Tropical forests store nearly a third of the planet’s aboveground, terrestrial carbon. But according to new findings published in Proceedings of the National Academy of Sciences this week, increasingly abundant woody vines called lianas could reduce forest-wide carbon uptake by slowing tree growth and causing early tree death.

Lianas depend on trees for support as they make their way upwards into the sunlit treetops, and previous work revealed that these long, climbing plants have a major negative effect on the growth and accumulation of biomass in forests. Now, to quantify the effect lianas have on a forest’s ability to store carbon, a team led by Marquette University’s Geertje van der Heijden compared carbon storage in forest plots that have been cleared of lianas with plots that haven’t been cleared. 

In March of 2011, the team cut all of the lianas at their base in eight experimental plots within the 60-year-old secondary forests of Barro Colorado Nature Monument in Panama; lianas were left intact in eight other plots. Over the next three years, the researchers measured the diameters of both trees and lianas, and they collected dead leaves, bark and other debris that fall to the forest floor.

After three years, the plots with lianas accumulated 76% less biomass than plots without them. With the vines around, not only was tree growth lower, tree mortality was also higher.

Furthermore, the team found that lianas shifted the relative productivity of leaves and woody stems: Compared with liana-free plots, those with lianas stored more carbon in leaves and less carbon in wood. But woody stems store carbon for a long time, while leaves rot quickly – releasing carbon back into the atmosphere. Based on the team's simulation of biomass changes over the next 50 years, lianas have the ability to reduce long-term carbon storage in forests by about 35%. 

Still, lianas are an important part of tropical forests. "In terms of carbon, lianas may be detrimental," study coauthor Stefan Schnitzer of Marquette University said in a statement. "However, lianas provide a wide range of resources for wildlife, such as fruits, seeds and fresh leaves, and by connecting trees together lianas provide aerial pathways that are used by the vast majority of arboreal animals to move through the forest."

Image in the text: Stefan Schnitzer and lianas in Panama's Barro Colorado Island on January 15, 2014. Sean Mattson/STRI