Alnus formosana, or Formosan Alder, is a species of alder native to Taiwan. It is a medium-sized tree, up to 20 metres (66 ft) in height. It is a common species growing on riverbanks from near sea level to 2,900 metres (9,500 ft).
Alnus formosana (Burkill) Makino, Bot. Mag. (Tokyo). 26: 390. 1912. Alnus maritima (Marshall) Nuttall var. Formosan Burkill in FB Forbes and Hemsley, J. Linn. Soc., Bot. 26: 500. 1899; A. japonica (Thunberg) Steudel var. Formosan Callier. Trees up to 20 m in height; dark gray-brown bark. Purplish brown branches, glabrous, grooved; barely pubescent twigs. Stipitate outbreaks, with 2 scales glabrous vaults. Petiole 1.2-2.2 cm, slender pubescent, densely furrow; Elliptic or oblong-lanceolate Limbo, rarely ovate-oblong, 6-12 × 2–5 cm, abaxially bearded in axils, lateral veins adaxially subglabrous, rounded or broadly cuneate base, irregularly minutely serrated margin, acuminate or acute apex; lateral veins 6 or 7 on each side of the midrib. One female inflorescence, or 2-4 in a cluster, ellipsoid, 1-2.5 cm; peduncle 3–5 mm, robust; bracts 3–4 mm, woody, cuneate base, rounded apex 5-lobed. Nutlet obovate, 2–3 mm, papery wings 1 / 4-1 / 3 as wide as nutlet. Fl. May–June, fr. July–September