Alnus oblongifolia Torr.

Alnus oblongifolia grows into a tall, straight-trunked, single or multi-stemmed tree with an open, rounded crown, typically to 15 m but exceptionally to 30 m. It is found in the mountains of the southwestern United States (Arizona and New Mexico) and northern Mexico (Sonora). It occurs in mesophytic habitats at relatively high elevation, especially amongst pine, fir and mixed oak forests, and along stream sides and in canyons. It belongs to a group of closely related species including A. acuminata, A. rhombifolia and A. jorullensis (Chen & Li 2004; Ren, Xiang & Chen 2010), and almost certainly intergrades with these other taxa where ranges overlap. It is most closely related to A. rhombifolia, both genetically and morphologically, and both are likely to be remnants of a once wider-ranging species. Both have only two stamens (rarely four), often rhombic leaves with serrulate margins, and smooth bark lacking prominent lenticels. A. oblongifolia leaves vary hugely within a single tree and can have the overall appearance of A. rhombifolia with rhombic leaves with small teeth, through to a more coarsely toothed ovate leaf resembling A. incana subsp. tenuifolia (Furlow 1979).

Alnus oblongifolia is a species that is somewhat difficult to distinguish, with a great deal in common with other taxa. The most reliable means of telling it from other species are: leaves narrowly ovate or lanceolate to narrowly elliptic, relatively small (50–90 × 30–60 mm) with large sharp to acuminate teeth; its stamens and perianth parts are in 2s (or rarely 4s, two large and two small). It can be distinguished from A. rhombifolia by its narrower, ovate (occasionally lanceolate, elliptic to rhombic) leaves with coarse uneven double-serrate teeth that are acute to dentate. A. oblongifolia can be distinguished from A. acuminata by its smaller leaves and blunter toothing, and from A. incana subsp. tenuifolia by its narrow leaves that are narrowly, not broadly ovate, with acute, not obtuse toothing. A. rubra is somewhat similar but this is larger in all parts with broadly ovate-elliptic leaves with recurved leaf margins. Murai (1964) placed A. oblongifolia in Sect. Glutinosae, which probably says little about its true relationships.

Arizona Alder appears to be rare in cultivation, despite making an attractive, large, drought tolerant tree suitable for many sites including stream sides and drier exposed areas. It is very easy to grow from seed and should be tested in cultivation more widely. The largest trees in the UK are a pair at White House Farm, Ivy Hatch, Kent, 12 m × 0.21 m dbh and 13 m × 0.34 m dbh tall in 2019 when the larger tree, derived from a Kenneth Ashburner collection from Arizona, was considered to be dying (Tree Register 2022).