Alnus trabeculosa Hand.-Mazz.

Alnus trabeculosa is a common and widespread species in wet swampy places along the coastal plain of eastern China and in lowland areas of Japan. It remains a small tree in the wild in Japan to 5 m (Ohwi 1965), or up to 10 m in China. In effect it appears much like a small A. japonica, which does occur in similar habitats. A. trabeculosa is believed to be tetraploid (2n=56) but this requires confirmation. Genetically the species is something of a mystery, with no consensus so far. One sequence shows it is related to A. nepalensis, A. cremastogyne, A. formosana or A. cordata (same sample used in Chen & Li 2004; Ren, Xiang & Chen 2010), while another sequence shows it most closely related to A. hirsuta. It is suggested this species is frequently misidentified and deserves more work to ascertain its true identity and taxonomic status.

Morphologically it is very similar to Alnus japonica but can be distinguished by its smaller size, leaves with typically 9–14 pairs of lateral veins, hairy domatia in vein axils, and leaf bases rarely broadly cuneate and often obtuse to cordate. A. × spaethii has much larger leaves, often more than 15 cm long, while A. × mayrii has twigs, petioles, and abaxial leaf surface pubescent, with cuneate leaf bases, reliably with glabrous to sparsely villous young twigs and adaxially glabrous leaves.

Alnus trabeculosa is rare in cultivation, possibly due to being inferior to A. japonica which grows into a larger, more stately tree, but it would make a useful addition in areas of wet marshy ground. It has little application as a utility tree except as a small nitrogen-fixing tree for site amelioration (Chen et al. 2016). It is unknown in cultivation in North America despite being introduced several times to the Arnold Arboretum, suggesting a continental climate is inappropriate. There are scattered trees in collections across Europe: for example, there is one at the National Botanical Garden of Belgium, Meise, originating from the Hangzhou Botanical Garden, accessioned in 1991. It also grows at the Botanischer Garten Berlin-Dahlem, and Freiburg-Günterstal Arboretum in Germany. In the UK it has been introduced successfully twice, with plants at Ness from Shanghai Botanic Garden, and a 1997 collection introduced by Ray Townsend and Mark Bridger from the Forest Tree Breeding Centre at Juo Town, Ibaraki Prefecture, Honshu, at 50 m asl. At Howick it has also been slow, one specimen there being less than 2 m tall when seen in 2005, but 5 m by 2019 (Tree Register 2022). The leaves are an attractive mid-green, and when young the midrib and principal veins are red.