Betula cylindrostachya Wall.

Flowering from April to June; fruiting from July to August.

Betula cylindrostachya is close relative of Betula rhombibracteata, but differs from the latter in its scales of buds villous (vs. glabrous), petiole densely yellow villous (vs. glabrous except villous in furrow), bracts oblong-lanceolate, 2-3 × 1.5-1.7 mm, pubescent at base (vs. rhombic, 4-5 × ca. 3 mm, glabrous), nutlet ca. 2 × 1-1.5 mm (vs. 3-5 × 1.5-3 mm).

Betula cylindrostachya is occurring in Sichuan, SE Xizang, NW Yunnan of China, Bhutan, India.

The phylogenetic relationships within the genus Betula (Betulaceae) were investigated using a part of the nuclear ADH gene and DNA sequences of the chloroplast matK gene with parts of its flanking regions (Järvinen et al., 2004). Two well-supported phylogenetic groups could be identified in the chloroplast DNA sequence: one containing the three American species B. lenta, B. alleghaniensis, and B. papyrifera and the other including all the other species studied. The ADH gene displayed more variation, and three main groups could be identified. The ADH phylogeny suggests that several independent polyploidizations within the genus Betula could have taken place. The ADH and chloroplast phylogenies were in part incongruent, the most likely reason for this seems to be cytoplasmic introgression.

Trees to 30 m tall; bark brown or grayish black. Branchlets yellow-brown, densely yellow villous. Petiole 0.8-1.5 cm, densely yellow pubescent; leaf blade ovate-elliptic, oblong, or ovate-lanceolate, 5-14 cm long, 2-8 cm wide, both surfaces densely villous when young, abaxially densely resinous punctate and bearded in axils of lateral veins, base rounded or subcordate, margin irregularly and doubly subincurved setiform serrate; apex acuminate; lateral veins 13-14 on each side of midvein. Female inflorescences 2, narrowly cylindric, 2.5-10 cm long, 5-7 mm wide; peduncle 7-10 mm, densely yellow pubescent; bracts oblong-lanceolate, 2-3 mm long, 1.5-1.7 mm wide, sparsely pubescent at base, ciliolate, spongy at base when mature, 3-lobed, middle lobe ovate-lanceolate, lateral lobes ± reduced, ca. 1/3 as long as middle lobe. Nutlet ovate or oblong, ca. 2 mm long, 1-1.5 mm wide, densely pubescent at apex, with membranous wings ca. 2 × as wide as nutlet.

Growing in subtropical and warm temperate broad-leaved forest; 1400-2800 m.