Greenhouse gas breeds venomous vines
Mounting carbon dioxide could fuel more poisonous ivy.
Forests could become thick with more toxic forms of poisonous ivy and other noxious vines, thanks to rising levels of carbon dioxide.
That's the conclusion from researchers in the United States who have shown that the higher CO2 levels expected in the next 50 years breed ivies that grow twice as fast, and, unexpectedly, manufacture a nastier form of poison. "It'll be more dangerous to go in the forest," says team leader Jacqueline Mohan of the Marine Biological Laboratory at Woods Hole, Massachusetts.
Poison ivy (Toxicodendron radicans), which grows as a shrub or tree-climbing vine, is already the scourge of gardeners and hikers in North America for the excruciating skin rash it can trigger. The plant makes a fatty toxin called urushiol in its leaves.
In the study, Mohan and her co-workers pumped extra CO2 over three large circular plots of North Carolina pine forest. For six years, the plants inside were exposed to an extra 200 parts per million of CO2 over today's atmospheric concentration of about 380 parts per million, roughly what we might expect from pollution by the middle of this century.
Other research has suggested that vines tend to grow particularly fast in response to higher CO2 levels, and that vines are increasing in abundance all over the planet. Unlike trees, which use extra carbon to grow more wood, vines use it to produce more leaves. The extra leaves help the plant to harvest even more CO2, the cycle continues and the vines flourish.
Mohan's experiment sought to check whether the plants shoot up in the wild, as they do in greenhouse experiments. "Yes, dramatically," was the answer. The poisonous ivies grew at double the rate of plants grown under regular CO2 levels, whereas woody species on average tend to grow around 31% faster. The elevated CO2 also created a nastier version of urushiol poison, the team showed.
Urushiol is made up of several different varieties of fat. A less poisonous variety of these fats is 'saturated', meaning that the molecule's carbon atoms only have single bonds to other carbon atoms, and the rest of their bonds are saturated with hydrogen. But most of the fats in urushiol are unsaturated, containing more than one chemical bond between carbons and less hydrogen. These are thought to be the most irritating to the skin.
By extracting urushiol from the plant's leaves, the researchers found that poison ivy grown in high CO2 churned out more than 150% more of one nasty, unsaturated form of urushiol and around 60% less of the mild, saturated form.
The researchers aren't sure why this chemical shift took place, but one idea is that the increased availability of carbon somehow favours the chemical reactions that produce the unsaturated forms of urushiol. Their results are published in the Proceedings of the National Academy of Sciences1.
Poison ivy causes an estimated 350,000 reported cases of skin rashes each year in the United States. Around 80% of people react to the toxin, and their reaction tends to get worse the more they are exposed to it. "I have colleagues who are so allergic that their dermatologists tell them they have to change professions," Mohan says.
Rising CO2 could also fuel growth of other noxious vines in the Toxicodendron family that occur around the world, Mohan says. "Forests in future might be dominated by different species than they are today."
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- Mohan J.E., et al. Proc. Natl Acad. Sci. USA , doi:10.1073/pnas.0602392103 (2006).
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