Ever noticed that poison oak in the Sierras doesn't look like the poison oak along the coast? It doesn't. I've long thought about this and today a link in Sequoia NP's Twitter feed prompted me to post this comparison:
Sierra poison oak at top, coastal on the bottom (images from NPS photo site and Yoga Slackers resp.) Different, huh? Though it's not to scale, you can tell how the Sierra variant has larger leaves that are much less rounded, much more like their namesake oak leaves. Coastal P.O. is really just small bumpy ovals. This difference may be relevant. Why? For one thing, do the oils from each necessarily induce cross-reactions? Is one worse than the other? It's possible they're the same species, but they look different when grown in cooler, more extreme seasonal conditions - or that there's an actual genetic difference, and we're looking at different species or subspecies that have long been incorrectly classified. I hate poison oak with the obvious motivation of personal experience, so if I can spare one other person (maybe you!) the nightmare of contact dermatitis, I'll be happy.
Note that the Sequoia NP folks said it's found up to 5,000' elevation. Up in the Bay Area we usually assume a cut-off of 4,000', and then further north, somewhere between Portland and Seattle the grow-line drops to sea level and you don't see it anymore. It also survives just fine in the desert (I've seen it in the Mojave) as long as it can grow in a shaded canyon near a water source.
I was surprised not to find any literature on these variants, but what's both frustrating and exciting is that you can't assume someone has properly studied plants like this. That's how it often goes: common organisms described centuries ago, if they don't cause disease or have an economic impact, don't get grant money dedicated to them. Consequently there are basic discoveries waiting under our noses even though we think science has already swept out all this space. So what's going on, West Coast naturalists? Help us out!
I should add that further confusing the nomenclature, a) other western states (non-coastal ones) use the term "poison ivy" for what appears to be the same plant as "Sierra poison oak", and b) true poison ivy (Toxicodendron radicans) and poison oak do in fact overlap in their ranges - I have encountered this in Austin, Texas as well as Moab, Utah. Sure, as a rule it makes sense not to touch plants with the word "poison" in the name, but it seems clear that we still have a lot to learn about the plants literally growing in our back yards.