Betula utilis D.Don

A group of similar birches common throughout the Himalaya from Afghanistan and Kashmir to western China have usually been referred to under the names Betula utilis, B. jacquemontii and B. albosinensis. Grown for their spectacular bark, which peels in scrolls and sheets, and varies in colour from bright white to pink or dark mahogany or pink, these birches are evidently closely related, and have been recognised in the past as separate species, subspecies or varieties. Together, they form a loose aggregate or complex with a more-or-less continuous distribution from Afghanistan and northern Pakistan in the west, along the Himalaya and through the mountains of Yunnan (as B. jinpingensis) into Sichuan and Hubei, as far east as Hebei province in northern China. Like the silver birches that comprise the B. pendula complex, they share the same chromosome number (2n=56, tetraploid), and although trees at the extreme ends of the cline are very different, intermediate states and continuous variation in characters makes it difficult to draw clean distinctions between the three taxa. For this reason, following McAllister & Ashburner (2013) they are here treated as subspecies of Betula utilis, namely subsp. utilis, subsp. occidentalis Kitam., subsp. jacquemontii (Spach) Kitam., and subsp. albosinensis (Burkill) Ashburner & McAll.

The most useful identifying character that unites mature individuals of the B. utilis aggregate is the presence of the conspicuously ‘knobbly’ male catkins which have a prominent raised boss on each scale. These are visible from late July, when the new catkins begin to mature, until flowering time the following spring. In the closely related Betula ermanii the male-catkin bosses are absent, the fruiting catkins usually erect, and the very sharpy toothed leaves are deltoid-ovate with the primary teeth at vein endings more than twice as long as the neighbouring teeth. The species actually most likely to be mistaken for B. utilis is the less closely related Betula papyrifera, though its leaves are never as glossy or leathery as in some B. utilis: again the knobbly male catkins provide the most useful means of distinguishing B. utilis, together with the seed wings, which are narrower (rather than broader) than the nutlet.

Betula utilis shows a classic Sino-Himalayan distribution, albeit extending rather further north and east than is usual; it is absent from the northerly refugia of Japan and from the adjacent Amur-Ussuri region on the Asian mainland (Ran, Wei & Wang 2006). Further research is needed in relation to the apparent discontinuity between subsp. albosinensis in Hebei province around Beijing, and B. ermanii (also tetraploid) at the south-western end of its distribution in the adjacent province of Liaoning (Li & Skvortsov 1999): it is unclear whether this break is real or whether there is some introgression at the boundaries. Although Betula utilis and B. pendula subsp. szechuanica grow close to one another in northwestern Sichuan, no hybrids have been recorded.

Betula utilis was originally described by David Don (1825) from material sent back by Nathanial Wallich from ‘Gosainthan’ in central Nepal – according to Tony Schilling, not the peak of Gossainthan itself but rather the Gosainkund mountains that form the southern side of the Langtang valley (Schilling 1984). John Lindley (in Wallich 1831) described Betula bhojpattra shortly thereafter, taking the epithet from the Hindi ‘bhojpatra’, the word for Betula utilis: parts of the plant are still used in Hindu ritual, as well as for medicine, and as a substitute for paper, especially for the writing of yantras (sacred diagrams).

From an ornamental perspective the different barks of Betula utilis are among the finest in the whole genus. Bark colours range from the purest white in subsp. jacquemontii from the west Himalaya, through copper-coloured to chocolate (with or without a white bloom) in subsp. utilis, to pure copper in some subsp. albosinensis from northern China. Those with a white bloom of betulin over a darker colour are particularly beautiful, and the pattern of the lenticels can also add to the effect: in subsp. jacquemontii they are often pink and set in relief against the white bark, whereas in subsp. utilis they stand out as white or buff streaks against the rosy-pink or brown bark. The male catkins are also large and showy, often expanding much later than those of other species. The trees have more garden presence than Betula pendula, their larger leaves, particularly in subsp. jacquemontii, tending to give a heavier and denser appearance to the canopy.

All variants of B. utilis are slower growing than B. pendula or B. papyrifera but will eventually make large trees. Provenances from the western end of the distribution, subsp. occidentalis and subsp. jacquemontii, often suffer from late spring frosts and severe dieback, resulting in trees of a very poor shape which may fail to fruit. The distinctive collections of subsp. albosinensis in cultivation (e.g. W 4106, Purdom 752) owe their characteristic appearance to their elongated leaves with up to 14 pairs of veins, and thinner matt leaves which are of a softer green than those of any subsp. utilis.