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.)

Genera 7, species ca. 575 (3 genera, 15 species in the flora).

To people in the northern hemisphere, the word mistletoe most often calls to mind a branch parasite in Viscaceae. In Europe, the Christmas mistletoe is typically Viscum album; whereas in North America, it is more often a species of Phoradendron. Mistletoe is associated historically with ancient Teutonic myths and Druidic rituals, but vestiges of these associations survive today as Yuletide kissing, a custom that dates to sixteenth-century England. Viscaceae represent just one of five clades that contain mistletoes, the others being Misodendraceae J. Agardh, Loranthaceae Jussieu, Santalaceae, and Amphorogynaceae Nickrent & Der (R. L. Mathiasen et al. 2008; D. L. Nickrent et al. 2010). Thus, the term mistletoe describes both a habit (an aerial parasite) and a taxonomic association (a member of Santalales). Early twentieth-century works (for example, A. Engler and K. Krause 1935) often treated Viscaceae as a subfamily of Loranthaceae, and indeed this practice persists today in some floras, herbaria, and popular treatments. But molecular phylogenetic work has greatly clarified relationships among the five mistletoe clades, showing that they originated independently at different time periods (R. Vidal-Russell and Nickrent 2008; J. P. Der and Nickrent 2008). Thus, the mainly tropical family Loranthaceae, whose members often have large, bird-pollinated flowers, is only distantly related to Viscaceae with small, insect-pollinated flowers.

In North America, Arceuthobium and Phoradendron are native and Viscum was introduced purposely and is now naturalized. By far, Arceuthobium is the most economically significant viscaceous mistletoe. In 1982, timber losses caused by species of Arceuthobium for the western North America were estimated as 130 million cubic meters (D. B. Drummond 1982; F. G. Hawksworth and D. Wiens 1996). Although these losses are severe, particularly in areas managed for timber production, species of Arceuthobium also play important ecological roles in natural forest ecosystems, for example its witches’ brooms provide nesting habitat for the northern spotted owl (R. Everett et al. 1997). Native trees as well as cultivated fruit and nut trees can be negatively impacted by Phoradendron in North America, Mexico, and Central and South America, but the extent of damage has not been quantified. In addition to its use as a Christmas holiday novelty, Viscum is also being explored for a variety of medicinal uses including treatments for some forms of cancer (A. Bussing 2000).

Viscaceae have been shown by molecular phylogenetic studies to be sister to Amphorogynaceae Der & Nickrent (previously a tribe of Santalaceae), which includes root and stem parasites (J. P. Der and D. L. Nickrent 2008; Su H. J. et al. 2015). Resolution of some relationships among the genera of Viscaceae had been seen from molecular studies conducted over a decade ago, but only recently has a clear picture emerged. Viscum and Notothixos Oliver are sister to the remaining genera, which include a clade consisting of Korthalsella Tieghem and Ginalloa Korthals and another of Phoradendron/Dendrophthora and Arceuthobium. From a biogeographic perspective, Arceuthobium is interesting because it is the only mistletoe to occur naturally in both the Old and New Worlds. A hypothesis by F. G. Hawksworth and D. Wiens (1972) is that Arceuthobium evolved in Asia and, following a trans-Beringian migration, underwent a massive radiation in the New World. Chloroplast spacer sequences show, however, that the New World species are most similar to Phoradendron whereas the Old World species have accumulated numerous deletions (Nickrent and M. A. García 2009). The most parsimonious hypothesis is that both Arceuthobium and Phoradendron first evolved in the New World, possibly in the Mexican highlands, where both genera are today quite diverse, and Arceuthobium then colonized the Old World.

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