University of Georgia forest ecologist Jacqueline Mohan tells us that not only is poison ivy already responding favorably to warming in Georgia, but its poison is getting more poisonous. The reason that poison ivy and other similar woody vines flourish in warmer climates (think Precambrian) is a symbiotic relationship with kind of fungi called arbuscular mycorrhiza. The myccorrhiza assists in the uptake of nutrients like phosphorus and nitrogen, enhancing growth.
But don’t go thinkin that old myth is true now, ya hear? Even in Gerogia, it’s supposed to get a lot drier with the warming, like most places. In addition, evaporation is greatly enhanced with warmer temperatures because the evaporation temperature ratio is not linear. More warmth means a LOT more evaporation. Drought can persist, or even grow worse in warmer times with normal or even above normal rainfall. So in general, and across large parts of the northern latitudes already, plants’ vegetative response is already declining, not increasing like the myth says on a planet with more CO2. The warmth and dryness have already overcome the beneficial fertilization aspects of increased CO2. It’s happening now with poison ivy and many other species in the subtropics where warming is less that elsewhere. In northern latitudes however, the CO2 fertilization effect has largely diminished.
Mohan et al., Biomass and toxicity responses of poison ivy (Toxicodendron radicans) to elevated atmospheric CO2, PNAS, April 2006.