We were pruning an Arizona Ash (Fraxinus velutina) the other day to remove mistletoe. One of my workers came over to me with a piece of the normal “Christmas” mistletoe that grows in the ashs, pecans and cottonwoods around here – (Phoradendron macrophyllum). It has what appears to be a clump of “desert” mistletoe – (Phoradendron californicum et al) growing on it! See this post for more information on the different types of mistletoe.
I forwarded photos of it to a botanist that I know, and it seems that this has been described for some mistletoe species, but is not very common. He has forwarded the photos on to some others and I hope to hear back from them as to the status of this unusual discovery! I have pressed the specimen and may forward it on if someone in the scientific community if there is interest in it.
Here is a macro photo (closeup) of the union of the two plants:
Dr. Daniel Nickrent in the Department of Plant Biology at Southern Illinois University wrote the following to me:
One of the most remarkable coevolutionary relationships that exists is the presence of one parasitic angiosperm upon another. Two forms of association can be distinguished, facultative and obligate. As suggested by Wiens and Calvin (1987) the term hyperparasite should be used to describe a facultative association between different parasite species. Probably the most frequent and generalized examples of hyperparasitism involve Cuscuta and Cassytha. These genera parasitize a variety of plants which, by chance, may include woody root hemiparasites such as Ximenia (Olacaceae), Santalum (Santalaceae), etc. Cuscuta and Cassytha have also been reported as hyperparasites of mistletoes such as Phoradendron and Struthanthus (Kuijt 1964).
In contrast, the obligate situation called epiparasitism is known from mistletoes of both Loranthaceae, Viscaceae and Santalaceae in both the Paleotropics and Neotropics. The sole epiparasitic genus in the latter family is Phacellaria of southeast Asia which is an obligate parasite of Loranthaceae (Danser 1939). All possible host-parasite combinations have been reported (e.g. loranth on loranth, viscoid on viscoid, viscoid on loranth, and loranth on viscoid), although certain combinations are more common in particular regions. For example, Viscaceae have rarely been reported on Loranthaceae in the New World, a rare exception being Phoradendron iltisii on Cladocolea pringlei (Kuijt 1990). Approximately ten species of Phoradendron have been reported to be epiparasites, and indeed the entire ‘Amplectens’ group may be so (Kuijt 1987). At least ten species of epiparasitic Viscum have been documented from Australia and Asia (examples include V. articulatum and V. loranthi) and Africa (e.g. V. loranthicola) with Loranthaceae being the most frequent hosts. In Africa, Loranthaceae such as Agelanthus pungu are frequently found parasitizing other members of both Loranthaceae and Viscaceae (Polhill and Wiens 1998). New World loranth-loranth combinations can be found in Ixocactus, Notanthera, Phthirusa, and Tristerix, however, only one report of the inverse exists, i.e. Oryctanthus occidentalis on Phoradendron crassifolium (Kuijt 1964). Even more remarkable are tripartite epi- or hyperparasitic associations, such asScurrula ferrugineus on Viscum articulatum on Elytranthe barnesii which was itself parasitic on Durio (Sands, 1924). Similarly, in South Africa, the following association has been reported: Viscum verrucosum on Tapinanthus quequensis (= T. leendertziae) on Agelanthus natalitius (= T. natalitius) upon Combretum apiculatum (Visser 1982). Field studies have shown that mistletoes maintain a water potential difference ca. 1000 kPa less than their nonparasitic hosts when both species are transpiring maximally. When an epiparasitic mistletoe is measured, its water potential is 1000 kPa less than its host mistletoe (Visser 1982). Water potential measurements have never been made on the component species in involved a three-way association, but the question can be asked “how great can water potential differences become?”]
So, epiparasitism seems to be common in Phoradendron, and maybe more common than we realize. I know the case I saw of this in Durango Mexico involving P. scabberimum (http://www.parasiticplants.siu.edu/Viscaceae/index.html) was very cryptic.