Alnus Mill.

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.