31 May 2006

Poison ivy is sexy

Research linking climate change with poison ivy isn't the only newsworthy story to be found in the latest issue of the Proceedings of the National Academy of Sciences, but it's easy to understand why it's the only one to attract heaps of media attention this past week.

What editor could resist running a story that says global warming will make poison ivy grow faster and nastier? Everybody understands the evils of poison ivy. I've got a bad case of Toxicodendron dermatitis, as it's technically known, right now. Probably picked it from the dog. It's such an easy sell that you could be excused for dismissing the whole thing as just a cynical attempt to attract attention. Kind of like the way environmentalists exploit the plight of polar bears, pandas, and other charismatic megafauna to galvanize support for their latest campaigns. (Not that that's necessarily a bad thing, although it is dishonest to imply that big cute animals are more worthy of saving than worms and other attractive, but ecologically important species.)

Anyway, the good news the poison ivy story is worth your time. I asked the study's lead author, Jacqueline Mohan of Harvard University and the Marine Biological Laboratory in Woods Hole, why she chose poison ivy, and not another plant. Kudzu, perhaps? She replied that poison ivy (Toxicodendron radicans) is "a fascinating species ecologically, chemically, and, unfortunately for us, medically."

Most of the media coverage dealt exclusively with the medical implications: more of us are going to run into the stuff, because it grows better in atmospheres with higher carbon dioxide levels, Fair enough. But I did find one story, a HealthDay News item in Forbes.com of all places, that managed to get at the larger significance of the research.
This is kind of sad news, not only for humans but for forests," Mohan said. "Increased vine abundance inhibits tree regeneration by killing young trees," she added.

One ecological expert thinks the findings are the first to link increased growth and toxicity with rising levels of C02.

"This is a very interesting paper," said Kevin L. Griffin, an associate professor at the Lamont-Doherty Earth Observatory of Columbia University. "The increase in the growth rate with elevated CO2 is very large. Similar rates have been reported for potted plants in short-term experiments, but for these to be maintained in the field with natural environmental variation is really quite surprising."

I say Mohan and her colleagues are making an important contribution to our understanding of the just how unpredictable the ecological effects of a warming planet are going to be. They are not alone. The current edition of SEED magazine includes a frightening feature on researchers in Costa Rica who are finding the opposite effect of more CO2. Down there it could actually slow the growth of trees in the rainforest, and eventually cause them to emit more CO2 as they decay.

This sort of positive feedback in the carbon cycle is exactly what we don't want to happen. CO2 is now 380 parts per million -- 35 percent higher than it was 150 years ago -- and it will almost certainly reach 500 ppm this century. Trying to figure out the impact of doubled CO2 is vital. The story so far is anything but clear, however.

We could see more of examples of positive feedback in addition to whatever happens to tropical rainforests. For one thing, melting permafrost could release vast quantities of methane, which is 20 times as effective at trapping heat as CO2. One of the overlooked PNAS papers just published concludes that oceanic coral reefs "may be more susceptible to climate change" than their continental counterparts -- a distinction that may surprise more than a few marine ecologists. On the other hand, there could be some negative feedback from cloud cover, and even a few good-news developments in the form of lengthened growing seasons here and there.

Anyway, it now looks like woody vines could invade new territory. Noting that other species in the Anacardiaceae family, including mango, cashew, and pistachio, also can be allergenic, Mohan's paper suggests it is "possible that these plants, too, may become more problematic in the future."

Great. Who thought more mangos could be bad thing?

The point is reliable big-picture, ecosystem-level predictions are a long way off. The poison ivy story just reminds us how little we know. And why what lies ahead could just as easily be worse than we expect as anything else. The last word comes the Health Day News story:
"The most worrisome message here is less about this particular plant and more about the whole forest," said Dr. David L. Katz, an associate professor of public health and director of the Prevention Research Center at Yale University School of Medicine. "We are upsetting a balance in ecosystems and that will have far-reaching effects, many of which we are first now beginning to guess," Katz said.

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