Insects may undermine trees’ ability to store carbon

Trees are often promoted as an important tool for combating climate change. That’s because trees take in carbon dioxide from the air and lock it away in wood and soil for years. But trees may not be as great of carbon sinks as we thought, a new study finds. Why? Blame hungry insects.

This revelation comes from a tree-growing experiment in Wisconsin. In the mid-1990s, scientists planted groves of aspen and birch trees. They then set up 12 rings of PVC pipes that vented gases into those dozen groves. Some tree patches got extra carbon dioxide, reaching levels of about 560 parts per million — about what might be expected in 2050 or sooner. Others were exposed to ozone levels about 50 percent higher than normal.

Then from 2006 to 2008, once the trees had grown tall, researchers collected leaves, scraps and insect fecal droppings so that they could measure how tree biomass changed, where nutrients were going and how insects were affecting the system. John Couture of the University of Wisconsin-Madison and colleagues published their findings March 2 in Nature Plants.

On average, stands of trees grown with high carbon dioxide saw 88 percent more damage to the tree canopy from hungry insects than those grown without the extra CO₂. As a result of the increase in insect feeding, trees in the higher-carbon forest stored about 70 grams less carbon per square meter than trees grown in open air, the team found.

“Insects have the potential to ‘level the playing field’ for forest stands under future atmospheric CO₂ conditions, disproportionately limiting stands that would grow rapidly under high CO₂ environments,” the researchers write.

The insects probably eat more because when grown under high CO₂, trees produce less nutritious leaves. Insects would have to eat more leaves to compensate for the lower nutrition.

However, the ultimate ability of forests to store our extra carbon will depend on more factors than simply carbon dioxide levels and hungry insects. And those other factors may change the equation. In the forest experiment, for instance, increased ozone resulted in an average 16 percent decrease in damage. And insects — and their droppings — move nutrients around the food web, further complicating things.

Trees may still be the easiest way to tackle our carbon problem, but it remains unknown how much — or how little — help they’ll be able to provide.