B. japonica var. szechuanica Schneid.; B. mandshurica var. szechuanica (Schneid.) Rehd.; B. platyphylla var. szechuanica (Schneid.) Rehd.; B. kenaica var. szechuanica (Schneid.) Lindquist
A tree to about 70 ft high in the wild, with a white bark and, as seen in cultivation, with spreading branches; young twigs with few or no glands. Leaves triangular-ovate to rhombic-ovate, rounded to cuneate at the base, acuminate, 13⁄4 to 31⁄4 in. long, 11⁄8 to 21⁄4 in. wide, dark bluish green above, dotted with glands beneath, simply toothed; petioles to about 1 in. long. Fruiting catkins cylindrical, to 2 in. long, about 1⁄4 in. wide; bracts with ascending lateral lobes; body of nutlet wider than the wings.
Native of western China from Kansu to Yunnan, and of south-east Tibet; introduced by Wilson in 1908 from west Szechwan. It has until recently been regarded as only varietally distinct from the other east Asiatic birches of section Betula, but was raised to species level by the Swedish botanist Jansson in 1962. He remarks that it varies geographically in its foliage: in Kansu the leaves are small, rhomboidal and irregularly toothed and in Yunnan widely ovate, long pointed and with a regular dense serration. The above description is of the introduced form from west Szechwan.
In cultivation this birch makes a vigorous but rather graceless tree, with a silvery white bark. 'This var. szechuanica is the only birch I know where the white comes off on the hands like old whitewash' (A. F. Mitchell).
specimens: Kew, near the Victoria Gate, pl. 1923, 48 × 31⁄4 ft (1979); University Botanic Garden, Cambridge, 56 × 41⁄2 ft (1981); Westonbirt, Glos., in Mitchell Drive, 36 × 21⁄2 ft (1980); Tortworth, Glos., 62 × 5 ft (1973); Hergest Croft, Heref., 82 × 51⁄2 ft (1985); Edinburgh Botanic Garden, an original from Wilson 983, pl. 1911, 50 × 41⁄4 ft (1981); Benmore, Argyll, 56 × 53⁄4 ft (1983); Mount Usher, Co. Wicklow, Eire, 59 × 61⁄4 ft (1975).
† B. rockii (Rehd.) Jansson B. japonica var. rockii Rehd.; B. platyphylla var. rockii (Rehd.) Rehd. – This birch was described by Rehder in 1928 from specimens collected by Dr Joseph Rock in the Kokonor region of western China, all from a single tree (Rock 13283). It is of uncertain taxonomic position, being according to Jansson intermediate between B. szechuanica and B. platyphylla. The provenance of the trees distributed as B. platyphylla var. rockii is uncertain. Judging from the foliage, they are a small-leaved form of B. szechuanica, which Rock also collected in Kansu and Yunnan.