Alnus japonica Sieb. & Zucc.

Alnus japonica is native to an extensive area of eastern Asia. It is most commonly associated with wet soils in lowland areas, but may be found in wet habitats up to 1500 m asl. It will tolerate anaerobic and waterlogged, but not compacted soils, both as young and mature plants, where it produces adventitious roots to cope with prolonged flooding (Yamamoto, Sakata & Terazawa 1995). It is noted as a pioneer light-demanding species but, unusually, will tolerate some shade. It grows into an upright, densely to rather sparsely crowned tree that holds its leaves late into the season, and remains evergreen in southern regions such as Taiwan. It is a useful species in wet areas and is frequently planted to stabilise land and reduce soil erosion, including reforesting old mine sites, and as a shelter crop for coffee. It is an important timber crop with a close-grained wood used for firewood, charcoal, dye making, and carving (CABI 2018). Its seeds are relatively short-lived, and high temperatures are needed to induce germination (Orwa et al. 2009).

Alnus japonica is likely have originated in the Oligocene (Chen & Li 2004) and is very variable across its range. There is ongoing debate as to its placement within the genus and the delineations of the species itself. Historically it was included as a variety of A. maritima but is clearly unrelated. Murai (1964) placed it as the type of Section Japonicae but this is unlikely to reflect its true relationships. Morphologically it is most similar to the sympatric A. trabeculosa, and the two are easily confused. A. japonica is distinguished by its leaves with cuneate bases, 6–10 pairs of side veins and often membranous domatia in vein axils. A. trabeculosa and A. × spaethii (A. japonica × A. subcordata) have leaf bases that are subrounded, subcordate, or broadly cuneate with 10–15 pairs of veins either side of midrib and lacking domatia. Genetically A. japonica appears to be most closely related to the North American A. serrulata (Chen & Li 2004; Ren, Xiang & Chen 2010). Cytologically, A. japonica is variously recorded as diploid, triploid and tetraploid, indicating that more work is needed to resolve identification and phylogenetic relationship issues.

Alnus japonica is relatively common in cultivation in larger gardens, arboreta and botanical collections across our area. It grows into an upright tree that holds its leaves late into the season, with robust upright fruit and purple-black immature male catkins. It appears to be completely hardy, but provenances of similar climate to a gardens’ own are likely to grow best. In the UK, continental origins should be avoided in favour of plants from South Korea or Japan. It is not known when it was first introduced, but plants obtained from Lee’s nursery were recorded fruiting at Kew in 1880 (Bean 1976). The largest plants in the UK can be found at Maurice Mason’s Old Larch Wood garden (20 m tall), Stanage Park (19 m), and at Sir Harold Hillier Gardens (19 m) (Tree Register 2021) with plants also found in collections across North America and Europe.