Does More CO2 Mean More Poison Ivy?

Nicole Heller

By Nicole Heller

Plants need carbon dioxide (CO2) to photosynthesize. So more CO2 in the atmosphere means faster plant growth and a greener world. That is a good thing, right?

Not necessarily.

There are websites promoting such thinking, but there are a number of problems with such a statement. For instance, many places are going to become a lot hotter and drier, and desertification does not lead to greener pastures. Even if we ignore everything we know about climate systems, and pretend that around the world a warmer climate would be miraculously productive for plant growth (or that we didn’t care about places like Arizona), that still doesn’t mean that more plant growth is going to be beneficial to people.

Last week’s Washington Post headline drove this point home for me: “Increased carbon in atmosphere may explain bumper crop of poison ivy.” If poison ivy likes extra CO2, then I don’t.

That is probably because when I was 12 years old, my eyes swelled shut, I was rushed to the hospital, and doctors told my family that my kidneys were shutting down! For about 15 minutes or so, I thought I was maybe dying, barely aware of what kidneys were but sure, given the alarm exploding around me, that they were important. Thank god it turned out to be just a strong reaction to poison oak exposure (the West Coast equivalent of poison ivy, (both Toxicodendron spp), and after a week of treatment with steroids, I was better. But that was only the first time of many serious rashes for me, or “allergic contact dermatitis,” in public health speak.

Given that this weed is a risk to public health it’s not surprising that a group of researchers have been busy the last few years trying to figure out how climate change is going to affect it. And as shown in the graphic below, poison ivy has a particularly strong and positive response to increased CO2 in the atmosphere.

Difference in poison ivy growth between recent conditions and a CO2-enriched environment.
Credit: Adapted from Mohan et al., 2006.

The graphic is an illustration of research by University of Georgia assistant professor Jacqueline Mohan, which was published in the Proceedings of the National Academy of Sciences in 2006. She worked in an experimental research facility at the Duke University Forest in North Carolina. This experiment is one of a number of free-air CO2 enrichment (FACE) experiments in the United States, which have been set up to help researchers study the response of ecosystems to increased CO2 environments.

FACE experiments are a cool hybrid kind of a system. Plants are grown in the “free-air” rather than in the laboratory, so their behavior is more realistic than it would be in an indoor laboratory, but researchers can still manipulate CO2 very carefully. At the Duke site, large plots were set up with pumps to add extra CO2 to the air.

To understand the impacts of more CO2, some plots are not enriched while others are exposed to extra CO2, so that the levels in the surrounding air are equivalent to the CO2 concentration that is expected to exist around 2050, given current emission rates. Mohan and her colleagues compared the size of plants grown in these different plots and found that poison ivy grown in elevated CO2 conditions were 149 percent larger than plants grown in today’s CO2 conditions.

Mohan’s research also showed that the response of poison ivy was much greater than tree species in the Duke Forest, supporting a theory that vines, like poison ivy, may be big winners in our CO2 enriched world. That is because vines, which basically use other plants for support, unlike a tree that has to grow a big trunk, can put more of their energy into growing leaves. And more leaves mean more photosynthesis and more growth.

The Duke University Forest in North Carolina, site of free-air CO2 enrichment (FACE) experiments.

This group did additional studies of CO2 and poison ivy in the laboratory, lead by Dr. Lewis Ziska, a plant physiologist with the U.S. Department of Agriculture. This work was published in 2007 in the journal Weed Science, and showed that not only was the plant bigger when grown in more CO2, but it had more leaves, and higher concentrations of urushiol — the toxic chemical that causes allergic contact dermatitis.

In other words, the stuff that causes the unbearably itchy, oozing rash to which I, along with two-thirds of Americans, are prone (if you’ve been lucky enough to avoid this, I invite you to type “poison ivy rash” into Google images and enjoy!).

However, just because a plant does well in an experiment doesn’t mean it does the same thing in the messy real world. Associate Professor Stefan Schnitzer from the University of Wisconsin, Milwaukee, and a group of colleagues, argue in an article published in the journal Ecology in 2008 that one needs to be careful extrapolating Mohan’s result. They looked at the natural abundance of poison ivy and other vines in forests in Wisconsin over 45 years (1959-2005), and found either no change or a decrease in vine abundance, despite a 40 percent increase in atmospheric CO2 over the last 150 years.

The difference between observations and experimental results in this case might be related to local climate factors or light availability, but most likely reflect a difference in herbivore populations. Deer love poison ivy. If you exclude deer from an area, poison ivy typically increases. If you increase deer, poison ivy is likely to get gobbled up. In Wisconsin, Schnitzer reports deer increased two-to-five-fold in the last half century, from 52 to 129 deer per square kilometer in the late 1960s to 259 deer per square kilometer in 2004. In the Duke Forest, deer were excluded from Mohan’s study.

To fully understand how natural systems are changing and what the future might look like, we need more observational studies that track how plants like poison ivy are changing as the climate warms and CO2 concentrations increase and other factors change as well, like deer populations and surrounding land-use.

In the meantime though, the experimental work from the Duke Forest and others like it provides clear evidence about the mechanism that could allow poison ivy to respond very rapidly to increased CO2.