Forest for the vines: Climbing lianas could squeeze tropical forest carbon sponges

Lianas vines climbing a tree (Credit: S.A. Schnitzer)

Scientists who can’t see the tropical forest for the trees ought to pay better attention to the vines, says Stefan Schnitzer, a Smithsonian research associate and professor at the University of Milwaukee

“Most people focus on trees and the role of trees in tropical forest ecology, and that’s a good place to start because there are a lot of trees and they’re really big,” Schnitzer said. After all, trees account for about 95 percent of carbon capture in tropical forests — and tropical forests hold a third of all carbon on Earth.

But trees are only part of the equation, according to Schnitzer. A type of woody vine called the liana weaves its way up tree trunks and through the canopies of tropical forests across the world. Lianas occupy a precarious space in tropical ecosystems. On one hand, they provide an aerial highway by which canopy critters can travel and forage without exposing themselves to predators. But on the other, lianas restrict tree growth and can cause a domino effect when one tree falls.

A new study in the journal Ecology demonstrates the effects of lianas on tropical forests’ abilities to capture carbon. Schnitzer, the study’s lead author, took to the forests of Panama to learn how lianas influence tree biomass, and what that means for global efforts to reduce carbon emissions.

In 1999, Schnitzer and a team of researchers walked beneath the hoary boughs of the Panama rainforest. Lianas, which make up one-third of Panama’s woody species, are tenacious plants, and start to regrow almost immediately after being cut. Armed with machetes, the team marked off plots of land and hacked away lianas in half of them — again, and again, and again.

“We knew what we were getting into,” Schnitzer said. “It was a lot of work, but it went really smoothly.”

Liana vine climbing a tree (Credit: S.A. Schnitzer)

Over the next eight years, the team measured tree growth, recruitment and survival rates. They returned to take measurements annually for the next few years, then slowing to return every three years. The study ended when changes in canopy composition placed liana management beyond the team’s control.

The study found that although lianas don’t hold much carbon, they prevent tree growth and increase tree mortality. Big trees, Schnitzer explained, absorb more carbon than small trees. Because trees continuously grow until their death, older trees play the most important roles in carbon mitigation.

Furthermore, the study disproved a common belief among ecologists regarding carbon transfer.

“Most people think this is a zero-sum game,” Schnitzer said, referring to the concept that all displaced carbon must eventually return to the environment. “But these lianas can compensate only about a quarter of what they displace in trees.”

The findings raised concerns about the future of tropical forests, and their role as “carbon sponges.”

“There are some important applied components of this work, because over the past two or three decades, we’ve found that lianas are increasing in abundance relative to trees,” Schnitzer said. “That’s going to affect large scale processes in forests.”

Liana vine (Credit: S.A. Schnitzer)

But Schnitzer isn’t about to lead a machete-armed mob against lianas. He fears that clearing out lianas en masse could have unintended consequences.

“I don’t think we have the data to advocate large-scale liana removal, Schnitzer said. “However, if your forest has a lot of disturbance, you may want to cut some of the lianas back because they can literally take over a forest.”

Schnitzer and the Smithsonian Tropical Research Institution are involved in a number of other studies focused on lianas and tropical forest carbon mitigation. Located in Panama, the Institution is dedicated to the study of biological diversity and provides research facilities for long-term ecological studies in the tropics.

“These forests are so important for lots of things,” Schnitzer said. “They house over 50 percent of all known animals, they have a huge role in storing carbon.”

“They’re not just important for the local countries, but for the whole planet.”