There are several naturally occurring hybrids in Alnus, many of which are unnamed and of uncertain origin. Alnus ×mayrii is allegedly the result of a cross between A. hirsuta Turcz. and A. japonica Siebold & Zucc. (Ohwi 1965), though it is not recognised in the World Checklist of Fagales (Govaerts & Frodin 1998). It resembles A. japonica (see Bean 1976a), but the leaves are broader (8–11 cm vs. 2–5 cm), and elliptic to oblong rather than lanceolate or narrowly ovate. The hybrid has dense, ferruginous pubescence on the branchlets, new shoots and lower leaf surfaces, as in A. hirsuta. Ohwi 1965. Distribution JAPAN: Hokkaido; KOREA. Habitat Marshy areas. USDA Hardiness Zone 5–6. Conservation status Not evaluated. Cross-reference S71.
Plants bearing this name are found in several arboreta in our area, specimens for the current work having been observed at Quarryhill, Howick and Kew. There is considerable variation between them, some showing more or less the characteristics of one or other parent, while others are intermediate. Most collections seem to have been made from hybrid parents in the wild, making the cultivated offspring the F2 generation, in which such segregation is to be expected. At Quarryhill, trees grown from a collection in Hokkaido in 1987 (Warner & Howick 510), planted in 1991, were 12–15 m tall when seen in 2004, looking very much like most alders, but bearing conspicuous clusters of attractive large cones. At Howick trees from the same collection are much shorter but still doing well in a considerably cooler locality. Kew has two trees of separate South Korean origin, the older (now 9 m) from a 1968 accession received from the US Department of Agriculture, the other (8 m) from the Morris Arboretum in 1985, and showing strong characters from A. japonica. Although many specimens of A. mayrii are attractive trees they have no merits greater than those of either parent.