Elevated concentrations of carbon dioxide in the atmosphere increase the loss of biomass due to plant-eating insects in northern temperate forests, reports a study published online this week in Nature Plants. The findings suggest that herbivorous insects could limit the capacity of these forests to function as carbon sinks in a high-CO2 world.

High levels of CO2 in the atmosphere are expected to promote the productivity of forests in some regions, so that they take up more carbon from the atmosphere. However, many factors can impair the ability of forests to act as carbon sinks, such as ozone in the troposphere and nutrient limitations.

John Couture and colleagues monitored levels of canopy damage in aspen and birch stands exposed to elevated concentrations of carbon dioxide and ozone, at the Aspen FACE facility in northern Wisconsin. Levels of insect-mediated canopy damage were markedly higher in the elevated-CO2 stands, but moderately reduced in the elevated-ozone stands.Using a model they estimate that herbivore-induced reductions in forest productivity more than doubled under elevated concentrations of CO2, but fell under elevated concentrations of ozone.