Alnus Mill.

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Credits

Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/). Accessed 2024-05-29.

Family

  • Betulaceae

Common Names

  • Alders
  • Al
  • Aulnes
  • Els
  • Erle
  • Leppä

Synonyms

  • Alnaster Spach
  • Alnobetula (W.D.J. Koch) Schur
  • Betula-alnus Marshall
  • Clethropsis Spach
  • Cremastogyne (H.J.P. Winkl.) Czerep.
  • Duschekia Opiz
  • Semidopsis Zumagl.

Glossary

berry
Fleshy indehiscent fruit with seed(s) immersed in pulp.
morphology
The visible form of an organism.
phenology
The seasonal timing of events in the life cycle of a plant or animal and the study thereof.
pistillate
Female referring to female plants (dioecy) or flowers (monoecy) or the female parts of a hermaphrodite flower.
section
(sect.) Subdivision of a genus.
staminate
Male referring to male plants (dioecy) or flowers (monoecy) or the male parts of a hermaphrodite flower.
acuminate
Narrowing gradually to a point.
apex
(pl. apices) Tip. apical At the apex.
bud
Immature shoot protected by scales that develops into leaves and/or flowers.
bullate
Puckered; with blister-like swellings on the surface.
cone
Term used here primarily to indicate the seed-bearing (female) structure of a conifer (‘conifer’ = ‘cone-producer’); otherwise known as a strobilus. A number of flowering plants produce cone-like seed-bearing structures including Betulaceae and Casuarinaceae.
craspedodromous
Pattern of leaf venation whereby the lateral veins run straight out to leaf margin. (Cf. camptodromous.)
divergent
Spreading from the centre.
emarginate
Notched at the apex.
introgression
Incorporation of genes from one species into the genotype of another through repeated hybridisation or repetitive backcrossing between a hybrid and one of its parents.
key
(of fruit) Vernacular English term for winged samaras (as in e.g. Acer Fraxinus Ulmus)
lanceolate
Lance-shaped; broadest in middle tapering to point.
monophyletic
(of a group of taxa) With a single ancestor; part of a natural lineage believed to reflect evolutionary relationships accurately (n. monophyly). (Cf. paraphyly polyphyly.)
morphology
The visible form of an organism.
ovate
Egg-shaped; broadest towards the stem.
ovoid
Egg-shaped solid.
pistillate
Female referring to female plants (dioecy) or flowers (monoecy) or the female parts of a hermaphrodite flower.
reticulate
Arranged in a net-like manner.
samara
Dry indehiscent winged fruit usually with a single seed (as in e.g. Acer Fraxinus Ulmus. Also called a ‘key fruit’.
section
(sect.) Subdivision of a genus.
subspecies
(subsp.) Taxonomic rank for a group of organisms showing the principal characters of a species but with significant definable morphological differentiation. A subspecies occurs in populations that can occupy a distinct geographical range or habitat.
symbiotic
(of two organisms) Dissimilar but closely associated living together in a mutually beneficial manner.
taxonomy
Classification usually in a biological sense.
venation
Pattern of veins (nerves) especially in a leaf.
vernalisation
The priming of a plant response (e.g. germination flowering) by exposure to low temperatures in winter.

References

Credits

Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/). Accessed 2024-05-29.

Deciduous (rarely semideciduous) monoecious trees and shrubs, 1.5–30 m. Bark smooth to rough. Branches terete with triangular pith and conspicuous circular to elliptic lenticels, sparsely to densely set simple hairs, glandular. Branchlets terete or winged. Winter buds resinous, ovoid to oblong or rounded, either on long stipe (actually a small axillary branch) with 2 or 3 equal valvate scales, or on very short stipe with 5 or more imbricate scales. Leaves alternate, simple, pinnately-veined, teeth varying from sparsely mucronate to coarsely double toothed, sometimes lobed, petiolate. Stipulate, deciduous or persistent, broad to narrow, ovate, elliptic, or obovate, glabrous to densely pubescent. Flowers unisexual, in modified pedunculate cymules, opening before leaves in spring and held partially developed over winter in bud, or in autumn. Staminate inflorescences are catkins, held erect or pendulous when first formed, and overwintering in this state except in autumn flowering species, pendulous and greatly elongating at anthesis, in short stalked racemes, singly or paniculate in axils of leaves (often appearing terminal on branchlet), with minute flowers of 2 or 4(–6) stamens and 4(–5)-partite perianth with terminal peltate bract comprised or 3 or 5 bracteoles. Pistillate inflorescences erect to pendent, solitary on main stem, ovoid to oblong or cylindric, below the staminate inflorescences, racemose, overwintering fully formed except in autumn flowering species and Section Alnobetula, bracts with 4 imbricate bracteoles; flowers 2 per bract, lacking perianth, with 1 pistil, inferior ovary, and 2 linear styles. Pistillate inflorescences maturing to persistent resinous woody cone-like fruit, ovoid to cylindric, mature scales hard (occasionally soft) 3– or 5–lobed. Seed an ovate to obovate or orbicular nutlet, with or without a membranous wing, or with thickened border, 1–2 per bract. (Furlow 1979; Furlow 1997; Iwatsuki 1993; Tutin et al. 1972; T. Baxter, pers. obs.).

Alnus is an ancient and complex genus originating in the late Cretaceous (Graham 1993; Chen, Manchester & Sun 1999). Something of the order of 40 species (their limits much debated) are distributed across the northern hemisphere, extending into the southern hemisphere in South America, where the genus reaches its southern limit in northern Argentina. Alders vary widely from tall, single-stemmed, fast-growing trees to smaller, multi-stemmed shrubs, sometimes with modest growth rates. The largest species on record is A. glutinosa (to 35 m) whilst the smallest is the A. alnobetula aggregate (subsp. alnobetula grows to 0.5–10 m tall (Tutin et al. 1964)). Almost all alders are light demanding; they are wind pollinated and wind dispersed, and many rapidly colonise suitable open habitats to form potentially pure stands. They also can be somewhat weedy: some are invasive outside their native range, especially where commonly planted, e.g. A. glutinosa in the USA, and A. cordata in North Wales slate quarries and elsewhere. Alders occur in a range of habitats. Many grow on wetter soils along river valleys and around water bodies (e.g. A. glutinosa, A. maritima), while others are found on gravelly soils high in mountains (A. jorullensis, A. matsumurae), as trees in cool temperate conditions (A. hirsuta) to tropical and sub-tropical forests (A. acuminata, A. nepalensis), to more arid regions (A. subcordata agg., A. alnobetula subsp. suaveolens). They occur from sea level (A. sieboldiana) to high mountains up to 3800 m (A. jorullensis). Much of the resilience and wide ecological tolerance of alders is due to their ability to fix nitrogen. All alders have a symbiotic relationship with nitrogen-fixing bacteria Frankia alni agg., a cosmopolitan species found in their root nodules. The genus is of high economic value across the world. Many species are fast growing and suitable in difficult sites, for land stabilisation, as a major timber crop, and some species are excellent in agro- or urban forestry. Many species are widely cultivated and some are of particular ornamental merit.

Morphology and Identification

Identification of some alders is straightforward, but certain groups are especially difficult. This is often the case where species aggregates exist, e.g. A. alnobetula, A. glutinosa. Plants of unknown provenance, such as found in gardens, can be exceedingly difficult or impossible to identify since hybrids are frequent. Whole-plant examination is important, and examination of form, together with vegetative and reproductive structures is usually needed for accurate determination. Wild populations are usually easier to identify than individual trees in cultivation. Phenology is important for some species, e.g. A. subcordata is one of the first of all spring-flowering species to flower, sometimes before the end of December (northern hemisphere). Care should be taken with purely vegetative characters as they vary between and within a species, as well as within a single plant, so only ‘typical’ structures should be used. Typical leaves used in determination are those that are pre-formed in bud, found either as the lowest leaf on extension growth or on short lateral shoots lower down the stem, growing in full sun. Other leaves on the plant are more variable and not necessarily reliable. Leaf venation is a useful character in Alnus: it may be craspedodromous (all secondary veins end at the leaf margin), eucamptodromous (all secondary veins terminate before the margin) or semicraspedodromous (only some secondary veins reach the margin). Trichomes of Alnus are the most variable within Betulaceae, demonstrating all six forms as defined by Hardin & Bell (1986). Reproductive characters vary considerably less, especially fruit. Most alders have a two-year flowering strategy, whereby inflorescences are initiated in summer and remain dormant over winter, flowering in spring the following year. However, a few species initiate flowering in spring and flower in autumn of the same year. Several species are characterised by female catkins usually being borne singly (A. maritima, Section Cremastogyne, A. sieboldiana) while in others the catkin and ‘fruit’ clusters are branched enough to be considered paniculate, rather than just racemose with the catkins being borne singly on the axis (e.g. A. nepalensis, A. formosana), although not all trees of a species may demonstrate these paniculate inflorescences. Fruit vary in size from small (A. glutinosa) to large (A. sieboldiana) and in shape from ovoid to elliptic to cylindric. Fruit scales are highly diagnostic, and can be soft and bulbous (A. nepalensis, Section Cremastogyne), hard, thick and somewhat divergent with often unequal lobes (A. glutinosa, A. japonica) or hard and thin with even lobes (Subgenus Alnobetula). The fruit is a tiny samara, with papery or leathery wings, although these are reduced in some species (Furlow 1979; Furlow 1982; Ashburner 1986; Li & Skvortsov 1999).

Classification

Alnus has been subject to diverse taxonomic treatments, and more work is needed to resolve the incongruence between morphology and genetics. There are several clearly related morphological groups, but there is ongoing debate regarding the majority of species and their broader relationships (Banaev & Bazant 2007; Chen & Li 2004; Colagar et al. 2016; King & Ferris 1998; Ren, Xiang & Chen 2010). The most common-sense morphological taxonomy was presented by Murai (1964), and this (as modified by Furlow 1979) is more or less followed here despite key omissions in that work. Recent genetic work has confirmed Alnus as a monophyletic genus (e.g. Bousquet, Strauss & Li 1992; Chen, Manchester & Sun 1999; Chen & Li 2004), most closely related to Betula, from which it is distinguished by the presence of persistent woody cone-like infructescences. Morphologically there is merit in splitting Alnus into three subgenera: Clethropsis, Alnobetula and Alnus (=Gymnothyrsus of Murai (1964)), with Subgenus Alnus further split into Sections: this is how they are tentatively presented here. Note, however, that molecular data do not wholly support this classification (see below). In particular, the autumn-flowering species of Subgenus Clethropsis do not form a natural, monophyletic group. Identification features for each group are summarized in a table below. For all but the a few recently described plants we place species following Murai, but go on to discuss other thoughts about their relationships in the text for each species.

Subgenus Alnus (=Gymnothyrsus), White Alders

Distinguished by flowering in spring, buds with long stalks (>2 mm), leaves with weak venation, pistillate flowers exposed in winter and fruit with thick scales. We tentatively recognize four sections following Murai (1964); these may well not reflect their evolutionary relationships.

Section Cremastogyne have pistillate inflorescences that are solitary in leaf axils. We describe AA. cremastogyne, ferdinandi-coburgii and lanata.

Section Fauriae have persistent stipules, with bullate, emarginate leaves.We describe AA. fauriei and serrulatoides.

Section Glutinosae are highly variable but all have seeds with broad wings. We describe AA. glutinosa, hirsuta, inokumae, lusitanica, matsumurae, oblongifolia, rohlenae, rubra, and serrulata.

Section Japonicae have leathery leaves with large fruit and seeds angular, 5-sided with narrow seed wings. We describe AA. acuminata, cordata, dvanshirii, japonica, jorullensis, orientalis, subcordata and trabeculosa.

We also describe the recently recognized A. dolichocarpa of Subgenus Alnus, without committing it to a section.

Subgenus Alnobetula, Green Alders

Distinguished by elliptic-lanceolate buds with short stalks (<1 mm), leaves that are strongly craspedodromous, pistillate inflorescences that emerge in spring, and fruit with thin scales that mature in autumn. This is a monophyletic subgenus and may represent the most primitive group within Alnus (Chen & Li 2004). We recognize two sections.

Section Alnobetula, with broad leaves and fruit scales that are ± equal in size and lack a recurved point. We recognize a single highly variable species A. alnobetula, with several subspecies, including A. hakkodensis as one of these, though its combination at this rank has not yet been formally published (March 2024).

Section Bifurcatus, with narrower leaves, and unequal sized fruit scales with a recurved terminal point. We describe AA. betulifolia, firma, pendula and sieboldii.

Subgenus Clethropsis

Flowering in autumn, with fruit that ripen the following summer or autumn (see notes below). This is not a monophyletic grouping. We describe AA. formosana (including A. henryi), maritima, nepalensis and nitida.

SubgenusAlnobetulaAlnus (Section Cremastogyne)Alnus (Sections Fauriae, Glutinosae, Japonicae)Clethropsis
Flowering timeSpringSpringSpringAutumn
HabitMostly large shrubsLarge upright treesTrees, rarely shrubsLarge trees or small shrub
BudsShort stalk (<1mm), scales numerous, spirally arrangedLong stalk (>2mm), scales (0–)2, leaves exposed over winterLong stalk (>2mm), scales (0–)2 (often absent in Section Japonicae), leaves sometimes exposed over winterLong stalk (>1mm), scales (0–)2, leaves often exposed over winter
LeavesBroadest at base (ovate) with typically deeply impressed lateral veins, leatheryObovate, apex abruptly narrow to acuminate tip, venation semicraspedodromousRounded, ovate to obovate to suborbicular, venation weak, blade typically matt and thinVery large (except A. maritima), ± elliptic venation various, often paler underneath
Pistillate flowersEmerge in springExposed in winter, open in spring, solitaryExposed in winter, open in springAutumn flowering
Staminate flowersSpring flowering, racemoseSpring flowering, solitary in leaf axilsSpring flowering, racemoseAutumn flowering, racemose or paniculate
FruitsOften large, ovoid, scales thin, maturing in autumnSmallish, ovoid to elliptic, glutinous, scales bulbous, thickOften large, ovoid, scales thick and woody, maturing in autumnOften small, ovoid to elliptic, glutinous, scales rather bulbous, thick, maturing the following summer to autumn
SeedLight and adapted for wind-dispersal, with wide, thin forward-pointing wingsSmall, very narrow wingOften dense, adapted for flotation, with thick corky wings, but sometimes thin (A. rubra)Very small, light, with narrow wing or thickened rim
WoodRemains white on exposureRemains white on exposureTurns bright brown on exposureRemains white on exposure

Molecular Phylogenetics and Evolution

Molecular phylogenetics has rather altered this picture, and more work is needed to produce a natural classification which takes account of both morphology and genetics. Molecular work has confirmed the monophyly of Subgenus Alnobetula and of Subgenus Alnus Section Cremastogyne (Chen & Li 2004). The cladistic placement of the morphologically similar autumn-flowering taxa is more disruptive, as there is no clear genetic relationship between these species. This is also true of other species including those in Section Japonicae, while A. serrulata appears more closely related to A. maritima than to others in Subgenus Alnus (Chen & Li 2004; Colagar et al. 2016; Ren, Xiang & Chen 2010). These complications may in part be the result of reticulate evolution, with populations and species becoming geographically isolated, diverging, then re-joining over time, followed by hybridisation and introgression, the whole cycle perhaps repeating several times. This model is especially pertinent to colder parts of the Northern Hemisphere through the climatic fluctuations of the Pleistocene (Milne & Abbott 2002). There are certain regions (e.g. SE Europe to Iran, and maritime East Asia) where there is a great deal of confusion. Certain species are clearly intermediate between described sections and share characteristics of both, perhaps the result of ancient hybridization, e.g. A. japonica (itself a very ancient species). Several previously named taxa (e.g. A. crispa (Aiton) Pursh) most likely represent ecological and regional variants of widespread and complex species. This has led to ongoing identification difficulties in Alnus, and several species have only been recently named, rightly or not, e.g. A. dolichocarpa, A. djavanshirii (Colagar et al. 2016). Alders frequently exhibit polyploidy and this can be crucial in determination of certain species, e.g. the tetraploid A. rohlenae and A. lusitanica are distinguished from the diploid or triploid A. glutinosa (Vit et al. 2017). In other taxa polyploids exist, but more work is needed to resolve problems, e.g. A. firma agg., A. hirsuta.

Cultivation

Alders are easy to cultivate and, indeed, are widely grown across the world. Their ability to fix nitrogen, and their rapid growth, makes them a useful genus for timber production, land stabilisation, and as street trees. Certain species are planted in great numbers, such as 1.5 million hectares of A. cremastogyne in the Yangtse valley (Tang, Ishii & Ohba 1996). The most commonly planted species throughout Europe is native A. glutinosa, with native A. incana and North American A. rubra also common. Alders have a reputation for needing moist soils, especially A. glutinosa, and although many perform better in moisture-retentive soils, several species are exceedingly drought tolerant. A. cordata, A. subcordata, A. rubra and A. oblongifolia are all proven to perform well in dry situations, including the urban environment. Provenance also has some bearing on this, and the more drought-sensitive A. glutinosa subsp. glutinosa should perhaps be replaced by the more drought tolerant A. glutinosa subsp. barbata or the similar A. rohlenae or A. lusitanica in drier or warmer situations, based on observations to date. Most species of alder are hardy in the cooler parts of temperate regions, apart from A. cremastogyne and A. lanata which appear suitable only in warmer regions due to their early leaf and flower emergence. Some species originate from cold continental climates, including A. incana, A. alnobetula and A. hirsuta, but these too can be susceptible to early or late frosts when grown in maritime climates. A. acuminata is a mostly tropical species, but provenances from high altitude in Mexico and northern Argentina have so far proven very promising in the UK, at least in milder areas. A. acuminata makes an excellent tree in areas such as New Zealand and Australia, where it has enormous potential as a timber tree.

Almost all alders are easy to grow from seed and quickly make large trees given the right care. Seed viability of species is variable, with many typically low, but some have high viability, particularly perhaps those which are self-compatible e.g. A. sieboldiana (as high as 95%). Seed is best collected as soon as ripe, winnowed and sown immediately, and left outside for vernalisation. Seedlings are usually quick growing given a soil-based compost inoculated with Frankia alni (Berry & Torrey 1985) to enable root infection. The addition to the seed compost of some ‘ordinary’ soil, perhaps from under alder trees (but note the risk of Phytophthora alni), or crushed nodules usually ensures nodulation. Well grown plants, especially in raised beds or under good care, should be at planting (standard) size within 2–4 years from seed. A. pendula and the smaller A. alnobetula are notable exceptions to this but should still grow 5–30 cm annually in good conditions. Alders can also be propagated from softwood cuttings under mist, typically from April to July depending on conditions.

Some of the most attractive alders are rarely seen except in large gardens or arboreta. Of particular ornamental merit are those within Subgenus Alnobetula. A. firma makes a small tree or large shrub with attractive glossy foliage. A. sieboldiana is very similar but generally larger in all aspects, and is especially notable for its abundant yellow male catkins. Both are drought tolerant. A. pendula is one of the most attractive foliage shrubs, with small pendulous female catkins. A. alnobetula is hugely variable but subsp. sinuata and plants from Ulleungdo are especially attractive in catkin, very drought tolerant and hardy. All of A. alnobetula s.l. would be especially useful shrubs in land reclamation work and can be used as nurse shrubs for a timber crop. A. glutinosa is the most commonly grown species, but many provenances not currently in general cultivation are potentially more attractive and drought tolerant, including those of Turkish origin with very glossy leaves (e.g. subsp. barbata TURX 205). A. cordata is already widely grown for its drought tolerance and glossy leaves, and adaptability to urban conditions, whilst A. subcordata is more vigorous and flowers over winter. A. cordata is the most drought tolerant alder based on leaf turgor loss data, and A. lanata the least (A. Hirons, pers. comm.).

Alders are susceptible to a number of pests and diseases. Most worrying is the Phytophthora alni complex, a water-borne fungus-like disease that can affect all species of alder. This disease has already claimed millions of trees in riparian habitats throughout Europe, particularly the UK, northeastern France and Bavaria, Germany. Symptoms include mid- to late-summer leaves that are abnormally small, yellow and sparse, and which may fall prematurely; twig and branch dieback; or unusually heavy fruit production. Typically around the base (but up to 3 m up the stem) tarry or rusty spots or black weeping cankers appear, beneath which the dead phloem is mottled reddish to purple brown, contrasting strongly with the adjacent healthy tissues. The disease is often fatal but some trees survive with continuing dieback, reducing the tree to a stump (Hansen 2012). Similar pathogens known in Alaska and Oregon are Phytophthora siskiyouensis and P. alni subsp. uniformis (Adams et al. 2008), causing basal rot and similar fatalities.

A significant pest is Alder Leaf Beetle (Agelastica alni) which affects many species of alder, especially A. glutinosa. It is not generally fatal but can cause severe defoliation, stressing the tree and making it more susceptible to other stress factors. Observational data appears to suggest that it affects species with thick or leathery leaves (including all of Subgenus Alnobetula) much less severely than thinner-leaved species. Many of the matt-leaved species (e.g. A. glutinosa) are susceptible to the alder leaf rusts Melampsoridium hiratsukanum and M. botulinum, both non-harmful rust fungi that discolour the underside of leaves; the first is an increasingly invasive species in many areas (CABI 2018). A ‘fun’ disease on alders is Alder Tongue (Taphrina alni), a fungal infection that causes tongue-like protrusions from green fruit, especially A. glutinosa. A little known component of alder biology is their mutualistic relationship to acarid mites (Maccracken, Miller & Labandeira 2019). Many species have hair- or bract-like structures in the vein axils of leaves (domatia) which house mites which are predatory or or have an anti-fungal effect on the tree. They are of significance in identification (e.g. A. fauriei has them in primary and secondary leaf axils) and also for ecology, with the mites endowing a beneficial effect on trees which have otherwise very palatable foliage. Those species with domatia are still very susceptible to Alder Leaf Beetle; this insect is clearly immune to any otherwise predatory effect of the mites.

Editor’s Note, March 2024

The authors, Tim Baxter and Hugh McAllister, have largely worked on Alnus at Ness Botanic Gardens, Wirral, UK, part of the University of Liverpool, where Hugh built up the extensive collection of Alnus as part of his long-standing interest in the Betulaceae. They gratefully acknowledge the facilities the garden has provided for this work over many decades. In this account Ness Botanic Gardens is frequently referred to by the colloquial ‘Ness’.

Identification key

1aPlants flowering in autumn; upright trees or multi-stemmed shrubs; female inflorescences racemose or paniculate; fruit woody, small, green, taking at least one full season to ripen (but often persisting through winter until summer to autumn)2
1bPlants flowering in spring, with fruit maturing in late summer-autumn; inflorescences and fruit not as above5
2aSmall rare shrub of North America; leaves with lateral veins craspedodromous or semicraspedodromous (usually mixed venation), often much branched near leaf margin or infrequently ending in the teeth; female inflorescences 1–(2) in leaf axils along the main stemsAlnus maritima
2bLarge upright trees; Himalaya to Taiwan and Vietnam; venation semicraspedodromous or eucamptodromous, reticulate; female inflorescences in clusters of two or more in leaf axils3
3aLeaves ovate to ovate-lanceolate, relatively small (55–130 × 30–50 mm), base cordate to truncate; male inflorescence of 2–5 catkins in raceme; common tree throughout TaiwanAlnus formosana
3bLeaves ovate to elliptic, larger, base cuneate to rounded; male inflorescence of 3–10 catkins in raceme or panicle; common throughout warm temperate regions of Himalayas east to Vietnam4
4aLateral leaf veins craspedodromous, reaching margins of leaves, terminating in mucronate teeth, with 8–12 veins either side of midrib; male inflorescence of 3–5 catkins in raceme; northern India and PakistanAlnus nitida
4bLeaves with 10–14 pairs of veins which do not reach leaf margins (eucamptodromous), with 10–14 veins either side of midrib; male inflorescence of 5–10 catkins paniculate; common tree in warm temperate regions of Himalayas and east to China, Thailand and Vietnam.Alnus nepalensis
5aUpright tree; leaves obovate and terminating abruptly in acute apex, semicraspedodromous; female inflorescences always solitary in leaf axils, fruit globose to oblong with scales more or less equal in size6
5bTree to small shrub, bark various; leaves various; female inflorescences in raceme, rarely solitary and if so then with fruit scale apices with recurved erect tip8
6aInflorescences on short erect peduncles, (perhaps becoming pendulous in fruit?) leaves broad and with buff-yellow hairs, especially in tufts in vein axils and along veins; seed wings very narrow (<1 mm)Alnus ferdinandi-coburgii
6bInflorescences pendulous and on long peduncles, leaves narrow and either covered in dense white hairs or more or less glabrous; seed wings broad (>1.2 mm)7
7aYoung twigs, buds, pedicle and leaves (especially abaxially) covered in long buff or white hairs, and long tufts in vein axilsAlnus lanata
7bPlant more or less glabrous, or if hairy, with short, white papillose hairs along veins, and short tufts in veins axils; leaves relatively narrow (as above), shiny dark greenAlnus cremastogyne
8aLarge or small shrubs to multi-stemmed trees; buds apparently sessile to very shortly (<1 mm) stalked, conical, shiny green and variously sticky with 2–6 unequal imbricate scales, leaves supervolute in bud; leaves with 10–26 pairs of straight, parallel lateral veins, serrately toothed; seeds elliptical with two broad forward-pointing wings and divergent styles9
8bShrubs to trees; buds various but often two-scaled with distinct stalk (often as long as bud), not sticky; seeds obovate with wing absent to broad lateral wings; leaves variable with 5–14 pairs lateral veins which are never parallel and straight, variously toothed to lobed, applanate or applanate in bud; circumboreal extending into South America18
9aLeaves ovate to ovate-elliptic, with 7–12 pairs veins, apex broadly acute to truncate, apical half with (0–)1–2 secondary teeth between the primary teeth; bud scales peeling back on opening of buds and retained until late in the season; staminate catkins) 1–5, often terminal in clustered groups, resinous, green; pistillate catkins below staminate, in clusters; fruit scales equal in size, without recurved point10
9bLeaves ovate to ovate-lanceolate, with 20–26 lateral veins on either side of midrib, apex acute to acuminate, apical half with 4–6 secondary teeth between the primary teeth; bud scales not peeling back, deciduous or not; cones in terminal or lateral groups 1–6, pendulous to erect; fruit scales with terminal recurved point18
10aLeaves large (to 200 × 150 mm), broadly ovate, base cuneate, truncate or cordate, apex acuminate to acute, even or coarsely double toothed11
10bLeaves generally smaller (<120 × <100 mm), ovate to orbicular; apex acute to sub-obtuse, toothing single or rarely double toothed13
11aLeaves mostly matt above and below, sharply toothed with secondary order of toothing relatively small and even, rarely slightly lobed between primary teeth; male inflorescences to 100 × 5–10 mm at maturity; cones relatively small (8–15 × 5–7 mm); eastern Russia (Sakhalin, Primorsky, Southern Kurils), and JapanAlnus alnobetula subsp. maximowiczii
11bLeaves glossy above, sharply toothed with second order of toothing uneven, often slightly lobed between primary toothing; male inflorescences 80–120 × 10–15 mm at maturity; cones generally larger (12–25 × 6–10mm); western USA, far eastern Russia, Ulleungdo/Jukdo12
12aLeaf toothing irregular but rarely slightly lobed between primary veins, blade fairly flat with wide slightly sinuous veins, teeth curved slightly upwards; cones large (15–25 × 7–10 mm); large shrub of Ulleungdo and Jukdo islands, South KoreaAlnus alnobetula from Ulleungdo
12bLeaf toothing irregular, often with leaves slightly lobed between secondary veins, blade distinctly rugose, veins straight, teeth not curved upwards; cones smaller (12–20 × 6–8 mm); shrubs of the mountainous and coastal north-western United States, Canada, and across to far eastern RussiaAlnus alnobetula subsp. sinuata
13aLeaves often broader than long, broadly ovate to orbicular14
13bLeaves longer than broad, ovate to ovate-elliptic, broad to narrow15
14aLeaves broadly ovate to orbicular, apex bluntly acute, leaves relatively large (30–70 × 40–65 mm); CorsicaAlnus alnobetula subsp. suaveolens
14bLeaves usually always wider than broad, with somewhat emarginate tip, leaves smaller (20–35 × 20–25 mm); Mount Hakkoda, JapanAlnus alnobetula subsp. hakkodensis
15aLeaves large ((30–)60–120 × (25–)40–95 mm), leathery with thick cuticle, broadly ovate to elliptic, base cuneate to rounded, apex broadly acute, toothing even and sharp, serrulate or finely serrate; cones pendulous, large (12–20 × 7–8 mm); northern parts of North America, Greenland and far north eastern RussiaAlnus alnobetula subsp.. crispa
15bLeaves generally smaller, ovate to elliptic, base cuneate, rounded or cordate, apex acute to apiculate, toothing even or irregular, sharp to blunt; cones pendulous to suberect, smaller (8–15 × 6–7 mm); Eurasia to western North America16
16aLeaves 10–50(–90) mm, usually more or less cuneate at base, with 4–8 pairs of lateral veins; cones erect to suberect; mountainous regions of EuropeAlnus alnobetula subsp.. alnobetula
16bLeaves 30–80(–120) mm, cuneate to subcordate at base, with 7–10 pairs of lateral veins. Secondary veins 7–10/side; cones sub-erect to pendulous17
17aLeaf blade broadly ovate, base cuneate to rounded, apex acute to short-acuminate, margins sharply and densely doubly serrate; cones sub-erect to pendulous; northern and eastern Asia and coastal western North AmericaAlnus alnobetula subsp. fruticosa
17bLeaf base rounded to cordate, margin rather regularly finely dentate; female inflorescences large, ovate-elliptic, broad; male inflorescences short (to 60 mm); southern and eastern ChinaAlnus alnobetula subsp. mandschurica
18aLeaves ovate to ovate-lanceolate, with apex acuminate, with 18–27 lateral veins on either side of midrib; female inflorescences pendulous with 2–6 catkins; cones small ; JapanAlnus pendula
18bLeaves ovate, with apex acute to acuminate, with 10–17 lateral veins on either side of midrib; female inflorescences semi-erect to pendulous with 1–6 catkins; Japan and Korea19
19aShrubs; male inflorescence compact, single; female inflorescences usually single or double in lateral branches above male inflorescences; Zhejiang Province, ChinaAlnus betulifolia
19bSmall trees to large shrubs; male inflorescences compact or elongate; female cones held in upper branches or in leaf axils below males, solitary in in groups of up to 6; Japan, Korea20
20aCones almost always solitary, branches glabrous, with 12–15 lateral veins on either side of midribAlnus sieboldiana
20bCones in groups of (1–)2–6; branches hairy and often white pilose when young; leaves with 13–17 lateral veins on either side of midribAlnus firma
21aBark shining grey or grey-brown with prominent numerous lenticels; stipules persistent (but if not, leaves as below); leaves bullate, emarginate (or with broadly acute tip and ±unlobed, especially on extension growth), orbicular to obovate; cones connate to ovate; shrubs; Japan22
21bBark various but often grey, not shining; stipules not persistent; leaves on long shoot typically not emarginate (or if so, blade flat, matt, lobulate or not); cones various, rarely connate (if so, leaves large and ovate – southern forms of Alnus acuminata)24
22aSmall to medium-sized shrub; leaves all longer than broad, never emarginate; fruits oblong to ovoid; high forests of JapanAlnus serrulatoides
22bTree or large shrub to 20 m; long shoot leaves orbicular (much broader than long), often with an emarginate apex; fruits connate and rarely ovate23
23aMulti-stemmed shrub; stipitate with large stipules, generally broader than long (15 × 25 mm) present late into season, especially on long shoots; leaves folded (conduplicate) in bud, opening to glossy green, especially aboveAlnus fauriei
24aShort shoot leaves unlobed or only slightly so, edges with uniformly sized teeth, often minute, serrate, denticulate or mucronate, always longer than wide25
24bShort shoot leaves often lobulate, toothing more conspicuous and often coarsely double-toothed, longer than wide to broader than long37
25aPistillate inflorescences single lower on shoot in leaf axils, or rarely terminal in clusters of 1-5; autumn flowering; North America26
25bPistillate inflorescences terminal if single or on short side shoots, 1–5; spring flowering29
26aFruits in pendulous to semi-erect racemes 2-5; Americas27
26bFruits in erect racemes 1–5; Eurasia30
27aLeaves large, 50–190 mm long, apex acuminate; infructescences 11–45 mm long; large trees of Mexico to South America28
27bLeaves usually smaller, 40–100 mm long, apex acute to obtuse; infructescences 10–17 mm long; large shrubs or small trees of North America29
28aLeaves broadest towards the middle to apexAlnus jorullensis
28bLeaves broadest towards the baseAlnus acuminata
29aLeaves elliptic to obovate, apex broadly acute to rounded, leaf texture papery to moderately coriaceous; staminate flowers with 4 stamens; large shrubs of eastern North AmericaAlnus serrulata
29bLeaves narrowly elliptic to rhombic, apex acute or obtuse, usually not rounded, texture thicker, slightly farinose; stamens 2, or 4 with 2 reduced in size; trees of mountainous western United StatesAlnus rhombifolia
30aLeaves generally large (60–150 × 40–80 mm), base subcordate to truncate, rarely cordate31
30bLeaves smaller (30–80 × 20–60 mm), base cordate to cuneate; lobulate or unlobed with varying toothing32
31aTwigs, petioles and leaf blade abaxially typically pubescent, leaf apex acute; male inflorescences to 150 mm, flowering December–FebruaryAlnus subcordata
31bTwigs, petioles and leaf blade abaxially glabrous and often glossy, leaf apex acute to caudate; male inflorescences 30–45 mm long, flowering February–MarchAlnus djavanshirii
32aLeaf base cordate (rarely subcordate); Italy and CorsicaAlnus cordata
32bLeaf base cuneate to truncate, rarely cordate33
33aTwigs glabrous, angled; buds with covering scale over winter; leaves thin textured, variously lobed and typically undulate; fruits ovoid-elliptic, small (10–20 × 8–15 mm); SE Europe to northern IranAlnus orientalis
33bTwigs pubescent with buff hairs, terete; buds with terminal bud with reduced or absent scales, exposing leaves over winter; leaves thick textured and somewhat glossy, unlobed with few minute teeth; fruits larger, ovoid; eastern Asia34
34aLeaves obovate, obovate-elliptic, or obovate-lanceolate, base cuneate, 6-10 veins either side of midrib, often with membranous domatia in vein axilsAlnus japonica
34bLeaves obovate-oblong, oblanceolate-oblong, or oblong, base subrounded, subcordate, or broadly cuneate; apex acute, to acuminate or caudate, 10–15 veins either side of midrib; lacking domatia in leaf axils35
35aLeaves large (120–300 × 50–100 mm)Alnus x spaethii
35bLeaves smaller (100–160 × 30–70 mm)36
36aVigorous plant with dense semi-erect branching; pubescent on shoots, petioles and abaxial leaf surface; leaf base cuneate, adaxially pubescent; Japan and ManchuriaAlnus x mayrii
36bUpright tree with an open crown; leaf base rounded to broadly cuneate, adaxially glabrous; eastern China, Japan, S KoreaAlnus trabeculosa
37aLeaves narrowly ovate or lanceolate to narrowly elliptic, 50–90 × 30–60 mm, major teeth sharp to acuminate, short to long; stamens and perianth parts 2 or 4, if 4, then 2 large and 2 smaller; trees of mountainous areas of Southern Arizona and northern New MexicoAlnus oblongifolia
37bLeaves broader, major teeth serrulate to dentate, often broadly so; stamens not as above38
38aLeaves lobed to denticulate, blade with margins strongly revolute at edges; large tree of western USAAlnus rubra
38bLeaves lobed or not, blade flat or not strongly revolute at edges; trees or shrubs39
39aTwigs somewhat to distinctly winged, glabrous; buds small (to 2mm), globular, dark brown to black; leaf base truncate to rounded, thin but leathery texture and glabrous; SE Europe to IranAlnus orientalis
39bTwigs terete; buds larger, green to dark brown; leaves lobulate or not, base cuneate to rounded, thicker and often hairy, if only abaxially on veins or in axils40
40aLeaf apex narrowly acuminate or caudate, margin of leaves with large triangular teeth in upper half and even smaller teeth in basal half, not lobulate; fruits large (30–40 × 10–20 mm), connate to elliptic; rare tree of IranAlnus dolichocarpa
40bLeaf apex broadly acuminate to emarginate, margin with even-sized teeth throughout or lobulate; fruits small or large41
41aShort shoot leaves margins finely serrate or serrulate, sometimes slightly lobed, apex broadly to narrowly acute; staminate flowers with 2 stamens (sometimes 4 with two reduced); large tree of western USAAlnus rhombifolia
41bShort shoot leaves coarsely serrate, denticulate, or lobed; stamens 442
42aLeaves large (50–180 mm long), margins slightly to moderately lobed, with acuminate to obtuse teeth on proximal side only, margins slightly revolute, lateral vein axils with hairy domatia; large tree of Central & South AmericaAlnus acuminata
42bLeaves smaller (40–100 mm long), margins variously lobed, serrate to coarsely double-serrate to deeply lobed, lateral vein axils lacking domatia45
43aYoung stems, petioles and abaxial leaf surface glabrous; tree of central and southern MexicoAlnus acuminata subsp. glabrata
43bYoung stems petioles and abaxial leaf surface variously hairy, often villous to velutinous44
44aLeaf margin regularly finely serrate; trees of South AmericaAlnus acuminata subsp. acuminata
44bLeaf margin double serrate; trees of Mexico and Central AmericaAlnus acuminata subsp. arguta
45aLeaves orbicular to broadly elliptic or obovate with apex rounded, obtuse, obcordate or emarginate, ± glabrous; Europe and Caucasus46
45bLeaves ovate to obovate, apex bluntly acute to apiculate; circumboreal48
46aLeaves orbicular, abaxially pubescent, often with stripes of white hairs parallel with lateral veins and along veins and in vein axils; shoots and buds pubescent; western Balkans, Albania and GreeceAlnus rohlenae
46bLeaves circular obovate, abaxially glabrous or sparsely pubescent, adaxially glabrous or sparse grey or rufous hairs in vein axils; shoots and buds glabrous47
47aLeaves 3–4(–7) times longer than petiole, often with emarginate apex; female catkin peduncle short (7–10) mm, female catkin ±1.5 times longer than wide; EurasiaAlnus glutinosa subsp. glutinosa
47bLeaves 2–3(–4) times longer than petiole, with rounded or obtuse apex; female catkin peduncle longer (12–16) mm long, female catkin ±1.75 times longer than wide; Iberian PeninsulaAlnus lusitanica
48aLeaves ovate-orbicular to broadly elliptic, base obtuse to truncate, glaucous beneath, veins abaxially prominent; fruits large (20–30 × 10–20 mm), ovate; western Asia49
48bLeaves narrowly ovate to broadly ovate-elliptic, base obtuse to cuneate, glabrous to densely pubescent abaxially, veins prominent or not; fruits usually small50
49aTwigs densely pubescent; stipule elliptic-acuminate; leaf veins abaxially very prominent, tertiary leaf veins convex; Japan onlyAlnus inokumae
49bTwigs glabrous; stipule broadly elliptic to obovate; leaf veins abaxially not prominent, tertiary leaf veins ±straight; eastern AsiaAlnus hirsuta
50aMedium sized trees or shrubs with grey or yellowish bark; young twigs smooth, pubescent; leaves ovate to elliptic, apex acuminate to subacute, pubescent abaxially; pistillate inflorescences with short peduncle51
50bLarge trees with dark fissured bark; young twigs viscid, glabrescent; leaves ovate to obovate, apex broadly acute to obtuse, glabrescent with hairy domatia in vein axils; pistillate inflorescences with long peduncle56
51aLeaf blade thick, shoot and long-shoot leaves variously hairy but normally sparsely to densely pubescent52
51bLeaf blade thin and papery, shoot and long-shoot leaves glabrous, glabrescent to sparsely pubescent, often glossy and sticky54
52aShort-shoot leaves coarsely double serrate, only ± lobulate on part of shoots, major teeth acute, long-shoot leaves lobulate; large shrubs of eastern North AmericaAlnus incana subsp. rugosa
52bShort-shoot leaves lobulate ± throughout53
53aLeaves ovate to elliptic apex acute to acuminate, densely pubescent to tomentose abaxially; throughout EuropeAlnus incana subsp. incana
53bLeaves elliptic, abaxially and adaxially pubescent especially when juvenile, densely so along veins; eastern Turkey and northern IranAlnus glutinosa subsp. antitaurica
54aTwigs and petiole glabrous to sparsely pubescent; major teeth rounded or blunt; large spreading shrubs or small trees of western North AmericaAlnus incana subsp. tenuifolia
54bTwigs and petiole glabrous to glabrescent; major teeth acute to denticulate55
55aLeaves lobulate with 5–7 lobes each side, long-shoot leaves secondary vein axils glabrous; Turkey onlyAlnus glutinosa subsp. betuloides
55bLeaves with 5–11 lobes each side, long shoot leaf veins pubescent with hairy domatia56
56aLeaves ovate to obovate, apex acute to obtuse or emarginate, matt and somewhat thin textured, with 5–9 veins either side of midribAlnus glutinosa subsp. glutinosa
56bLeaves elliptic to oblong, apex acute or acuminate, often thick and somewhat glossy textured, with 8–11 veins either side of midrib; south eastern Europe to northern IranAlnus glutinosa subsp. barbata