Flora of North America species comparison

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.)

Species ca. 244 (7 in the flora).

Although not particularly diverse in the United States, Phoradendron underwent a massive radiation in Mexico, Central America, and South America. The first modern taxonomic treatment of the genus was by W. Trelease (1916), who named 240 species. More recently, J. Kuijt (2003) produced a monograph that included 234 species, but the similarity in number belies the fact that only 18 of the names accepted by Trelease were retained.

Phoradendron is closely related to Dendrophthora Eichler (a genus of 125 species), separated from it only by the presence of 2-locular versus 1-locular anthers. Whether these two genera are monophyletic remains to be tested using molecular methods (V. E. T. M. Ashworth 2000). Together Phoradendron and Dendrophthora are important components of mesic and arid environments in the New World, particularly because their fruits provide food for various bird species that disperse their seeds.

In the key and descriptions that follow, basal pyllotaxy refers to the orientation of the basal pair of leaves or cataphylls on a lateral branch. When those leaves or cataphylls are in the same plane as the main and lateral branch, basal phyllotaxy is median; when they are at right angles to that plane, it is transverse. Flower seriation refers to the arrangement of flowers on fertile internodes (J. Kuijt 2003). Different seriation may occur in staminate and pistillate inflorescences of the same species (for example in Phoradendron californicum). Flowers typically are arranged in one or more columns above each bract, and each set of columns is topped by a single median flower. Each fertile internode has two sets of flower columns, one above each of the opposite bracts. When two columns of flowers occur above the bract, the condition is called biseriate. When three columns of flowers occur, the condition is triseriate. Uniseriate and multiseriate conditions exist, but only outside the flora area. When only three flowers occur above each bract, seriation cannot always be determined (for example in P. juniperinum).

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