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Eastern Poison-ivy (Toxicodendron radicans) is found throughout the eastern United States and adjacent Canada south to Guatemala. Poison-ivy and its close relatives are well-known for possessing skin-irritating oil (urushiol), which can cause severe allergic reactions in humans.
The taxonomy and nomenclature of North American Toxicodendron has been in flux for over a century, largely due to confusing within-species variation in growth form, leaf and leaflet shape, and other features (e.g., Gillis 1971; Gartner 1991). This has resulted in an abundance of synonyms, but five species are now generally recognized: Common Poison-ivy (T. radicans), Western Poison-ivy (T. rydbergii), Eastern Poison-oak (T. pubescens), Western Poison-oak (T. diversilobum), and Poison-sumac (T. vernix) (Senchina 2006). Eastern Poison-ivy is the only Toxicodendron species in eastern North America that is typically a climbing (or straggling) vine with aerial roots (Western Poison-ivy, Eastern Poison-oak, and Poison-sumac do not climb but are instead erect or suberect shrubs) (Gleason and Cronquist 1991). Older plants often develop thick, reddish brown stems that ascend tree trunks. For reasons that are not yet clear, Eastern Poison-ivy grows more abundantly on some tree species than on others (Talley et al. 1996). Eastern and Western Poison-ivy may intergrade to some extent, making identification less clear in some areas.
Toxicodendron radicans is supposedly also present in eastern Asia, with the North American form known as T. radicans radicans and the Asian form known as T. radicans hispidum. However, molecular phylogenetic analyses by Nie et al. (2009) suggest that these two taxa are not each other's closest relatives and that the Asian form is, instead, the sister group to a clade including all four trifoliate (three-leafleted) North American Toxicodendron (Poison-sumac has 7 to 13 leaflets per leaf and its apparent closest relative lives in eastern Asia). This would imply that what is currently known as T. r. hispidum should be treated as a distinct species.
Senchina (2008) reviewed the literature on animal and fungal associates of Toxicodendron in North America with a particular eye toward identifying potential biological control agents. Interest in finding new ways to control poison-ivy and its relatives may increase in coming years given data suggesting that these plants may become more abundant and more ‘‘toxic’’ in the future, potentially affecting global forest dynamics and human health (Mohan et al. 2006).
(Gillis 1971; Nie et al. 2009 and references therein)