Betula luminifera H.J.P.Winkl.

Betula luminifera is a common species in western China, occurring from Yunnan as far north as Shaanxi and Gansu, and east to Jiangsu; it was collected by both Père Delavay and Père Farges, but not described as a distinct species until the publication of Winkler’s account of the genus (Winkler 1904). Wilson (in Sargent 1916) described it as ‘the low-level birch of western Hubei and of Sichuan’, found at altitudes up to 2500 m asl, rarely exceeding 20 m in height and never forming pure stands but always growing in association with other trees. Flora of China cites the flowering period as May to June, but in cultivation in Britain (at Ness Botanic Gardens) a tree of Sichuanese provenance (EMR 4121 from Baoxing) is one of the earliest birches to break bud, the young leaves and catkins expanding in early April.

Seedlings and saplings of Betula luminifera have very densely hairy young shoots and leaves, but the shoots of older plants are almost glabrous. In leaf shape and catkin length the species is quite variable. The Farges specimens have broadly ovate and almost cordate leaves. Betula wilsoniana C.K.Schneid. from near Ichang is tomentose with very long catkins, while B. hupehensis C.K.Schneid. is nearer glabrous, with shorter catkins.

Betula luminifera was introduced to cultivation by Wilson in 1901 (Henry, in Elwes & Henry 1906–1913) and again in 1907; more than a dozen of Wilson’s numerous collections were fruiting, so the precise provenances of this material cannot easily be identified (Henry gave northeast Sichuan as the source of the 1901 introduction, but this does not correspond to any recorded Wilson collection). At Kew these trees did not prove long-lived, the last of them dying in 1960, having reached 14 m in height.

Bean (1976) described Wilson’s trees at Kew as ‘curiously distinct in the resinous sheen beneath the leaves, which became more apparent as the leaf dried’, this characteristic corresponding to Diels’ variety resinosa (originally published under B. cylindrostachya) from Nanchuan, Chongqing (formerly Sichuan), now treated as a synonym of B. luminifera, in which the leaves are hairless and glutinously shiny. The resinous quality of the young branchlets, leaves and petioles in B. luminifera is noted as characteristic in Flora of China (Li & Skvortsov 1999) but not mentioned in the treatment by McAllister and Ashburner (2013), which is largely based on observations from plants in recent (British) cultivation.

Betula luminifera is not especially noteworthy from a horticultural perspective, though the very long thin male catkins provide an delicately attractive display in very early spring. At least one clone has been distributed as grafted plants by Hilliers (of Winchester, England) for many years and other collections have recently been introduced to cultivation.

Skvortsov regarded Betula cylindrostachya as being most similar and closely related to B. luminifera, despite the female catkins being borne singly in B. luminifera and in clusters of two to six in B. cylindrostachya (not necessarily a reliable distinguishing character, however, since weak shoots of B. cylindrostachya often bear single fruiting catkins). Limited investigation of chromosome counts by McAllister and Ashburner (2013) suggest that B. luminifera is diploid, making regular interbreeding with the tetraploid B. cylindrostachya unlikely, though some gene flow might be expected.