Flora of North America species comparison

Throughout most of its range, Prunus pensylvanica appears distinct from P. emarginata. The leaves of P. pensylvanica are generally larger and lanceolate to ovate-lanceolate rather than oblanceolate to elliptic; the leaf apices are usually acuminate (sometimes acute) versus rounded to obtuse (rarely acute) in P. emarginata. Inflorescences of P. pensylvanica are corymbose to umbellate with central axes shorter than pedicels; in P. emarginata the inflorescences are corymbose to racemose with central axes longer than pedicels. Where their ranges overlap in British Columbia and western Montana, intermediates are found with the corymbose inflorescence of P. emarginata and leaves more comfortably accommodated within the variation of P. pensylvanica. Some of these specimens have been identified as P. corymbulosa, based on a type from Montana, here included within P. pensylvanica.

Along the eastern slopes of the Rockies and throughout the northwestern Great Plains, Prunus pensylvanica is shrubby and has smaller leaves (less than 60 mm) than it does farther east. Compared to leaves of eastern plants, those of western specimens are also broader in proportion to their length (1.7–2.3:1 versus 2–4.3:1), have acute rather than acuminate apices, and tend to be more coarsely toothed. Although sometimes segregated as var. saximontana, or subsp. corymbulosa, when having a corymbose inflorescence, these plants fit within the variation in plant habit, leaf size, leaf shape, and margin serration seen throughout the broad range of P. pensylvanica.

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

Of our native plum species, Prunus americana is the most widely distributed and abundant in terms of individual plants. As might be expected, it is variable. U. P. Hedrick (1911) reported that about 260 horticultural varieties had been derived from it, far more than from any other North American plum. Not infrequently, individual specimens possess characteristics intermediate between P. americana and other plum species, probably the result of gene exchange, which seems to be common among American plums (J. Shaw and R. L. Small 2005).

The most obvious variation in Prunus americana is the presence or absence of indument on twigs, pedicels, hypanthia, sepals, petioles, and abaxial surfaces of leaves. The hairy form has been known by the epithets mollis and lanata, both as varieties of P. americana and at species rank. The type of P. mollis, upon which all of these names are ultimately based, is conspecific with that of P. nigra, and thus mollis and lanata are misapplied as infraspecific epithets of P. americana (L. H. Shinners 1956b). The hairy form is most abundant in the western part of the species range, where it is at least as prevalent as the glabrous form; hairy individuals are found throughout the range of the species. As in other species of Prunus, variation in indument is an easily noticed difference among individuals but not one of taxonomic significance.

In Missouri, Arkansas, and neighboring states, hairy Prunus americana intergrades with P. mexicana. In this zone of sympatry, they hybridize and produce intermediate forms, making determination of some herbarium specimens from this region difficult, if not arbitrary. One solution, not chosen here, would be to combine P. americana and P. mexicana into a single species. Tradition and morphologic distinctions argue for keeping them separate until more data can clarify the entire North American plum complex. Typically, P. americana is a shrub or tree to 80 dm tall and the plants sucker to form thickets; P. mexicana is a single tree 30–120 dm tall, rarely suckering. The leaves of P. americana are longer in proportion to their width than those of P. mexicana, and the leaf bases are cuneate in P. americana versus rounded or subcordate in P. mexicana. In hairy individuals of P. americana, the form most likely to be confused with P. mexicana, twigs are almost always hairy; about 70% of all P. mexicana specimens examined have glabrous twigs.

The following names have been misapplied to Prunus americana, though the types belong with that of P. nigra: Prunus americana var. lanata Sudworth, P. americana var. mollis (Torrey) Torrey & A. Gray, P. lanata (Sudworth) Mackenzie & Bush, and P. mollis Torrey.

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