Betula costata Trautv.

Betula costata is a beautiful tree with delicate, handsomely ribbed leaves with numerous veins and elegantly drawn-out apices. Mature trees in the wild are very striking in their fine haze of winter twigs which, in their slenderness, distinguish this species from Betula ermanii. Old trees often have magnificent rounded crowns. A taxonomically isolated species, Betula costata exhibits an intriguing mixture of characters of different taxonomic groups. In vegetative characteristics it resembles a more delicate form of B. ermanii, with thinner twigs, a much shorter petiole relative to the leaf blade, and a more elongated, less sharply toothed leaf with usually more numerous veins and a single order of toothing.

The species is usually classified with B. ermanii and species of the B. utilis aggregate though it is distinct from this group in its rigidly erect small fruiting catkins and short petioles. In the present treatment, Betula costata is treated as the type species of section Costatae, taken to include only the B. utilis group and B. ermanii. However, most previous classifications – e.g. that of Rehder (1940) – had treated sect. Costatae as including all the species here placed in subgenus Asperae; and indeed the species does exhibit some notable similarities with the members of subgenus Asperae, notably the erect catkins, relatively short petioles, and large number of leaf veins. On the other hand, it differs from all of these in its thin winged nutlets and white bark, its twigs lack the oil of wintergreen characteristic of section Lentae, and the wings of the nutlets differentiate it from species of section Asperae. The smooth bark is described as grey-brown (Rehder 1940; Nakai 1915), but a grove of trees in the Ussuri forests have beautiful creamy-white bark (McAllister & Ashburner 2013). The bark of the older trees seen in mixed forest in the Odae-San National Park in central South Korea was platy, strongly reminiscent of mature trees of B. ermanii in Hokkaido, while many younger trees in the same area were comparatively smooth-barked.

It is interesting that it is the only presumed relict species from the northeast Asian refugium (Ran, Wei & Wang 2006) that does not also occur in Japan. It was named by Ernst Rudolf von Trautvetter (1809–1889) from specimens collected by Carl Maximowicz on his expedition to the Amur river area of eastern Siberia in July–August 1856 (Maximowicz 1859).

Betula costata appears to be sensitive to drought, and well-established 4 m trees have died of drought at Ness. In Devon, at Stone Lane Gardens, it was found impossible to cultivate young trees without annual die back. A group of apparently thriving trees at least 2 m tall observed a few years ago at Westonbirt are now becoming threadbare with die-back, probably due to late spring frosts or winter dieback due to inadequate cuticle thickness as a result of what is often called insufficient ‘ripening’ of the new twigs, caused by too short or too cold a growing season (OECD 2003; Barclay & Crawford 1982).