Rising carbon levels feed these weeds

Times Staff Writer

Rapidly rising carbon dioxide levels in the atmosphere are driving noxious poison ivy and those annoying patches of dandelion to grow taller, lusher and more resilient, according to two new studies.

Poison ivy growing in lab chambers set to present carbon dioxide conditions swelled to twice the size of samples grown under conditions from the 1950s, according to a study published in the current issue of the journal Weed Science.

In an unrelated experiment, dandelions in carbon dioxide conditions elevated to projected 2100 levels produced 32% more seed-carrying fruit and longer white pappus hairs, which improve seeds’ ability to float with the wind.

“These papers ... suggest many weedy species are going to respond strongly to elevated carbon dioxide,” said Jeff Dukes, an ecologist at the University of Massachusetts Boston, who was not involved in the studies.


Carbon dioxide levels have been on the rise since the dawn of the Industrial Revolution more than 200 years ago. Over that period, atmospheric levels have risen from 280 parts per million to about 380 parts per million, the highest point in 650,000 years.

Plants are acutely sensitive to carbon dioxide because they use it for food, converting it into carbohydrates, lipids and other compounds during photosynthesis.

A previous study of poison ivy found that the plants grew faster and larger and produced more of the skin irritant urushiol when an additional 200 parts per million of carbon dioxide was piped into a forest near Duke University.

Lewis Ziska, a U.S. Department of Agriculture weed ecologist based in Beltsville, Md., and colleagues wanted to refine the Duke findings by testing poison ivy in sealed aluminum chambers.

They set carbon dioxide levels in various chambers at 300, 400, 500 and 600 parts per million. Those levels corresponded to concentrations from the 1950s, the present, 2050 and 2090. Temperature and light conditions were the same in all chambers.

After eight months, the researchers found the stems and leaves dramatically increased in mass with more carbon dioxide, though an increase in the concentrations of urushiol was deemed statistically insignificant.

The rate of growth diminished as carbon dioxide rose above 400 parts per million.

“The take-home message is the change in carbon dioxide that has already occurred in the 20th century was enough to significantly stimulate the growth of poison ivy, much more than originally anticipated,” Ziska said.

When scientists plucked off leaves to simulate animals feeding, the plants under current carbon dioxide conditions grew back their leaves twice as fast as those at 1950 conditions. Plants growing at estimated 2090 carbon dioxide levels grew back at triple the rate.

Previous studies have shown that plants increased their mass an average of about 30% with the rise in carbon dioxide over the 20th century.

Ziska’s study showed that poison ivy did much better, increasing its mass by about 105% given a similar increase in carbon dioxide concentrations.

This disproportionate growth favors the success of poison ivy in the future over more preferable plants, such as wheat or shade trees.

“From what we’ve seen so far, all the weeds we have examined seem to respond much more to carbon dioxide than other species do,” Ziska said.

The idea was supported by plant ecologist Xianzhong Wang’s experiment on dandelions, also reported in the current issue of Weed Science.

“In the future, we think humans will have to spray lawns more often or they may need to increase the dosage of the herbicides to kill the dandelions,” said Wang, of Indiana University-Purdue University Indianapolis.

More dandelions, he added, could also increase the discomfort of seasonal allergy sufferers.