University of Wisconsin-Milwaukee professor Stefan Schnitzer, a research associate at the Smithsonian Tropical Research Institute, is seen among the woody vines he studies in the Barro Colorado Nature Monument, Panama. Credit: Marcos Guerra / Smithsonian Tropical Research Institute

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Stefan Schnitzer wields an unusual instrument for a scientist — a machete.

Schnitzer, of the School of Freshwater Sciences at the University of Wisconsin-Milwaukee and the Smithsonian Tropical Research Institute, studies woody vines in the tropical forests of Panama — work that has him traipsing through the forest and hacking away at vines to study the impact of their growth on the trees around them.

The results of his research could have implications for global climate change. The vines, Schnitzer discovered, reduce the carbon intake of the forests by preventing the trees from growing, allowing more carbon to remain in the atmosphere where it can wreak havoc on the environment.

Schnitzer joined the faculty of UWM in 2003 after obtaining his doctorate from the University of Pittsburgh and a stint at the University of Minnesota. His research group includes three UWM graduate students, two postdoctoral researchers and several UWM undergraduate students. In 2006, he became a research associate with the Smithsonian Tropical Research Institute, a unit of the Smithsonian Institution that has its headquarters in Panama City, Panama.

The vines Schnitzer studies, known as lianas, have woody, climbing stems and put out lots of leaves — think grape vines, but beefier. They climb trees to reach the top of the forest, where they spread out their leaves to soak up the sun.

"They're really abundant, and they're really diverse," Schnitzer said.

Despite this, research on the vines has long been neglected in favor of the more charismatic trees. The vines, however, can have a big impact on the growth of tropical forests, Schnitzer reports in the journal Ecology.

Lianas compete with the trees and slow their growth. They monopolize valuable resources, such as water, nutrients and sunlight, and piggyback on the trees' structure instead of creating their own, making them "almost like a structural parasite," Schnitzer said.

'A huge sponge'

Tropical forests are an important factor in understanding the carbon cycle on the planet because they contain more than a third of the planet's terrestrial carbon and are constantly using carbon and releasing it as they grow and die.

"Tropical forests are like a huge sponge," Schnitzer said. "They kind of soak up the carbon and then rerelease it slowly."

Much of the growth in tropical forests occurs in places where trees have recently fallen. Tropical forests are dominated by trees that tower up to 44 yards high, with leafy canopies that capture nearly all of the sun's rays, making light at the forest floor a scant resource. When a tree dies, it opens up a hole in the forest canopy through which light can reach the forest floor. New trees spring up quickly to take advantage of the light.

But trees aren't the only plants trying to take advantage of the new space — lianas also grow in abundance. Schnitzer wanted to see what would happen if the vines were removed from the equation.

For eight years, Schnitzer and his team cut away vines in forests in the Isla Barro Colorado Nature Monument in Panama, in areas where trees had recently fallen. When they measured the impact on the nearby trees, they found that trees in areas where the vines were removed had nearly three times as much growth as those in areas where the vines were allowed to grow naturally.

The lianas, however, didn't make up for the trees that they replaced. Because lianas stems aren't as dense as trees, the lianas added only a quarter as much mass as the trees lost due to competition with the lianas — an important quantity because mass is directly related to the amount of carbon stored in the forest.

"Most people were thinking that this is a zero-sum game," Schnitzer said, but his research shows that it's not.

Schnitzer's method is significant because such experimental studies are rare in ecology, said Saara DeWalt, associate professor at Clemson University, who was not involved in the research.

"Lots of people talk about competition between different species or groups of plants, but not many people actually experimentally test the hypothesis that there is competition," she said.

Less tree growth due to competition with vines translates to less carbon storage, and therefore less carbon removed from the atmosphere, where it can contribute to climate change. When averaged over the whole forest — assuming that about 2% of trees fall every year — the scientists estimate 18% less growth in forests where lianas are present.

"That's a lot, considering the contribution of these forests to the global carbon balance," Schnitzer said.

Francis Putz, professor at the University of Florida, calls the study "an important piece of work," particularly in light of logging of tropical forests. Many of the forests outside of protected areas are maintained as forests because they can use used for timber, which is economically lucrative. If lianas prevent the growth of new trees, he said, "it may promote deforestation by reducing the value of the forest."

'Something is changing'

Worryingly, scientists also have found that vines have been increasing in abundance over the past several decades. Schnitzer's research implies that this change could reduce how much carbon the forests can store, and may therefore play a role in climate change.

The reason for the increase in vines is still undetermined, but possible factors could be decreased precipitation in the tropics, increased carbon dioxide concentration in the atmosphere or increasing numbers of tree falls in the forest.

If the vine population increase is a result of climate change, the process could produce a feedback loop that would enhance the impact of climate change.

"Something is changing in these forests," Schnitzer said.

Understanding that change will be an important step toward untangling the effects of climate change.