4. Amelanchier alnifolia (Nuttall) Nuttall ex M. Roemer, Fam. Nat. Syn. Monogr. 3: 147. 1847.
Alder-leaved shadbush, saskatoon, saskatoon berry, amélanchier à feuilles d'aulne
Aronia alnifolia Nuttall, Gen. N. Amer. Pl. 1: 306. 1818
Shrubs, 1–12 m. Stems 1–50, solitary or colonial. Leaves mostly unfolded; petiole (3–)6.8–19.1(–28) mm; blade usually elliptic to oval to suborbiculate, sometimes quadrangular, (14–)24–47(–67) × (7–)17–36(–55) mm, base usually subcordate to truncate, sometimes ± tapering or ± cuneate, each margin with 0–3(–9) teeth on proximal 1/2 and (0–)3–5(–8) teeth in distalmost cm, largest teeth more than 1 mm, apex rounded to truncate or occasionally acute or mucronate, abaxial surface sparsely to densely hairy (or glabrous) by flowering, sparsely to moderately hairy (or glabrous) later, adaxial glabrous or sparsely (moderately) hairy later. Inflorescences (4–)6–11(–16)-flowered, (8–)14–43(–62) mm. Pedicels: (0 or)1 or 2(or 3) subtended by a leaf, proximalmost (2–)3–20(–29) mm. Flowers: sepals erect to recurved after flowering, (1.4–)2.2–4(–4.9) mm; petals oblanceolate to oval or obovate to elliptic, (5.7–)9.5–14(–18.8) × (2.2–)3.3–5.2(–6.6) mm; stamens (10–)15–21(–22); styles (3 or)4 or 5(or 6), (1.3–)2–2.9(–3.9) mm; ovary apex moderately to densely hairy (or glabrous). Pomes black or purple, 8–15 mm diam.
Varieties 3 (3 in the flora): w North America.
Amelanchier alnifolia is widespread and polymorphic, and its taxonomic and geographic limits have been viewed differently. L. Cinq-Mars (1971) considered this species to range eastward to the Gaspé Peninsula. Disagreements about the boundary between A. alnifolia and A. humilis are evident in herbarium specimen annotations. The distinctness of the varieties of A. alnifolia has also been questioned. G. N. Jones (1946) treated the three varieties recognized here as distinct species and noted that the leaves of var. alnifolia and var. semiintegrifolia (A. florida) are virtually indistinguishable, and, although petal lengths of the two do not overlap, occasional larger-flowered var. alnifolia and occasional smaller-flowered var. semiintegrifolia occur, so the petal length distinction is not an absolute one. Geographically these two varieties are largely separate, with var. alnifolia occurring in the Great Plains and Rocky Mountains and var. semiintegrifolia on the Pacific slopes of mountains from Alaska to northern California.
Amelanchier alnifolia is thought to hybridize with Sorbus scopulina (×Amelasorbus jackii Rehder).