Betula utilis subsp. utilis

Typical Betula utilis subsp. utilis is found mainly from the central Himalaya through western China into western Sichuan, east of the area occupied by subsp. jacquemontii and southwest of the area occupied by subsp. albosinensis. The bark is predominantly, but not invariably, brown or pink, though somewhat white-barked individuals or populations occur even as far east as Sichuan (e.g. Forrest 19505). To the east of the Kali Gandaki gorge in Nepal, trees of Betula utilis tend to become darker-barked with firmer-textured, more leathery leaves with more numerous and more deeply indented veins and more cordate leaf bases (Ashburner & Schilling 1985). However, there appears to be considerable overlap in these features. Both west and east of the Kali Gandaki gorge there are whiter-barked trees on the drier northern slopes and darker-barked populations in the wetter forest on south-facing slopes. Many individual trees, particularly in central Nepal may be impossible to assign to either subspecies, as there appears to be no sharp dividing line between the two. Unlike the white-barked trees which often form tree-line forest, the darker-barked trees occur scattered through the species-rich deciduous and mixed deciduous-coniferous temperate rainforest growing alongside species of Acer, Sorbus, Rhododendron, Viburnum and Abies, never forming pure birch forest. Further east (southeast Tibet and Yunnan, as far as western Sichuan) this pattern continues with these birches mostly scattered through the forest. Ashburner & Schilling (1985) concluded that the distinction between B. jacquemontii and B. utilis could not be maintained at species level, and recognised jacquemontii as a variety of B. utilis; however, the present treatment follows Mcallister & Ashburner (2013) in treating both as infraspecific taxa within a broadly conceived B. utilis.

In southeastern Tibet, typical Betula utilis occurs rather infrequently in dense mixed forest from about 3000 m to the tree line at about 3800 m asl, in association with Picea linzhiensis, Abies fordei, and Acer spp. with an understorey of bamboos and a range of shrubby genera such as Viburnum and Rhododendron. At lower altitudes the trees are usually very tall, up to about 20 m, and sometimes very large with trunks to 80 cm in diameter. Fruiting catkins are borne at the very tops of the trees, and can be collected only from the occasional fallen tree. These trees have very attractive, copper-orange coloured bark flushed with white and large leaves with 11–14 pairs of veins, impressed above. This combination of characters is typical for the species east of the Kali Gandaki; towards the tree line the trees became smaller but are otherwise similar. The name Betula utilis var. prattii – described by Burkill from specimens collected by A. E. Pratt near Tachienlu, today Kanding, and originally covering trees with orange-brown to grey or orange-grey bark – has usually been attached to forms which have very dark, shining bark, often overlaid with numerous white lenticels. As was noted in the case of Nepalese trees, these dark-barked forms tend to occur as large, single specimens in mixed forest, at lower altitudes than most B. utilis in the same area, and are found from central Bhutan (Schilling 1993) eastwards to western Sichuan. Lancaster (2008) illustrates a tree with similar bark at Huadianba, in the Cangshan near Dali, and similar trees are common in the wet forests of Gongga Shan in western Sichuan (M. Rix, cited in McAllister & Ashburner 2013).

Betula jinpingensis was described by Li (1979) from the Jin Ping Shan in southeast Yunnan adjacent to North Vietnam; it is said to differ from B. utilis primarily in its shorter peduncle, wider nutlet wing, wider fruiting catkin, and the absence of glands on the branchlets. The illustration provided in Flora of China suggests, as was proposed by Skvortsov (footnoted in Li & Skvortsov 1999), that it should be treated as a synonym of B. utilis differing only in minor characters which could apply equally to other variants of subsp. utilis.

Bean (1976) reports Betula utilis as having been first introduced from Sikkim in 1849 by Joseph Hooker, and later from China by E. H. Wilson (W 4035, 4087, and 4089) and George Forrest (Forrest 15381). Contrary to what Bean reported, collections of subsp. utilis from the wet climates of the eastern Himalaya do seem to thrive in the British Isles, being slow to break bud, and thus avoiding damage from late frosts. Experience suggests that they are more sensitive to drought than Betula pendula in cultivation. A wide range of recently (and not-so-recently) introduced material is available, from which many selections have been made.