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Tree to 20 m. Bark dark grey to brown. Branchlets sparsely pubescent and purplish brown. Buds with two glabrous, ribbed scales. Leaves deciduous, 6–12 × 2–5 cm, elliptic to oblong, rarely ovate, upper surface almost glabrous, lower surface with dense pubescence in the axils of the lateral veins, six to seven lateral veins on each side of the midvein, margins with minute, irregular serrations, apex acute or acuminate; petiole with dense pubescence restricted to the furrow, 1.2–2.2 cm long. Staminate inflorescences catkin-like, 3–8 cm long; pistillate inflorescences solitary or in racemes of two to four, pedunculate, ellipsoid, 1–2.5 cm long. Cone woody, 1.1–1.8 × 0.8–1.2 cm, bracts 0.3–0.4 cm wide. Flowering January to March, fruiting July to September (Taiwan). Li & Skvortsov 1999. Distribution TAIWAN. Habitat Typically, forming pure stands by riverbanks, between 0 and 2900 m asl. USDA Hardiness Zone 8. Conservation status Not evaluated. Taxonomic note This species appears to be more closely related to the eastern North American species A. maritima Nutt. and A. nitida Endl. than to other Asian alders (Chen & Li 2004), and it has indeed been known as A. maritima var. formosana!
Alnus formosana is not only a large handsome tree but apparently hardy throughout most of the British Isles: it should be planted more widely. There are some beautiful specimens at the Royal Botanic Garden Edinburgh, where a Treecreeper Certhia familiaris was enjoying the long straight bole of the largest (16 m, dbh 38 cm, planted 1926) while notes were being taken for this book; another tree there, of 15 m, has a much thicker, fluted bole (110 cm dbh, TROBI). The spreading branches create a pyramidal shape, and bear a dense canopy of narrow, long-pointed leaves; younger specimens resemble Prunus padus. Alnus formosana has been gathered on several occasions in the past few decades and is represented in many collections in the United Kingdom and elsewhere, including Quarryhill.