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Thirty-five species of Alnus are recognised, distributed across the northern hemisphere, though with an extension through Central America into the Andes (Govaerts & Frodin 1998). Two subgenera (Alnus and Alnobetula) are recognised within the genus, and some important distinctions are summarised in the table below (p. 136). Alders are deciduous or occasionally evergreen trees or shrubs with distinctive root nodules, which contain nitrogen-fixing mycorrhizae. The bark is typically smooth, though in mature specimens it can be furrowed, corky or broken into flat plates. The Latin American species (A. acuminata, A. jorullensis) have prominent constrictions in the stem. Winter buds form at the apex of short branches and are usually protected by two bud scales (some species have no bud scales, merely reduced leaves: for example, A. acuminata, A. formosana, A. ferdinandi-coburgii). These are pubescent, glandular, and covered in a resinous secretion. Initially, the expanding leaves are plicate (folded), though they flatten later. The leaves are simple, petiolate and with double-serrated margins (rarely entire). They are typically ovate, pubescent and deeply grooved. All alders have glandular leaves, the glands concentrated on the lower surface and petiole. Stipules are present in most species and range in size between 0.5 and 15 mm long. Alnus is monoecious; the staminate inflorescences (catkins) are cylindrical and pendent, with numerous inconspicuous flowers, the pistillate inflorescences erect, solitary or in groups, with numerous, persistent, five-lobed woody bracts, overlapping to form cone-like structures (referred to as ‘cones’ in the account below). This contrasts with Betula, where the pistillate inflorescences are catkin-like. The pistillate flowers are also inconspicuous. 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. The fruit is a tiny samara, with papery or leathery wings, although these are reduced in some species (Furlow 1979a, 1979b, 1982, Ashburner 1986, Li & Skvortsov 1999).
Alnus is an often overlooked genus, its members forming part of the backdrop of trees in many places but seldom receiving much attention except as useful, fast-growing temporary fillers – even car-park trees in the case of A. cordata, or for awkward damp places. This is unfortunate, because some alders are truly beautiful in shape and foliage features, and many have attractive catkins in early spring. For flowering effect A. maximowiczii, especially the provenance from Ullung-do, South Korea, is outstanding, with fat green catkins in late winter, but it is curiously scarce and seldom seen. Alnus sieboldiana and A. cordata are also very showy in spring catkin.
Most alders are pioneer species, germinating on open, often disturbed ground and growing rapidly until out-competed by other species that establish in their shade. In cultivation they require a similarly open site, and are particularly useful for stabilising reclaimed land. Although most will do best in moist soil they do not necessarily need to be planted in marshy conditions and many will be happy in drier sites, although their growth rate may be slower. The recently introduced species described here are in general still rather scarce in cultivation and represented only in specialist collections. In the United Kingdom there are three National Plant Collections of Alnus – in Jersey (Jersey Association of Men of the Trees: not visited), at Blagdon, Northumberland (Lord Ridley), and at Stone Lane Gardens, Devon (Kenneth Ashburner). In addition there are good collections of Alnus at the Hillier Gardens, Howick, Kew and Ness. Most arboreta in Europe and North America have a moderate representation of the genus, but with few of the more recent introductions.
Key characters for Alnus subgenera Alnus and Alnobetula, from information supplied by K. Ashburner
Alders – mostly trees (e.g. A. glutinosa)
Bud scales two, but absent in the A. japonica group
Leaves usually large, rounded
Pistillate flowers exposed in winter, scales open in spring
Fruits often dense, adapted for flotation, with thick corky wings, but sometimes thin (A. rubra, A. lanata and possibly A. ferdinandi-coburgii)
Cone scales thickly woody
Wood turns bright brown on exposure
Green Alders – mostly shrubs (e.g. A. viridis)
Bud scales numerous, spirally arranged
Leaves usually narrow, pointed, with deeply impressed lateral veins
Pistillate flowers emerge in spring
Fruits light, adapted for wind-dispersal, with wide, thin wings
Cone scales thinner
Wood remains white on exposure
Alnus jorullensis from Mexico was described by Clarke (1988) but with a rather negative report on its hardiness. More recent plantings seem to be more successful, and in these warmer times it should be tried more widely.
Seed, sown on the surface and kept moist, provides the best means of propagating alders, but hybridisation can be a problem in collections. It is important to transplant seedlings with care so as not to disturb their roots, and to use a well-aerated, non-sterile or inoculated compost to ensure nodulation (H. McAllister, pers. comm. 2007). Alnus seedlings are somewhat prone to damping off (K. Ashburner, pers. comm. 2007), so appropriate precautions should be taken. Early summer cuttings rooted under mist are perhaps the best option for propagating selected clones or rare individuals (Ashburner 1986).
The alders are deciduous trees and shrubs closely allied to, and only likely to be confounded with, the birches (Betula). Leaves with stipules, alternate, more or less toothed in all the cultivated species. Winter buds nearly always stalked. Male and female flowers borne on the same tree but on separate catkins. Male catkins long and slender, usually in clusters of two to six; the flowers small, with a four-lobed calyx, no petals, and usually four (sometimes one to three) stamens. Female catkins shorter, clustered, or rarely solitary, developing into woody, cone-like “fruits”, correctly, ‘strobiles’, 1⁄3 – 11⁄4 in. long. The “seed” (the true fruit) is a minute, flattened nutlet, often with thin membranous wings at the sides. Apart from three species – A. maritima, A. nepalensis, and A. nitida – which flower in autumn, the cultivated alders form their catkins in the late summer and autumn; these expand the following spring, either very early before the leaf-buds begin to grow, or along with the leaves; the fruits develop during the summer and persist until the succeeding spring. From the alders the birches are distinguished by the fruits being longer, not woody, and falling to pieces (those of the alders falling whole), and the flowers of birches have never more than two stamens.
In gardens and parks the alders are chiefly valuable for growing in wet situations unsuited to the majority of trees. Some, however, such as A. japonica, nitida, cordata, and firma, succeed quite well in ordinary good soil. All are best propagated by seed except the garden varieties, which may be grafted on their respective types, or, better still, rooted from cuttings made as soon as the leaves fall, and put in sandy soil, as willow or poplar cuttings are – compared with which, however, they do not strike root so readily. The following is a selection of the best worth growing, irrespective of their use in damp places: cordata, firma, nitida, rubra; glutinosa ‘Imperialis’; incana ‘Laciniata’ and ‘Aurea’.
The American species are revised by John F. Furlow in Rhodora, Vol. 81, no. 825, pp. 1-121 and no. 826, pp. 151-248 (1979). See also the article by the same author in International Dendrology Society Year Book 1981, pp. 115-19.