Limeledge Botanical Garden and Arboretum

Philip Crim

1-25-23

The genus Prunus is one of the most important groups of woody agricultural plants in the world. Prunus is best known from encompassing the familiar stone fruits: cherries, peaches, plums, apricots, and almonds, but it is also home to more obscure groups such as the Maddencherries, a small clade of poorly-known Asian species formerly treated in their own genus, Maddenia. Other groups such as the laurelcherries (or cherry-laurel in Europe) are primarily utilized for horticulture, since many of them are relatively cold-hardy broadleaved evergreens with attractive foliage. The most recent phylogenetic study of the genus that I have studied (Shi et al. 2013) divides Prunus into three subgenera: Padus, containing the bird cherries as well as the Maddencherry and laurelcherry clades; Cerasus, which includes many of the agricultural cherry species; and Prunus, which includes the remaining stone fruit species.

American black cherry (Prunus serotina Ehrh.) is one of the bird cherries, belonging to the subgenus Padus. It also goes by the alternative common names “fire cherry” (to which the species epithet serotina refers) and “rum cherry” for its role in flavoring certain beverages. Black cherry has a very extensive native range; if all of its subspecies are considered, it currently occupies vast areas of both North and South America, although its populations south of Mesoamerica are likely due to human assistance.

A novel variegated black cherry (Prunus serotina) seedling.
This variegated American black cherry seedling was collected on a property near Limeledge. Since it was discovered while investigating a plant that had been vandalized, I will name this plant ‘Karma’ if the variegation is stable. Ironically, it could be far more valuable than the plant that was destroyed.

This extensive native range has provided dendrologists with the opportunity to witness evolution in progress, as the species possesses a wide range of regional morphological variability. In some of these cases, this variation is great enough to warrant taxonomic status for the local plants… and completely flummox taxonomists seeking to impose well-defined boundaries on these entities. Even within the “typical” American black cherry (Prunus serotina ssp. serotina), many variants have been found and propagated, mostly in Europe. These include weeping and cutleaf forms. I have personally observed a great deal of variation in this species for traits such as fall color and canopy form. In Summer of 2022, I observed a variegated seedling, but time will tell whether or not the color pattern is stable. I am aware of at least one other plantsman that has discovered a similar seedling, and if stable I look forward to examining the ornamental qualities of each.

References that appear to be based on field research and scientific publications generally recognize five subspecies of black cherry. Many U.S. government-affiliated references list these taxa as varieties rather than subspecies; however, this may be due to American references historically associating subspecies classifications with animals and varieties with plants, and may not reflect any of the biological realities of the group. Due to the complexity of the black cherries, a good molecular study for the entirety of the complex is necessary to best address the status of these classifications. My examination of the literature to get a handle on this group began with the work of McVaugh (1951). His concepts of subspecies (hereafter abbreviated as ssp.) eximia and ssp. virens, supplemented by more recent work, were helpful in understanding some of the southwestern black cherries. The entity known as ssp. eximia (Small) Little, commonly referred to as escarpment black cherry, appears to be a fairly narrow endemic occupying the Lampasas Cut Plain and Edwards Plateau in Texas. Meanwhile, ssp. virens (Wood & Standl.) McVaugh commonly called Mexican black cherry is an abundant, generally southwestern variant occupying more arid parts of Arizona, New Mexico, Texas, and adjacent Mexico. Mexican black cherry also appears to be the best place to include McVaugh’s var. rufula, which appears to be distinct enough to warrant recognition at the lower level of variety. Although Prunus serotina ssp. virens var. rufula McVaugh (Southwestern black cherry) is certainly a mouthful, it allows us to recognize the variation present in the southwestern states and northeastern Mexico while minimizing the loss of information that could come from lumping it into a broad, variable collection of traits. This treatment is also adopted by Segura et al. (2018). Caution is necessary with these names; Flora of North America considers McVaugh’s var. rufula as a synonym of the entity discussed here as ssp. virens, and accepts var. rufula as the correct name for plants of both types. What a mess!

A Mexican black cherry (Prunus serotina ssp. virens) northwest of Fort Davis, TX. This appears to be typical ssp. virens and not var. rufula. Seeds were collected for assessment and comparison to other black cherries at Limeledge.

The southeastern U.S. is another region hosting distinct black cherry variants. The entities alabamensis, australis, and cuthbertii are often seen listed at the species level, but it appears that they are best treated within ssp. hirsuta (Ell.) McVaugh, the Alabama black cherry. By all accounts I have seen, this taxon is distinct in the field and can be found in Alabama, Georgia, and Florida. It is characterized by flower characteristics, downy branchlets, leaf undersides, petioles and like ssp. serotina, it can be a large forest tree. The fifth and final subspecies is ssp. capuli, the capulin cherry, an entity found in South and Central America that is valued for its fruit. As a result of its extensive history of cultivation, the true extent of its original native range before humans interfered may never be known but it appears to have Mesoamerican origins. Perhaps thanks to cultivation, ssp. capuli grows sympatrically with other Prunus serotina taxa, and in some areas, there appears to be a continuum of morphological variation rather than the distinct boundaries that humans prefer for identification.

Flora of North America omits any mention of the South and possibly Central American ssp. capuli and sinks every subspecies into synonymy and/or reduces them to the level of variety. Indeed, this may be a reasonable treatment; additional molecular studies would be very helpful in identifying the taxonomic levels at which these entities are classified, as well as identifying any potentially cryptic species lurking within the variation of this complex. Rohrer in FNA (2014), a source that often utilizes the taxonomic level of variety in place of (rather than subordinate to) subspecies notes that ssp. eximia should be a synonym of their Prunus serotina var. serotina since its morphological variation falls within the broader concept of the latter… but admits that the isolated and disjunct Edwards Plateau plants are very consistent in their combination of those trait variations. This is exactly the sort of biological (and related ecological) information and context that is preserved by recognizing these entities, even at a low subordinate level within other taxa. Indeed, Guzman et al. (2018) present molecular evidence that ssp. eximia is the most genetically distinct of the black cherries, and that varietal designations may be more appropriate for some of the subspecies that were poorly delineated by their results. Unfortunately, this work did not include ssp. hirsuta. In summary, a robust molecular study would be informative on whether the eximia entity is a distinct lineage, and how best to organize the infraspecific taxa of the Prunus serotina complex. Morphology often deceives, but quantitative molecular work can allow us to pierce the veil of cryptic taxa, patterns of introgression, and measure the relationships between genes and environmental variables.

Escarpment black cherry (Prunus serotina ssp. eximia) on the Edwards Plateau between Camp Wood and Leakey, TX

Here is a helpful summary table of the infraspecific black cherry taxa. These are the classifications that seem to have the most biological and ecological evidence to justify them, although there is evidence that some of the subspecies may be demoted to variety status under subspecies serotina in the future.

TaxonColloquial Name
Typical Form:
Prunus serotina ssp. serotina (Ehrh.) McVaugh
American black cherry
Prunus serotina ssp. capuli (Cav.) McVaugh (=P. salicifolia; P. serotina var. salicifolia)Capulin cherry
Prunus serotina ssp. eximia (Small) LittleEscarpment black cherry
Prunus serotina ssp. hirsuta (Ell.) McVaugh (= P. alabamensis; P. australis; P. cuthbertii; P. serotina var. alabamensis)Alabama black cherry
Prunus serotina ssp. virens (Wood & Standl.) McVaugh (=P. serotina var. rufula)Mexican black cherry
Prunus serotina ssp. virens var. rufula (Wood & Standl.) McVaughSouthwestern black cherry
Escarpment black cherry (Prunus serotina ssp. eximia) seedling from the tree in the previous photo. Note the jagged margins, a trait that this taxon is known for. This plant is not known to be in cultivation; the individuals at Limeledge may be the first in a public garden.

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