Betula corylifolia Regel & Maxim.

Betula corylifolia is a very distinct birch, probably not closely related to any other living species – Skvortsov (2002) created the monospecific subgenus Nipponobetula for it. The foliage gives the tree an appearance quite different from that of any other birch, the leaves having whitish undersides and matt, bluish-green upper surfaces, and with the major teeth (in which the secondary veins end) being broad and often lacking subsidiary teeth between them. The male catkins are also quite unlike those of other species, most resembling those of B. calcicola or B. chichibuensis in their thin, hardly fleshy scales with elongated triangular apical portions. These are recurved, giving the male catkins a very different appearance from the smooth glossiness of those of most birches. The fruiting catkins and their scales are large and only one nutlet is borne in the axil of each scale – these are glabrous with a distinct beak and narrow wing, distinct from all others in the genus.

Betula corylifolia has a very restricted range in the wild, found only at high altitudes in central Japan. In the subalpine forests of central Honshu it occurs with B. ermanii in areas dominated by Abies mariesii and A. veitchii, with Tsuga diversifolia regenerating in openings in the forest (Yamamoto 2000). In the Nikko region in Honshu north of Tokyo, trees of Betula corylifolia are found scattered at woodland edges, inconspicuous on account of their dark and only moderately shining bark.

Betula corylifolia was first collected by Tschonoski Sungawa (1841–1925), a Japanese collector who assisted Maximowicz during his stay in Japan from 1860 to 1864, and who is commemorated in such familiar species as Carpinus tschonoskii, Acer tschonoskii, and Malus tschonoskii (Ohwi 1965). Betula corylifolia was introduced to cultivation by E. H. Wilson (W 7651) in 1914 from Honshu, Shimotsuke province, at Yumoto, and has been represented in Britain since then by a single tree at RBG Edinburgh – other trees cultivated under the name were wrongly identified (Bean 1976).

Even in the presence of a wide range of other species this tree has produced only seedlings of true Betula corylifolia, suggesting that it cannot hybridise with other birches. Seed collected from the Edinburgh tree, probably in 1981, germinated to give six seedlings, which grew to over a metre in three years; five of them at Ness Botanic Garden died, largely of drought, and one at Stone Farm grew to four metres before dying in 2006. There is now another, younger tree in Edinburgh, probably grown from seed obtained from a Japanese nursery (as B. maximowicziana) of unknown wild source (McAllister & Ashburner 2013), and several plants in Edinburgh and at Benmore from a 2005 expedition to Nigata (Brownless, McNamara, Bolton, Jamieson & Tsukie 174). A recent introduction of seed from Mt Goyo, Iwate Prefecture, gave rise to two seedlings which were planted in a more favourable moist site at Ness: both grew well to 6 m and fruited, one producing viable seed by self-pollination. In spring 2006 many branches on both trees failed to break bud and the trees have since died, perhaps from waterlogging.

As a young tree, Betula corylifolia has a neat pyramidal habit and makes an attractive specimen with its sea-green leaves with whitish undersides. The large, erect fruiting catkins, which remain on the branches for most of the winter, are an unusual and interesting feature. It is relatively slow growing, and like many birches, perhaps especially those from mountainous areas, it appears to be very intolerant of shade and seems to grow best in moist, humus-rich soils in a relatively humid climate, in a sunny situation. Soft cuttings, taken in early summer, root readily under mist.