News•March 11, 2015

CO2 Boosts Trees, But Ups Damage From Forest Pests

Greenhouse gas pollution often gives forests helpful growing boosts, but burgeoning bounties of carbon dioxide-fertilized canopies can also whet the appetites of leaf-munching bugs.

Scientists warn that damage inflicted by tiny forest pests could worsen as carbon dioxide levels rise. Early research suggests that attacks by caterpillars, beetles, termites and other insects in some forests could be enough to cancel out projected increases in tree growth rates.

Wisconsin woodlands contain birch and aspen trees.
Credit: Wisconsin Department of Natural Resources/Flickr

Forests are critical sponges for sucking carbon dioxide out of the air, and storing the carbon in their trunks and soils — keeping it out of the atmosphere and combating climate change — and the findings from a three-year experiment in northern Wisconsin warn of an emerging global warming feedback riding in the guts of countless creepy crawlies.

“Models that predict how much carbon storage will be in forests in the future under elevated levels of carbon dioxide typically do not include background levels of canopy damage by insects,” John Couture, a scientist in the University of Wisconsin at Madison’s forestry and ecology department, said. “They do include potential outbreaks but, generally, background levels that are seen every year are overlooked and considered negligible.”

To help fill in gaps in science’s understanding of how leaf-eating insects could respond to changing greenhouse gas levels, Couture and three of his colleagues tracked everything from leaf damage and indicators of forest growth to insect droppings in trembling aspen and paper birch stands exposed to different levels of air pollution. Carbon dioxide and ozone levels were controlled at a long-running outdoor experiment site called the Aspen FACE Experiment.

The researchers reported last week in Nature Plants that canopy damage was markedly higher in stands that were exposed to levels of carbon dioxide pollution expected to be widespread by 2050.

“Atmospheric change can influence tree foliar quality, and we know that has a strong influence on the way herbivorous insects act,” Couture said. “In elevated carbon dioxide environments, we saw there was a substantial increase in damage rates. It was variable across years, but the impact of damage on the forests was pronounced. It was not negligible.”

A caterpillar in Wisconsin.
Credit: Kelly Sikkema/Flickr

In some cases, the observed negative effects of insects on trees more than doubled after carbon dioxide levels were ratcheted up, reducing the ability of trees to pack carbon onto their wiry frames. Couture said the reason is likely based on a combination of factors, all of which require further investigation. He said it might be a response by the insects to falling nutrient concentrations in faster-growing trees. Or it might be related to changes affecting insect predators and parasites. Or, perhaps the insects just bred faster. Further studies are also needed to determine how other types of forests will be affected by rising carbon dioxide levels, he said.

The study was described as “intriguing, and maybe concerning” by University of Western Sydney ecology lecturer Scott Johnson. He said the research “illustrates the potential for insect herbivores to moderate expected responses of plants to atmospheric change, but on a scale that has not been previously appreciated.”

Johnson wasn’t involved with the new study, but he is another pioneer of studies that investigate how insect pests could be affected by rising carbon dioxide levels. In late 2013, he reported in PLOS One that soil-dwelling insects had inflicted worsening damage on experimental Eucalypt seedlings grown in glass houses as carbon dioxide levels increased. This severely cut into the benefits of the increased pollution on tree growth. In some cases, the negative and positive effects of the elevated levels of carbon dioxide almost canceled each other out.

“Stimulation of growth and classical responses to elevated carbon dioxide simply disappeared when herbivores were thrown in the mix,” Johnson said.