Betula alnoides Buch.-Ham. ex D.Don

This widespread Himalayan birch is a component of the tropical to subtropical forests from Nepal eastwards to Vietnam, mostly below 2000 m, though perhaps reaching 2600 m in Yunnan. It is unlikely to be hardy over most of the British Isles and has probably only been introduced to cultivation very recently from Vietnam (T. Hudson, in McAllister & Ashburner 2013). It was described by Francis Buchanan-Hamilton in David Don’s introduction to the Nepalese Flora (Don 1825); the name alnoides refers to its broad resemblance to an alder.

Betula alnoides has been much confused with B. cylindrostachya: the two species overlap in altitudinal range and sometimes grow together in the Himalaya. Betula alnoides is autumn-flowering and broadly tropical to subtropical while B. cylindrostachya is warm temperate and flowers in spring, but many collectors and observers have not distinguished between the two and they remain widely confused. In the field the most useful distinguishing characters of B. alnoides are probably the flowering time and spring fruiting (February to April), the very glandular shoots, and the sparse marginal toothing. Skvortsov (1997) reported that wild Betula alnoides near Kalimpong, West Bengal, developed new shoots in mid-October while still retaining their old leaves, implying that the species is probably evergreen in its natural environment.

The differences may be summarised as follows (McAllister & Ashburner 2013, Skvortsov 1997):

Skvortsov (1997) additionally mentions differences in fruit wing shape – more or less symmetrical in Betula alnoides; forward-pointing and extending well beyond the nutlet in B. cylindrostachya – but this character seems to be very variable even within a catkin.

Published chromosome counts for Betula alnoides are diploid (2n = 28) (Mehra & Sareen 1973), but tetraploid in B. cylindrostachya. These cytological findings need further testing and confirmation (particularly with Himalayan material), but such data has been found diagnostically useful: two collections of ‘Betula alnoides’ from Bhutan by Keith Rushforth, and one from the Garwhal area of northwest India by Kenneth Ashburner, were referred to B. cylindrostachya, on the basis of chromosome number in combination with other characters (spring flowering, broader fruiting catkins and larger nutlets). The different flowering times and distinct ploidy levels of the two species greatly reduce the opportunities for interbreeding, explaining why these closely related species remain distinct and why no hybrids have been reported.

Although both Betula alnoides and B. cylindrostachya (as well as the closely related B. luminifera) grow in tropical to warm-temperate forest, they are also reported to behave as primary colonisers following disturbance such as landslips or road building in the same way as the white-barked birches of section Betula, and they are commonly found along rivers where gravels have been exposed by floods. The production of vast numbers of very small, very broadly winged seeds ensures that they are widely dispersed.

As a (sub)tropical species, Betula alnoides is rarely if ever encountered in temperate collections, and little is known of the parameters for successful cultivation. All four species of this subgenus seem to be relatively tolerant of a range of soil texture and soil moisture. In subtropical or warm temperate climates Betula alnoides, B. cylindrostachya and B. luminifera are rapid-growing, attractive trees likely to be suitable for areas with adequate summer moisture.