commonly recognised — subgenus Alnus (15–25 species), Clethropsis (about three species), and Alnobetula (about four species) — for a total of roughly 25–35 species, with regional treatments differing somewhat on whether to recognise certain segregates and hybrids. Alnus is most readily separated from sister genus Betula by its woody, persistent, five-lobed infructescence scales, in contrast to the thin, three-lobed, deciduous scales of birch.
Conservation
The genus has no genus-level entries in the ISSG Global Invasive Species Database at the time of access. Regionally, however, Alnus glutinosa and Alnus viridis are classed as environmental weeds in New Zealand, where they spread along watercourses and into disturbed ground.
