Alnus Mill.

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.

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’.