Flora of North America species comparison

Varieties 2 (2 in the flora).

Because of variation in the degree of hairiness on branches and leaves, thickness of leaves, leaf shape, sizes of racemes and flowers, and colors of fruits, Prunus virginiana has been split into forms, varieties, and segregate species. When viewed on a continental scale, most of the key characteristics used to separate these taxa intergrade enough so as to be useless.

The most useful distinction is between chokecherries found east of the Rocky Mountains and those found from the mountains westward. They differ in leaf shape and raceme size; the variation is imperfectly correlated. The eastern plants generally have their larger leaves obovate with lengths less than two times widths, racemes typically less than 7 cm, and petals to 4 mm. Plants of the Rockies and west have longer racemes and larger petals, and generally their larger leaves are elliptic to oblanceolate leaves with lengths two or more times widths. Distinctions between the eastern and western varieties are least obvious from Montana to eastern Washington north into British Columbia, where the varietal ranges overlap. Plants of the High Plains show intergradation between those of the Rockies and those of the East. The taxonomic solution provided here is a single species of chokecherry in North America with an eastern and a western variety that overlap in range along a line from northeastern New Mexico to southwestern British Columbia.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

H. J. Scoggan (1978–1979, part 3) included Prunus emarginata within P. pensylvanica. Given that much of the problem of separating P. emarginata from P. pensylvanica arises from intermediate plants found in British Columbia and western Montana where the ranges of the two species meet, this makes some sense from a Canadian perspective. When viewed from a continental perspective, P. emarginata normally does not look much like P. pensylvanica (see notes on their differences under 10. P. pensylvanica).

Individuals of Prunus emarginata vary considerably in their habit, size and shape of leaves, and indument on branchlets, inflorescences, and leaves. Many segregate species have been described based on this variation. Most often, western botanists have recognized one species with two varieties based on habit or indument; those characters are not well correlated. Some keys use habit to separate the varieties (var. emarginata, a shrub, versus var. mollis, a tree, commonly used in Pacific Northwest floras); others have used indument (var. emarginata, glabrous, versus var. mollis, hairy, more commonly used in floras of the arid West). On the mountain slopes in the more arid portions of its range, P. emarginata is a thicket-forming shrub to about 6 m with the leaves expanding to no more than 50 mm; in the moister, milder climates of coastal Oregon to British Columbia, as well as the interior wetbelt of northern Idaho, northwestern Montana, and adjacent British Columbia, the plants are trees to 15 m with larger leaves expanding to 80 mm. Hairy plants occur throughout the range of P. emarginata, and glabrous plants are found from British Columbia to Arizona, generally not west of the Cascades.

Hybrids between Prunus emarginata and naturalized P. avium (P. ×pugetensis Jacobson & Zika) have been reported from the lowlands of western British Columbia, Oregon, and Washington.

(Discussion copyrighted by Flora of North America; reprinted with permission.)