A Rash of Consequences

Natural History Magazine, 1 October 2006

S.R.

As if there weren't enough ominous consequences of global warming, here's a little bitty consequence that's just plain nasty: bigger, badder poison ivy. Already the irksome vine is responsible for more than 350,000 cases of dermatitis annually in the United States alone-and those are just the reported cases. A new study shows that as atmospheric carbon dioxide (CO2, a major cause of global warming) becomes more abundant, poison ivy proliferates and makes a more toxic form of urushiol, the substance that triggers the rashes.

From 1996 until 2004 investigators pumped CO2 into a pine plantation at Duke University in Durham, North Carolina. The idea was to simulate the 54 percent increase in atmospheric CO2 expected by mid-century from the continued burning of fossil fuels. Jacqueline E. Mohan, a biologist at Duke, and six colleagues report that in the last five years of the study the poison ivy in the plantation grew nearly three times as fast as the plants normally do. And the toxicity of its urushiol shot up by some 33 percent.

Extra CO2 boosts photosynthesis in all plants, but vines such as poison ivy invest relatively little of the extra energy they gain in growing more wood for support. Instead, vines can channel most of their extra energy into making even more photosynthetic greenery, a positive feedback loop that is bad news for trees: when vines are on the increase, as they are in many parts of the world, they often interfere with tree growth, and they even kill trees outright by shading or choking them.
Oh, and did I mention the part about more itching and scratching? (PNAS 103:9086-9, 2006)