Arizona alder, New Mexican alder, aliso (Mexico)

Trees , to 30 m; trunks often several, crowns spreading. Bark dark gray, smooth, becoming blackish and breaking into shallow vertical plates in age; lenticels inconspicuous. Winter buds stipitate, ovoid, 4--8 mm, apex rounded; stalks 1.5--4 mm; scales 2, equal, valvate, sometimes incompletely covering underlying leaves, moderately resin-coated. Leaf blade narrowly ovate or lanceolate to narrowly elliptic, 5--9 × 3--6 cm, leathery, base narrowly to broadly cuneate or narrowly rounded, margins flat, sharply and coarsely doubly serrate, rarely evenly and densely short-serrate, major teeth sharp, acuminate, secondary teeth distinctly larger, apex long to short-acuminate, rarely acute; surfaces abaxially glabrous to sparsely pubescent or infrequently villous, moderately resin-coated. Inflorescences formed season before flowering and exposed during winter; staminate catkins in 1 or more clusters of 3--6, 3.5--10 cm; pistillate catkins in 1 or more clusters of 2--7. Flowering before new growth in spring. Infructescences ovoid, ellipsoid, or nearly cylindric, 1--2.5 × 0.8--1.5 cm; peduncles 5--10 mm. Samaras elliptic to obovate, wings narrower than body, irregular in shape, leathery.

Flowering early spring. Sandy or rocky stream banks and moist slopes, often in mountain canyons; 1000--2300 m; Ariz., N.Mex.; Mexico (n Chihuahua and n Sonora).

Alnus oblongifolia is closely related to the Mexican and Central American A . acuminata , with which it has sometimes been confused. It is found only in scattered populations in the temperate deciduous forest vegetation zone of high mountains in the arid Southwest.