Alnus acuminata Kunth

Alnus acuminata has an extensive distribution through the Americas, from Mexico south to northern Argentina. In the classification followed here Alnus jorullensis is the only other species to occur in central America, but A. acuminata alone extends the genus’ distribution into South America proper. South American trees identified as A. jorullensis belong here. It is believed these species evolved from A. rubra and migrated south (Furlow 1979). The main morphological character to separate the two is leaf shape and secondary leaf vein number: A. acuminata is broader towards the leaf base and has 5–10 pairs of secondary leaf veins; A. jorullensis has leaves which are broader towards the apex (obovate-elliptic) and with 7–17 pairs of veins. (A. oblongifolia is similar to A. acuminata but is distinguished by narrower, lanceolate leaves, flowers typically with 2 rather than 4 stamens, and is restricted to southern Arizona and NW Mexico in dry regions at high altitude). Introgression may occur between all three species.

Alnus acuminata is a highly variable species and most of its range is tropical or subtropical, making it an unusual subject in cultivation in northern temperate regions. It is often confused with A. jorullensis, especially for specimens lacking precise origin details. The morphology of both is variable and appears to be related (not necessarily clearly) with habitat. All American alders, including both A. jorullensis and A. acuminata,are diploid (2n=28) (Furlow 1979; Restrepo & Gervais 1994). Being wind-pollinated and sharing similar phenology they are likely to interbreed where both occur together, potentially compounding confusion further. However, this does not bely the fact that more work is needed, especially in A. acuminata, to determine whether other named taxa such as A. castaneifolia Mirb. actually merit taxonomic distinction rather than being considered local variants as stated by Furlow (1979) and Govaerts & Frodin (1998). Populations sometimes segregated as A. castanaeifolia appear to occur in a distinct north-easterly arm, somewhat separate to typical A. acuminata, and have narrow lanceolate-elliptic leaves. Molecular evidence supports that A. acuminata and A. jorulensis (as examined) are at least closely related (e.g. Ren, Xiang & Chen 2010; Ren & Liu 2006).

The treatment to subspecies level follows the generally accepted line in the World Checklist of Selected Plant Families (Govaerts & Frodin 1998) and Furlow (1979), with three subspecies recognised here. Christenhusz – a notorious lumper – did not distinguish between subspecies in Flora Mesoamericana (Christenhusz 2009). Morphological distinctions between them are slight and only accessions of known geographical provenance can be identified with certainty.

Alnus acuminata appears rare in cultivation in northern temperate climates, but all three subspecies are represented in several European and American collections. All are best grown in a warm garden with mild winters, on moist soils, in areas with moderate to high rainfall, but even in optimum conditions trees are unlikely to be evergreen in a temperate climate. Cold tolerance appears to be dependent on provenance: At Blagdon Hall (Northumberland, UK) for example, subsp. glabrata has proved to be hardier than subsp. arguta (M. Ridley in Grimshaw & Bayton 2009). This subspecies survived unscathed in the UK’s cold 2010–11 winter, but precise hardiness data is not known. It is moderately susceptible to Alder Leaf Beetle, despite the thick leaf blade.