There is a strong tendency in most species to form underground suckers, so that a grove of aspens or poplars may represent a single clone. Furthermore, leaf shape is quite variable and further complicated by hybridization in at least some species. Although typical material is distinctive and readily identifiable, specimens of suckers or sprouts, hybrids, or other atypical states may cause confusion.
The key is not specifically designed for use with specimens bearing aments before the appearance of any leaves, but sufficient data on flowers, buds, and habit are given in the key or text that incorrect identification of flowering material may usually be avoided.
1. Mature leaf blades beneath and petioles densely felted with white pubescence; leaves of long shoots ± sinuate with 5 or fewer irregular rounded lobes on a side; trees spreading from cultivation.
1. Mature leaves glabrous or nearly so (white-felted when young in P. heterophylla and P. grandidentata); leaves usually with more than 5 teeth on a side; trees native or spreading from cultivation.
2. Petioles terete, often somewhat grooved above.
3. Outline of leaf blade usually slightly concave toward very acute or short-acuminate tip; buds very strongly glutinous, varnished with a fragrant gum; expanding leaves glabrous or at most minutely puberulent; flowers sessile or nearly so in the aments, pedicels less than 2 (–2.5) mm long.
3. Outline of leaf blade usually slightly convex toward blunt or rounded tip; buds usually ± pubescent, not (or only slightly) glutinous; expanding leaves (and often edges of mature midribs beneath) densely cottony-tomentose; flowers on pedicels ca. 2–8 mm long.
2. Petioles strongly compressed laterally, especially near blade.
4. Leaf blades clearly broadly triangular or ± diamond-shaped, the margins with a firm colorless border thicker than adjacent major veinlets, the incurved teeth with callous tip; stamens more than 15; scales in ament glabrous, fringed with many thread-like segments.
5. Glands prominent at junction of blade and petiole; larger blades broadly triangular, mostly (6–) 7–12 (–20) cm broad, the margin (especially when young) usually ± densely minutely ciliate with minute hairs (at least on the teeth).
5. Glands absent at junction of blade and petiole; larger blades ± diamond-shaped, usually less than 8 cm broad (but broader than long), the margin always glabrous.
4. Leaf blades orbicular to reniform at most obscurely triangular, the margins often without colorless thin border or with one no thicker than adjacent main veinlets, the teeth with or without a glandular tip; stamens 12 or fewer; scales in ament pilose, fringed with only ca. 3–7 (–10) linear or narrowly lanceolate segments.
6. Leaf margins closely undulate-dentate with fewer than 10 (–12) large teeth on a side (except on large-leaved sprouts); buds and new growth (leaves, branchlets) ± pilose or tomentose with white to gray pubescence; scales fringed with mostly 5–7 segments.
6. Leaf margins closely crenulate-serrate with 15–35 or more (–70) teeth on a side; buds and young growth glabrous or nearly so; scales fringed with mostly 3–5 narrow segments.