Understanding the relationships within the genus Viburnum has always been a challenge for scientists. Widespread, it occurs from the tropics to the far north in both the eastern and western hemispheres. This wide range has resulted in an extraordinary amount of diversity within the genus, and approximately 200 species are currently recognized. Complicating matters further, many viburnums belong to groups of closely related species, and determining whether to classify members of these complexes as separate species or under others as subspecies or varieties is a difficult question. Even the name of the family it belongs within has sparked fierce debate, with the current prevailing view that the name Viburnaceae is correct and proper, rejecting the (still) widely used name Adoxaceae.
Enter the Porphyrotinus Clade of Viburnum species. Building on previous work that sought to understand the relationships in the genus, Landis et al. (2021) utilized nuclear and plastid DNA, morphological measurements, modern geographical ranges of extant Viburnum species, and the fossil record. The discussion of clades and subclades here follows the naming conventions used in that paper. Their work enabled the researchers to construct a family tree of viburnums and provided evidence to divide the species between ten principal clades, some of which could be further divided into subclades. Of these, the species within the Porphyrotinus Clade are particularly familiar to ecologists, naturalists, and horticulturalists as harboring the species complexes widely referred to as “arrowwoods”.
The Porphyrotinus Clade is itself divided into three subclades in the Landis et al. paper: Subclade Dentata, Subclade Mollotinus, and Subclade Oreinotinus. While the latter is primarily a tropical and subtropical Mesoamerican group, the first two clades contain our familiar arrowwood species. Even to many novices, the name Viburnum dentatum L. is instantly associated with the term arrowwood. This colloquial name for members of the species complexes within these two subclades refers to the ability of some of these plants to produce very straight branches that are a good size for making the shafts of arrows and were used for this purpose by indigenous peoples and colonists. While not of much use for this purpose in the present, the shrubs are widespread throughout many regions of North America and are frequently utilized in horticulture as they showcase many excellent landscape traits.
Subclades Dentata and Mollotinus each contain their own “arrowwood” species complexes. The treatment of perhaps the best-known “arrowwood”, V. dentatum L., part of a group of similar and closely related species within Subclade Dentata, has caused a great deal of confusion; look no farther than its Wikipedia page for outdated taxonomy. To be fair, “official” databases such as USDA Plants and ITIS also fail to incorporate the most recent scientific information on the taxonomy of this group.V. dentatum L. had been previously considered to have a large range and encompass great morphological and ecological variability, but in recent years V. recognitum Fernald, previously considered a variety (aka V. dentatum var. lucidum Aiton) has been recognized as the predominantly Northern arrowwood species while typical V. dentatum L.is more southern. In addition, there is strong evidence that the taxon known as V. dentatum var. venosum (Britton) Gleason, which reaches into New York on Long Island and downstate, is a distinct species: Viburnum venosum Britton. Where the two plants grow together in the field on Long Island, Steve Young’s observations support this view, as the two taxa seem to remain distinct with no obvious intermediate forms. The plants flower at different times, which would impose a reproductive barrier between the species and explain the lack of obvious hybrids in the field. Likewise, Sorrie (2010) apparently presented a compelling presented a compelling case for raising V. alabamense (McAtee) Sorrie, V. carolinianum Ashe, and V. scabrellum (Torr. & A.Gray) Chapm. to species status as well as V. venosum Britton; however, this was apparently in an unpublished manuscript. Sorrie and Weakley (2011) and Weakley et al. (2011) summarize the updates and recognition of species within this complex.
Subclade Mollotinus, with species that morphologically bear strong resemblance to the “traditional” V. dentatum L. arrowwood, is also rather messy and has been poorly understood until recently. A taxon that is seen in the literature and occasionally enters horticulture due to its excellent ornamental traits has been called Viburnum dentatum var. deamii (Rehder) Fernald. While this plant is often considered a synonym of V. dentatum L., Sorrie (2012) accepts this as a good species and notes that it actually has morphological affinities with V. bracteatum Rehder, which would place it in Subclade Mollotinus. Therefore, V. deamii (Rehder) Sorrie is the correct name. Weckman (2002) elevated V. ozarkense Ashe to species status as distinct from V. molle Michx.; however, the taxon is closest to V. bracteatum. Estes (2010) performed a morphological study indicating that V. ozarkense and V. bracteatum could not be separated on the basis of the traits studied and sunk V. ozarkense into synonymy with the latter. However, given that V. ozarkense populations are disjunct from the nearest V. bracteatum by over 500-km, I would prefer to see a more thorough ecological and molecular study to indicate whether or not these taxa are truly conspecific. For now, I will retain the V. ozarkense name at species level with the caveat that it is likely a subspecies or variety of V. bracteatum, if it’s worthy of distinction at even those levels. Regardless of their exact taxonomic level, plants of V. bracteatum-ozarkense are narrow endemics in dire need of conservation.
If you’re interested in growing arrowwoods and enjoying some of the diversity present in this fascinating group of species, here is a guide that I have synthesized from some of the most recent studies on this little corner of the genus Viburnum:
|Porphyrotinus Clade||Colloquial Names|
|alabamense (=recognitum var. alabamense)||Alabama arrowwood|
|deamii (=dentatum var. deamii)||Glossy arrowwood|
|recognitum (= ashei; dentatum var. lucidum)||Northern arrowwood|
*There is evidence that this is synonymous with the disjunct V. bracteatum, but more research is necessary.