Betula cylindrostachya Lindl. ex Wall.

Described by John Lindley (in Wallich 1831) from specimens collected in Uttarakhand, northern India, Betula cylindrostachya is a component of the warm temperate broad-leaved Himalayan forests, its range and altitudinal distribution overlapping with that of Betula alnoides, with which it has been (and continues to be) much confused. The two species often grow in close proximity (Skvortsov 1997) and can occupy similar habitats, but B. cylindrostachya flowers in spring, while Betula alnoides is autumn-flowering and more markedly subtropical in character. Nevertheless, many collectors and observers have not distinguished between the two taxa and they remain widely confused – most recent introductions to cultivation in Britain named Betula alnoides appear to be B. cylindrostachya, helpfully found to be tetraploid in contrast to the reportedly diploid B. alnoides (Mehra & Sareen 1973). In the field the most useful distinguishing characters are probably the spring flowering time (rather than autumn to winter), larger nutlets, much more closely toothed leaf margin, and the hairy and usually almost eglandular young shoots (note, however, that the glands may be difficult to detect under the dense hair, and glandular specimens might be referable to B. alnoides or possibly hybrids). For a table enumerating the distinguishing features, see the entry for B. alnoides.

As a low-altitude, southerly species, Betula cylindrostachya is less hardy than most birches – like other members of subgenus Acuminatae it is unable to fall back on dormant buds to replace new growth damaged by late spring frost or dessicating winds: the buds on young trees break simultaneously, and if these are killed the trees appear to be incapable of generating new buds from the base of their trunks. Probably for this reason, the species is rather rare in cultivation outside its native area, though it promises to be an attractive horticultural subject for warmer temperate climates. In mild, coastal districts, young trees grow rapidly to form large spreading trees – two plants raised from a collection by Peter Cox and Steve Hootman (C&H 9042) have thrived at Tregrehan, Cornwall, reaching 10 m by 2007, with attractive dark brown, shiny trunks, and long showy catkins in early spring (T. Hudson, cited in Grimshaw & Bayton 2009). This collection is also grown at Howick Arboretum, Northumberland. Keith Rushforth’s collections of B. cylindrostachya from Bhutan (KR 876, 976) and Tibet (KR 6000) and Kenneth Ashburner’s north Indian introduction from Garhwal have survived without significant damage for more than ten years at Ness Botanic Garden, forming vigorous spreading, freely fruiting trees which look as if they may eventually grow large and attractive. Given the species’ role as a primary coloniser in its native habitat, the possibility that it could naturalise and become invasive should be borne in mind.

Any batch of seedlings shows a range of variation in time of bud break, and those with later breaking buds may be the only ones that survive late frosts and wind damage in extrazonal temperate conditions. The possibility of selecting strains of Betula cylindrostachya adapted to higher latitude temperate climates is therefore worth serious attention. At Ness, for example, material originating from Garhwal seed has undergone natural selection, many plants having died at the seedling stage from late frosts. Additionally, as noted by Hacke & Sauter (1996) in a discussion on timing of bud break, epigenetic factors – that is, inherited characteristics affecting gene expession but not involving changes in DNA – may result in trees grown from seed from cultivated trees being less susceptible to frost damage than trees grown in the same area from seed of wild origin. Now that B. cylindrostachya is fruiting in western temperate collections it may be possible to produce seedlings adapted to higher latitudes.

The only difference between type Betula cylindrostachya and the relatively recently described B. fujianensis (Zeng, Li & Chen 2008) – treated in synonymy by McAllister and Ashburner (2013) – seems to be the glandular shoots and absence of hair tufts in the secondary vein axils on the abaxial leaf surface (both variable characters in B. cylindrostachya). These Fujianese populations had previously been regarded as B. alnoides (Li & Skvortsov 1999; Zeng, Li & Chen 2008), but the deciduous character, spring flowering, densely hairy young shoots, often subcordate leaf base, suborbicular to ovate nutlet, and greater hardiness reported by Zeng, Li and Chen led those authors to deduce that they did not belong to that species and they described them as a new species, B. fujianensis, which they compared with B. alnoides and B. luminifera but not B. cylindrostachya.