Betula utilis subsp. albosinensis (Burkill) Ashburner & McAll.

From western Sichuan, northwards into Gansu and Shaanxi and eastwards into Hubei, and in provinces to the east and north, the predominant bark colour in trees of Betula utilis is orange-copper, often with a bloom of white; the leaves are often (but not always) longer, thinner, more delicate looking, and with a matt rather than a glossy upper surface; the young shoots also soon become glabrous. This is Betula utilis subsp. albosinensis, which occasionally forms pure birch forests, but is usually seen as scattered individuals in mixed forest (Wilson in Sargent 1916).

Betula albosinensis was described by Burkill, citing specimens collected by Père Farges in eastern Sichuan, near Tchenkeoutin. In the same year and from the same collection, it was described by Franchet as var. sinensis of B. bhojpattra. In Flora of China, Li & Skvortsov (1999) accepted Betula albosinensis as a separate species, based on its orange bark colour and the absence of hairs in the vein axils on the leaf underside. However, the bark character is very inconsistent – many central Nepalese populations of B. utilis have quite coppery bark with a bloom of white, not dissimilar to populations of B. albosinensis in northern China (Shaanxi). Similarly, axillary pubescence is not a reliable distinguishing character: study of a large number of specimens has shown that hair tufts are particularly well-developed and persistent on leaves on long shoots, but may be inconspicuous or absent on those of spur shoot. Thus neither the bark colour nor the axillary hair character show clear geographical correlation and they cannot be used to distinguish B. albosinensis from B. utilis.

On these grounds, and in view of the many intermediates not assignable to either Betula utilis or B. albosinensis found in British collections, McAllister and Ashburner (2013) thought it most appropriate to treat these north-eastern variants as subspecies albosinensis of Betula utilis. Examination of recent living collections has likewise shown that there are many populations in northwest Yunnan and Sichuan which cannot confidently be placed in either subsp. utilis or subsp. albosinensis because they have either intermediate characters or various combinations of the different characters. Howick & McNamara 1480 from Mt Luoji, 30 km south of Xichang in central Sichuan has bark typical of subsp. albosinensis, but its leaves are almost exactly intermediate in shape between those of subsp. utilis and subsp. albosinensis. Other collections which could not be easily assigned to either subspecies include KR 1364, SICH 2296, BLRM 219. Still others have a mixture of characters, for example leaves corresponding to subsp. utilis in shape, but matt in texture, for example W 990 and KA 1994–1938 (small leaves with only 6 or 7 pairs veins). SBEC 816 from the Cangshan in western Yunnan has elongated leaves similar to those of trees from much further north and east. In the Lijiang area in northwest Yunnan there are trees with long narrow matt leaves of the subsp. albosinensis type (SBLE 396, KEG 1644) and many others with the appearance of typical subsp. utilis. Specimens from Hubei (SABE 31, 897) have spur shoots with relatively short (52 × 35 mm) leaves with few (8) pairs of veins (very similar to those of subsp. occidentalis) and some axillary hairs, but long shoot leaves with up to 13 pairs of veins and heavily bearded axils

Betula albosinensis var. septentrionalis was described by C. K. Schneider from specimens collected by E. H. Wilson near Tachienlu (Kanding) in western Sichuan. It was said to differ from type albosinensis by its glandular young shoots, leaves silky-hairy beneath, especially on the midrib, veins and vein axils, and deeply cleft catkin scales). Wilson described the bark –’orange-brown or orange to yellowish orange or orange-grey’ – as ‘singularly beautiful’ (Bean 1976). A clone of var. septentrionalis has been widely cultivated, probably derived from a seedling of Purdom 752 from western Gansu (see ‘Septentrionalis Purdom’). In Flora of China, Li & Skvortsov (1999) treated Betula albosinensis var. septentrionalis as a synonym of B. utilis, whilst retaining B. albosinensis as a distinct species; however, the present treatment follows McAllister and Ashburner (2013) in placing it in synonymy with B. utilis subsp. albosinensis.

Cultivated trees of this subspecies are usually grown under the names Betula albosinensis and B. albosinensis var. septentrionalis, names given when very few collections were available for study in the west. The lenticel pattern is always very bold and striking – as white gashes and streaks on a shining dark background, often coalescing, with quite extraordinary effect. The bark of the commonly cultivated clones of subsp. albosinensis is a most attractive coppery colour, overlain with a bloom of white. The first introduction appears to have been Wilson’s collection for Veitch from western Hubei in 1901 (Wilson for Veitch 1157), though the oldest trees existing trees appear to belong to W 900 (introduced in 1908), W 4106 (sent in 1910 from western Sichuan (Bean 1976), near Wenchuan in the Min valley), and Purdom 752 (var. septentrionalis from western Gansu). These distinctive collections, particularly W 4106, owe their characteristic appearance to their elongated and thinner matt leaves which are of a softer green colour that those of any subsp. utilis, and have up to 14 pairs of veins. The origin of the clone distributed by Hillier’s is uncertain, and may be from either W 4106 or Purdom 752. The trees in Edinburgh belonging to Yü 14547, Rock 13648 and W 900 (var. septentrionalis) are rather different, with dirty-looking, dark greyish-black bark – not particularly attractive. It is surprising that the better clones have not become more popular and more widely planted, as has happened with some white-barked forms of subsp. jacquemontii.