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Western Poison-oak (Toxicodendron diversilobum) is found at elevations below around 1650 m in westernmost North America from British Columbia (Canada) to Baja California (Mexico), including the U.S. states of Washington, Oregon, California, and Nevada. It may grow as a shrub (sometimes tree-like), 0.5 to 4 m tall, or (when externally supported) as a vine more than 30 m long--or any form in between (Gartner 1991a,b). The (typically) 3-leafleted leaves turn bright red in autumn. Poison-oak 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).
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-oak 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).