Alnus oblongifolia Torr.

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Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus oblongifolia' from the website Trees and Shrubs Online ( Accessed 2024-07-20.


  • Alnus
  • Subgen. Alnus, Sect. Glutinosae

Common Names

  • Arizona Alder


  • Alnus serrulata var. oblongifolia (Torr.) Regel


Narrowing gradually to a point.
Sharply pointed.
Diameter (of trunk) at breast height. Breast height is defined as 4.5 feet (1.37 m) above the ground.
With evenly triangular teeth at the edge. (Cf. crenate teeth rounded; serrate teeth saw-like.)
Lance-shaped; broadest in middle tapering to point.
(of a plant) Growing in moist (mesic) habitats.
Egg-shaped; broadest towards the stem.
Calyx and corolla. Term used especially when petals and sepals are not easily distinguished from each other.
Diamond-shaped. rhomboid Diamond-shaped solid.
With saw-like teeth at edge. serrulate Minutely serrate.


Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus oblongifolia' from the website Trees and Shrubs Online ( Accessed 2024-07-20.

Tree to 15–30 m, crown open, rounded. Bark smooth, light grey, becoming corky and darker with age. Branchlets red-brown, not differentiated into long and short shoots. Buds ovoid, 4–8 × 1.5–4 mm, moderately to heavily resin coated, stalk 1.5–4 mm long, 2 valvate scales. Leaves narrowly ovate, or lanceolate, elliptic to rhombic, 3–11 × 2–7 cm, apex long to short acuminate or acute, base cuneate or rounded, sharply to coarsely double-serrate to serrulate; major teeth acuminate, adaxially sparsely pubescent, villous to glabrous, abaxially villous, resin-coated, hairy domatia, 9–13 pairs of lateral veins, craspedodromous, chartaceous to coriaceous, mid green. Petioles 7–18 mm long, villous to velutinous. Stipules ovate, elliptic or obovate, 5–7 × 1.5 mm. Peduncles densely yellowish hairy. Pistillate inflorescences borne in erect racemose group 4– 5 on short branchlets, erect, 4–5 × 2.5 mm. Staminate catkins in pendulous terminal racemose clusters 3–6, at anthesis 3–8.5 × 5–8 mm, on peduncles 3–13 mm long, stamens 2. Fruit ovoid, ellipsoid, or cylindric, 1.5–2.4 × 0.5–1.5 cm, scales 3 × 3 mm, the terminal lobe-tip acute. Seeds narrowly winged, broadly elliptic to obovate, 1.8–3 × 1.2 –2.3 mm with papery wings 2.2–3.5 mm, styles persistent. Flowering early spring. (Furlow 1979).

Distribution  Mexico Sonora United States Arizona, New Mexico

Habitat Riparian habitats including sandy or rocky stream banks and moist slopes, often in mountain canyons, 1000–2300 m asl.

USDA Hardiness Zone 7

Conservation status Least concern (LC)

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).