Flora of North America species comparison

This treatment follows J. Kuijt (2003), who considered Phoradendron bolleanum to be a widespread and variable species complex. The three main taxa that often have been recognized as species are Phoradendron bolleanum in the narrow sense, P. densum, and P. pauciflorum,. Phoradendron bolleanum in the narrow sense has small, narrow leaves and frequently parasitizes Juniperus as well as Arbutus. The P. pauciflorum variant has broad leaves and mostly parasitizes Abies concolor. The P. densum variant tends to have leaves that are intermediate between those of the other two variants; it ranges from Oregon to Mexico and parasitizes Cupressus and Juniperus. Molecular analyses indicate that P. bolleanum is not monophyletic unless P. minutifolium Urban is synonymized with the other variants. Hybrids between P. bolleanum and P. juniperinum produce plants closely resembling P. minutifolium (D. Wiens and M. DeDecker 1972). As pointed out by Kuijt, molecular studies will be required to determine species boundaries within this complex.

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

J. Kuijt (2003) used the name Phoradendron serotinum, based on the name Viscum serotinum Rafinesque (1820), not P. leucarpum, which is based on the earlier name by the same author, V. leucarpum (1817). A proposal to conserve the later name (D. L. Nickrent et al. 2010b) was not accepted, thus the name P. leucarpum has priority.

Phoradendron leucarpum has a convoluted taxonomic history, reflecting not only various species concepts but also complex evolutionary and ecological processes. Among the 234 species of Phoradendron, J. Kuijt (2003) recognized subspecies only in P. leucarpum (as P. serotinum). In addition to the typical subspecies from eastern Texas eastward, they are subsp. augustifolium from Mexico, subsp. macrophyllum from eastern Texas through New Mexico and Arizona to California and Oregon, and subsp. tomentosum, with about the same distribution as subsp. macrophyllum but also extending into Mexico. Kuijt noted that in some geographic areas, such as east-central Texas, the putative subspecies show a continuum of morphological intergradation.

A population genetic and morphometric study of this complex was undertaken by A. K. Hawkins (2010). Principal component analyses using the characters that J. Kuijt (2003) considered to be diagnostic of the subspecies, such as leaf size, color, and venation, as well as the type and density of hairs present on young vegetative and reproductive tissues, in addition to host species, did not result in clusters corresponding to the four described subspecies. Moreover, FST analyses of microsatellites showed significant interpopulational differentiation that did not match the subspecies that Kuijt recognized. Because morphological and molecular analyses show that subspecies, at least as defined by Kuijt, cannot be differentiated in Phoradendron leucarpum, no subspecies are accepted here.

Phoradendron leucarpum is the only species of the genus found east of Texas. It parasitizes over 60 species of native and introduced trees, especially Acer, Fraxinus, Juglans, Nyssa, Platanus, Populus, Quercus, Salix, and Ulmus.

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