Alnus Mill.

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Credits

Tim Baxter & Hugh A. McAllister (2021)

Recommended citation
Baxter, T. & McAllister, H.A. (2021), 'Alnus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/). Accessed 2022-10-02.

Family

  • Betulaceae

Common Names

  • Alders

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.
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.
phenology
The seasonal timing of events in the life cycle of a plant or animal and the study thereof.
section
(sect.) Subdivision of a genus.
bract
Reduced leaf often subtending flower or inflorescence.
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.
domatia
Cavity or tuft of hairs that acts as a shelter for insects or other creatures.
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.
ovoid
Egg-shaped solid.
phloem
Sap-carrying vascular tissue.
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’.
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 (2021)

Recommended citation
Baxter, T. & McAllister, H.A. (2021), 'Alnus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/). Accessed 2022-10-02.

New text for Alnus is currently being edited and prepared for publication during 2022 (JMG, February 2022).

Deciduous (rarely semideciduous) monoecious trees and shrubs, 2.5–30 m. Bark smooth to rough. Branches terete with triangular pith and conspicuous circular to elliptic lenticels, and sparsely to densely simple hairs, glandular. Branchlets terete or winged. Winter buds resinous, ovoid to oblong or rounded, either on long stipe (actually 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 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 without perianth, 1-pistil, ovary inferior, 2 linear styles. Pistillate inflorescences maturing to persistent resinous woody cones, ovoid to cylindric, mature scales hard (occasionally soft) 3 or 5 lobed. Seed ovate to obovate or orbicular nutlet, with or without a membranous wing, or with thickened border, 1–2 per bract. 

Alnus is a complex and ancient genus originating in the late Cretaceous (Graham 1993; Chen, Manchester & Sun 1999). There are 49 taxa belonging to 34 species distributed across the northern hemisphere and in South America as far south as northern Argentina. They vary widely from tall single-stemmed fast-growing trees to smaller, multi-stemmed shrubs. The largest species is A. glutinosa (to 35 m) whilst the smallest is A. alnobetula agg. (ssp. alnobetula grows 0.5 to 10 m (Tutin et al. 1964)). Almost all are light demanding. They are wind pollinated and wind dispersed and many colonise rapidly to form potentially pure stands in suitable open habitats. They also can be somewhat weedy, and 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 including many in wetter soils along river valleys and around water bodies (e.g. A. glutinosa, A. maritima), to 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 ssp. 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 in their root nodules. All alders have a symbiotic relationship with the nitrogen-fixing bacteria Frankia alni agg., a cosmopolitan species found in nodules in their roots. 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.

Alnus has been subject to various taxonomic treatments, often resulting in quite different classifications, with more work clearly needed to resolve the incongruence between morphology and genetics. There are several clearly related morphological groups, but there continues to be 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 is more or less followed here despite key omissions in that work. Recent genetic work has confirmed Alnus is a monophyletic genus (e.g. Bousquet, Strauss & Li 1992; Chen, Manchester & Sun 1999; Chen & Li 2004), most closely related to Betula, and distinguished by the presence of persistent woody cones. Morphologically there is much sense in splitting Alnus into three subgenera: Clethropsis, Alnobetula and Alnus, and this is how they are tentatively presented here.

Subgenus Alnobetula is distinguished by elliptic-lanceolate buds with short stalks (<1 mm), leaves that are strongly craspedodromous, pistillate inflorescences that emerge in spring, and cones with thin scales that mature in autumn. Subgenus Alnobetula is split into two sections, Section Alnobetula and Section Bifurcatus. Section Alnobetula (1 species, 6 taxa) has broad leaves and cone scales that are ± equal in size and lack a recurved point. Section Bifurcatus (4 species) has narrower leaves, and unequal sized cone scales and a terminal recurved point.

Subgenus Clethropsis (4 species, 7 taxa) flower in autumn, with cones that take until the following summer or autumn to ripen (see notes below).

Subgenus Alnus consists of four Sections: Cremastogyne; Fauriae; Glutinosae; and Japonicae, and is distinguished in flowering in spring, buds with long stalks (>2 mm), leaves with weak venation, pistillate flowers exposed in winter and cones with thick cone scales. Section Cremastogyne (3 species) have pistillate inflorescences that are solitary in leaf axils. Section Fauriae (2 species) have persistent stipules, with bullate, emarginate leaves. Section Japonicae (7 species, 12 taxa) have leathery leaves with large cones with seeds angular, 5-sided with narrow seed wings. Section Glutinosae (16 species, 23 taxa) are highly variable but all have seeds with broad wings.

The application of modern molecular techniques has confused this picture, and there is clearly more work to be done. Such investigations have confirmed the monophyletic Subgenus Alnobetula and Section Cremastogyne (Chen & Li 2004). The cladistic placement of the morphologically similar autumn-flowering taxa is especially confusing as there is no clear genetic relationship between species. This is also true of other species including Section Japonicae and A. serrulata that appears to be more closely related to A. maritima than others in Subgenus Alnus (Chen & Li 2004; Colagar et al. 2016; Ren, Xiang & Chen 2010). The cause of these complications may be the reticulate evolution of very ancient species, with populations and species becoming geographically isolated then re-joining over time, with considerable mixing where they meet. This has led to variable taxa whose ranges have overlapped, allowing natural hybridisation and introgression to occur. Certainly there are certain regions (e.g. SE Europe to Iran and maritime east Asia) where there is a confused picture. Certain species are clearly intermediate between described sections and share characteristics of both, e.g. A. japonica, in 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 some identification difficulties in Alnus and several species have 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.

Identification of many species of alders is generally straightforward but certain groups are especially difficult. This is often the case where species aggregates exist, e.g. A. alnobetula s.l., A. glutinosa agg. Plants of unknown provenance, such as found in gardens, can be exceedingly difficult or impossible to identify as hybrids of novel parents are frequent. Whole-plant examination is important and examination of form, vegetative and reproductive structures are 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, and 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 on the stem growing in full sun. Other leaves on the plant are more variable and not necessarily reliable. 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 cones. 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 ‘cone’ 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. Cones vary in size from small (A. glutinosa) to large (A. sieboldiana) and in shape from ovoid to elliptic to cylindric. Cone 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).

Alders are very easy to cultivate and are widely cultivated worldwide. Their ability to fix nitrogen and rapid growth has made 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 A. glutinosa, with A. incana and 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 ssp. glutinosa should perhaps be replaced by the more drought tolerant A. glutinosa ssp. barbata or the similar A. rohlenae or A. lusitanica in drier or warmer situations. Most alders are completely hardy but there is huge variation in their cold tolerance. Some species originate from cold continental climates, including A. incana, A. alnobetula and A. hirsuta, but these may also be susceptible to early or late frosts. 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 grow into large trees given the right care. Seed viability of species is variable, with many typically low, but some having high viability, perhaps particularly 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 infested alder trees (but note the risk of Phytophthora alni), or crushed nodules may usually ensure 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 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.

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. Already this disease has claimed millions of trees in riparian habitats throughout Europe, particularly the UK, north-eastern 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 heavy cone 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 in Alaska and Oregon are P. siskiyouensis and P. alni subsp. uniformis (Adams et al. 2008) that cause basal rot and similar fatalities. Another problem species is Alder Leaf Beetle, (Agelastica alni) that affects many species of alder, especially A. glutinosa. It is not generally fatal but can cause severe defoliation, stressing the tree and ruining its aesthetic value. It does not appear to affect species with thick or leathery leaves including any Subgenus Alnobetula. Many of the matt-leaved species (e.g. A. glutinosa) are susceptible to alder leaf rusts, Melampsoridium hiratsukanum and M. botulinum, both non-harmful rusts that discolour the underside of leaves. the first is 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 cones, especially A. glutinosa. Another little known component of alder biology is their mutualistic relationship to mites. In many species, they contain hair or bract-like structures in the vein axils of leaves which house mites that have a predatory or 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 the tree which have otherwise very palatable foliage. Those trees with domatia are also very susceptible to Alder Leaf Beetle, and this non-native species is clearly immune to any otherwise predatory effect of the mites.

Some of the most ornamentally attractive species 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, 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 Ulleung-do 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, but many provenances not currently grown 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 s.l. is more vigorous and flowers over winter.

1aPlants flowering in autumn; upright trees or multi-stemmed shrubs; female inflorescences racemose or paniculate; cones small, green, taking at least one full season to ripen2
1bPlants flowering in spring, with cones maturing in late summer-autumn; inflorescences and cones not as above5
2aSmall rare shrub of North America; leaf venation 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; leaves semicraspedodromous or eucamptodromous, reticulate; female inflorescences in clusters of two or more in leaf axils3
3aLeaves ovate to ovate-lanceolate, relatively small (55-130 x 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
4aVenation craspedodromous, reaching margins of leaves, terminating in mucronate teeth, with 8-12 pairs of veins; 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),; male inflorescence of 5-10 catkins, paniculate; common tree in warm temperate regions of Himalayas and east to China, Thailand and VietnamAlnus nepalensis
5aUpright tree; leaves obovate and terminating abruptly in acute apex, semicraspedodromous; female inflorescences always solitary in leaf axils, cones 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 cone scale apices with recurved erect tip8
6aInflorescences on short erect peduncles, leaves with buff-yellow hairs, especially in tufts in vein axils and along veins; seed wings very narrow (<1mm)Alnus ferdinandi-coburgii
6bInflorescences pendulous on long peduncles, leaves covered in dense white hairs or more or less glabrous; seed wings broad (>1.2mm)7
7aYoung twigs, buds, pedicle and leaves (especially abaxially) covered in long torulose white hairs and long tufts in vein axilsAlnus lanata
7bPlant more or less glabrous, or if hairy, short white papillose hairs along veins, and short tufts in veins axilsAlnus cremastogyne
8aLarge or small shrubs to multi-stemmed trees; buds apparently sessile to very shortly (<1mm) stalked, conical, shiny green and variously sticky with 2-6 unequal imbricate scales; leaves supervolute in bud, 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; leaves variable with 5-14 pairs lateral veins which are never parallel and straight, variously toothed to lobed, applanate or supervolute in bud21
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; cone 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 of 1-6, pendulous to erect; cone scales ±unequal with terminal recurved point18
10aLeaves large (to 200 x 150mm), broadly ovate, base cuneate, truncate or cordate, apex acuminate to acute, evenly or coarsely double toothed11
10bLeaves generally smaller (<120 x <100mm), ovate to orbicular; apex acute to sub-obtuse, toothing single or rarely double13
11aLeaves mostly matt above and below, sharply toothed with secondary order of toothing relatively shallow and regular; male inflorescences to 100 x 5-10mm at maturity; cones relatively small (8-15 x 5-7mm); eastern Russia (Sakhalin, Primorsky, Southern Kurils), and JapanAlnus alnobetula subsp. maximowiczii
11bLeaves glossy above, sharply toothed with second order of toothing uneven; male inflorescences 80-120 x 10-15mm at maturity; cones generally larger (12-25 x 6-10mm); western USA, far eastern Russia12
12aLeaf toothing irregular but rarely slightly lobed between primary veins, blade fairly flat with wide slightly sinuous veins; cones large (15-25 x 7-10mm); large shrub of Ulleungdo and Jukdo islands, South KoreaAlnus sp. nov. (ulleungensis)
12bLeaf toothing irregular, often with leaves slightly lobed between secondary veins, blade distinctly rugose, veins straight; cones smaller (12-20 x 6-8mm); 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 x 40-65mm); CorsicaAlnus alnobetula subsp. suaveolens
14bLeaves usually always wider than broad, with somewhat emarginate tip, leaves smaller (20-35 x 20-25mm); Mount Hakkoda, JapanAlnus hakkodensis
15aLeaves large ((30-)60-120 x (25-)40-95mm), 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 x 7-8mm); 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 x 6-7mm); 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 60mm); southern and eastern ChinaAlnus alnobetula subsp. mandschurica
18aLeaves ovate to ovate-lanceolate, apex acuminate, with 18-27 pairs of veins; female inflorescences pendulous with 2-6 catkins; cones small (8-20 x 4-12mm)Alnus pendula
18bLeaves ovate, with apex acute to acuminate, with 10-17 pairs of veins; female inflorescences semi-erect to pendulous with 1-6 catkins; cones large (12-35 x 10-25mm)19
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 or in in groups of up to 6; Japan, Korea20
20aCones almost always solitary, branches glabrous, leaves with 12-15 pairs of veinsAlnus sieboldiana
20bCones in groups of (1-)2-6; branches hairy and often white pilose when young; leaves with 13-17 pairs of veinsAlnus firma
21aBark shining grey or grey-brown with numerous prominent 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 ovate to elliptic; 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 ovate (if so, leaves large and ovate – southern forms of A. acuminata)24
22aSmall to medium-sized shrub; leaves all longer than broad, never emarginate; cones oblong to ovoid; high forests of JapanAlnus serrulatoides
22bTree to 20m or large shrub; long shoot leaves orbicular (much broader than long), often with an emarginate apex; cones ± cylindrical and rarely ovate23
23aMulti-stemmed shrub; stipules large, generally broader than long (15 x 25mm) present late into season, especially on long shoots, leaves folded (conduplicate) in bud, opening to glossy green, especially aboveAlnus fauriei
23bUpright single stemmed tree, or multi-stemmed shrub; stipules elliptic-lanceolate, falling very early in the season; leaves plicate in bud, opening flat to matt, above and belowAlnus matsumurae
24aShort shoot leaves unlobed or only slightly so, edges with uniformly sized teeth, often minute, serrate, denticulate or mucronate, leaves always longer than wide25
24bShort shoot leaves often lobulate, toothing more conspicuous and often coarsely double-toothed, leaves 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 in clusters of 1-5; spring flowering29
26aCones in pendulous to semi-erect racemes of 2-5; north and south America27
26bCones in erect racemes 1-5; Eurasia30
27aLeaves large, 50-190 mm long, apex acuminate; infructescences 11-45mm long; large trees of Mexico to South America28
27bLeaves usually smaller, 40-100 mm long, apex acute to obtuse; infructescences 10-17mm long; large shrubs or small trees of north America29
28aLeaves broadest around the middle to apexAlnus jorullensis
28bLeaves broadest in proximal halfAlnus 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 x 40-80mm), base subcordate to truncate, rarely cordate31
30bLeaves smaller (30-80 x 20-60mm), base cordate to cuneate; lobulate or unlobed with varying toothing32
31aTwigs, petioles and leaf blade abaxially typically pubescent, leaf apex acute; male inflorescences to 150mm, flowering December-FebruaryAlnus subcordata
31bTwigs, petioles and leaf blade abaxially glabrous and often glossy, leaf apex acute to caudate; male inflorescences 30-45mm 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; cones ovoid-elliptic, small (10-20 x 8-15mm); 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; cones larger, ovoid; eastern Asia34
34aLeaves obovate, obovate-elliptic, or obovate-lanceolate, base cuneate, 6-10 pairs veins, 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 x 50-100mm)Alnus × spaethii
35bLeaves smaller (100-160 x 30-70mm)36
36aVigorous plant with dense semi-erect branching; pubescent on shoots, petioles and abaxial leaf surface; leaf base cuneate, adaxially pubescent; Japan and ManchuriaAlnus × 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 x 30-60mm, 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 IraAlnus 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; cones large (30-40 x 10-20mm), connate to elliptic; rare tree of IranAlnus dolichocarpa
40bLeaf apex broadly acuminate to emarginate, margin with even-sized teeth throughout or lobulate; cones 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 (50-180mm 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 and south AmericaAlnus acuminata
42bLeaves smaller (40-100mm long), margins variously lobed, serrate to coarsely double-serrate to deeply lobed, lateral vein axils lacking domatia43
43aLeaves orbicular to broadly elliptic or obovate with apex rounded, obtuse, obcordate or emarginate, ± glabrous; Europe and Caucasus44
43bLeaves ovate to obovate, apex bluntly acute to apiculate; circumboreal46
44aLeaves 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
44bLeaves ovate to obovate, abaxially glabrous or sparsely pubescent, adaxially glabrous or sparse grey or rufous hairs in vein axils; shoots and buds glabrous45
45aLeaves 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
46aLeaves ovate-orbicular to broadly elliptic, base obtuse to truncate, glaucous beneath, veins abaxially prominent; cones large (20-30 x 10-20mm), ovate; western Asia47
46bLeaves narrowly ovate to broadly ovate-elliptic, base obtuse to cuneate, glabrous to densely pubescent abaxially, veins prominent or not; cones usually small48
47aTwigs densely pubescent; stipule elliptic-acuminate; leaf veins abaxially very prominent, tertiary leaf veins convex; Japan onlyAlnus inokumae
47bTwigs glabrous; stipule broadly elliptic to obovate; leaf veins abaxially not prominent, tertiary leaf veins ±straight; eastern AsiaAlnus hirsuta
48aLeaf blade thick, shoot and long-shoot leaves variously hairy but normally sparsely to densely pubescent49
48bLeaf blade thin and papery, shoot and long-shoot leaves glabrous, glabrescent to sparsely pubescent, often glossy and sticky51
49aShort-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
49bShort-shoot leaves lobulate ±throughout50
50aLeaves ovate to elliptic apex acute to acuminate, densely pubescent to tomentose abaxially; throughout EuropeAlnus incana subsp. incana
50bLeaves elliptic, abaxially and adaxially pubescent especially when juvenile, densely so along veins; eastern Turkey and northern IranAlnus glutinosa subsp. antitaurica
51aTwigs and petiole glabrous to sparsely pubescent; major teeth rounded or blunt; large spreading shrubs or small trees of western North AmericaAlnus incana subsp. tenuifolia
51bTwigs and petiole glabrous to glabrescent; major teeth acute to denticulate52
52aLeaves lobulate with 5-7 lobes each side, long-shoot leaves secondary vein axils glabrous; Turkey onlyAlnus glutinosa subsp. betuloides
52bLeaves with 5-11 lobes each side, long shoot leaf veins pubescent with hairy domatia53
53aLeaves ovate to obovate, apex acute to obtuse or emarginate, matt and somewhat thin textured, with 5-9 pairs of veins either side of midribAlnus glutinosa subsp. glutinosa
53bLeaves 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