Alnus

Alders are trees of the genus ''Alnus'' in the birch family Betulaceae. The genus includes about 35 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, as well as the northern and southern Andes.

Description

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'', another 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 . By contrast, the widespread '' Alnus alnobetula'' (green alder) is rarely more than a shrub.

Phylogeny

Classification

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''
** subsp. ''acuminata''
** subsp. ''arguta''
** subsp. ''glabrata''
* '' Alnus cordata''
* '' Alnus cremastogyne''
* '' Alnus firma''
* '' Alnus glutinosa''
** subsp. ''barbata''
** subsp. ''glutinosa''
** subsp. ''incisa''
** subsp. ''laciniata''
* '' Alnus hirsuta''
* '' Alnus incana''
** subsp. ''incana''
** subsp. ''kolaensis''
** subsp. ''rugosa''
** subsp. ''tenuifolia''
* '' Alnus japonica''
* '' Alnus jorullensis''
** subsp. ''lutea''
** subsp. ''jorullensis''
* '' Alnus lusitanica''
* '' Alnus matsumurae''
* '' Alnus nepalensis''
* '' Alnus oblongifolia''
* '' Alnus orientalis''
* '' Alnus rhombifolia''
* '' Alnus rohlenae''
* '' Alnus rubra''
* '' Alnus serrulata''
* '' Alnus subcordata''
* '' Alnus tenuifolia''
* '' Alnus trabeculosa''

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''
* '' Alnus maritima''
* '' Alnus nitida''

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'' (synonym-''Alnus viridis'')
** subsp. ''alnobetula''
** subsp. ''crispa''
** subsp. ''fruticosa''
** subsp. ''sinuata''
** subsp. ''suaveolens''
* '' Alnus firma''
* '' Alnus mandshurica''
* '' Alnus maximowiczii''
* '' Alnus pendula''
* '' Alnus sieboldiana''

Not assigned to a subgenus

* '' Alnus fauriei''
* '' Alnus ferdinandi-coburgii''
* '' Alnus glutipes''
* '' Alnus hakkodensis''
* ''Alnus henryi''
* '' Alnus lanata''
* '' Alnus mairei''
* '' Alnus paniculata''
* '' Alnus serrulatoides''
* '' Alnus vermicularis''

Species names with uncertain taxonomic status

The status of the following species is unresolved:

* ''Alnus balatonialis''
* ''Alnus cuneata''
* ''Alnus dimitrovii''
* ''Alnus djavanshirii'' – Iran
* ''Alnus dolichocarpa'' – Iran
* ''Alnus figerti''
* ''Alnus frangula''
* ''Alnus gigantea''
* ''Alnus glandulosa''
* ''Alnus henedae''
* ''Alnus hybrida''
* ''Alnus laciniata''
* ''Alnus lobata''
* ''Alnus microphylla''
* ''Alnus obtusifolia''
* ''Alnus oxyacantha''
* ''Alnus subrotunda''
* ''Alnus vilmoriana''
* ''Alnus washingtonia''

Hybrids

The following hybrids have been described:

* '' Alnus × elliptica'' (''A. cordata'' × ''A. glutinosa'')
* '' Alnus × fallacina'' (''A. incana'' subsp. ''rugosa'' × ''A. serrulata'')
* '' Alnus × hanedae'' (''A. firma'' × ''A. sieboldiana'')
* '' Alnus × hosoii'' (''A. maximowiczii'' × ''A. pendula'')
* ''Alnus × mayrii'' (''A. hirsuta'' × ''A. japonica'')
* '' Alnus × peculiaris'' (''A. firma'' × ''A. pendula'')
* '' Alnus × pubescens'' (''A. glutinosa'' × ''A. incana'')
* '' Alnus × suginoi''

The status of the following hybrids is unresolved:

* ''Alnus'' × ''aschersoniana''
* ''Alnus'' × ''koehnei''
* ''Alnus'' × ''ljungeri''
* ''Alnus'' × ''purpusii''
* ''Alnus'' × ''silesiaca''
* ''Alnus'' × ''spaethii'' (''A. japonica'' × ''A. subcordata'')

Fossil record

The oldest fossil pollen that can be identified as ''Alnus'' is from northern Bohemia, dating to the late Paleocene, around 58 million years ago.

* †'' Alnus fairi'' - Miocene; Western North America
* †'' Alnus heterodonta'' – Oligocene; Fossil, Oregon
* †'' Alnus hollandiana'' - Miocene; Western North America
* †'' Alnus largei'' - Miocene; Western North America
* †'' Alnus parvifolia'' - Ypresian; Okanagan Highlands
* †'' Alnus relatus'' - Miocene; Western North America

Etymology

The common name ''alder'' evolved from the Old English word ''alor'', which in turn is derived from Proto-Germanic root ''aliso''. The generic name ''Alnus'' is the equivalent Latin name, from whence French ''aulne'' and Spanish ''Alamo'' (Spanish term for " poplar").

Ecology

Alders are commonly found near streams, rivers, and wetlands. Sometimes where the prevalence of alders is particularly prominent these are called alder carrs. In the Pacific Northwest of North America, the white alder (''Alnus rhombifolia'') unlike other northwest alders, has an affinity for warm, dry climates, where it grows along watercourses, such as along the lower Columbia River east of the Cascades and the Snake River, including Hells Canyon.

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 leaves and especially the roots are important to the ecosystem because they enrich the soil with nitrogen and other nutrients.

Nitrogen fixation and succession of woodland species

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 to follow.

Because of its abundance, red alder delivers large amounts of nitrogen to enrich forest soils. Red alder stands have been found to supply between of nitrogen annually to the soil. From Alaska to Oregon, ''Alnus viridis'' subsp. ''sinuata'' (''A. sinuata'', Sitka alder or slide alder), characteristically pioneer fresh, gravelly sites at the foot of retreating glaciers. Studies show that Sitka alder, a more shrubby variety of alder, adds nitrogen to the soil at an average rate of per year, helping convert the sterile glacial terrain to soil capable of supporting a conifer forest. Alders are common among the first species to colonize disturbed areas from floods, windstorms, fires, landslides, etc. Alder groves often serve as natural firebreaks since these broad-leaved trees are much less flammable than conifers. Their foliage and leaf litter does not carry a fire well, and their thin bark is sufficiently resistant to protect them from light surface fires. In addition, the light weight of alder seedsnumbering allows for easy dispersal by the wind. Although it outgrows coastal Douglas-fir for the first 25 years, it is very shade intolerant and seldom lives more than 100 years. Red alder is the Pacific Northwest's largest alder and the most plentiful and commercially important broad-leaved tree in the coastal Northwest. Groves of red alder in diameter intermingle with young Douglas-fir forests west of the Cascades, attaining a maximum height of in about sixty years, and then are afflicted by heart rot. Alders largely help create conditions favorable for giant conifers that replace them.

An alder root nodule gall.JPG, alt=An alder root nodule, Whole root nodule
A sectioned alder root nodule gall.JPG, alt=A sectioned alder root nodule, Sectioned root nodules

Parasites

Alder roots are parasitized by northern groundcone.

Uses

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 such as coffee, salmon, and other seafood.

Alder is notably stable when immersed, and has been used for millennia as a material for pilings for piers and wharves. 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. Some Native American cultures use red alder bark ('' Alnus rubra'') to treat poison oak, insect bites, and skin irritations. Blackfeet Indians have traditionally 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, is used by some Indigenous peoples of the Americas in 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 those manufactured by the Fender Musical Instruments Corporation, have been built with alder bodies since the 1950s. Alder is appreciated for its tone that is claimed to be tight and evenly balanced, especially when compared to mahogany, and has been adopted by many electric guitar manufacturers. It usually is finished in opaque lacquer (nitrocellulose, polyurethane, or polyester), as it does not have a prominent grain.

As a hardwood, alder is used in making furniture, cabinets, and other woodworking products. In these applications, its aforementioned lack of prominent grain means that it is often veneered, either by stained light woods such as oak, ash, or figured maple, or by darker woods such as teak or walnut.

Alder bark and wood (like oak and sweet chestnut) contain tannin and are traditionally used to tan leather.

A red dye can also be extracted from the outer bark, and a yellow dye from the inner bark.

Culture

Ermanno Olmi's movie '' The Tree of Wooden Clogs'' (''L' Albero Degli Zoccoli'', 1978) refers in its title to alder, typically used to make clogs as in this movie's plot.Ontano nero
(''tr. Black Alder'') accessed 17 November 2020 ''uomoenatura.it''

References

Further reading

*

External links

Flora Europaea: ''Alnus''

Flora of Bolivia: ''Alnus''

Flora of China: ''Alnus''

Flora of North America: ''Alnus''

Flora of Pakistan: ''Alnus''

{{Authority control

Taxa named by Philip Miller