By Kurt Chandler
Ecologist Stefan Schnitzer first heard about liana vines while working on his doctorate in 1996. The spindly, fast-growing vines were rapidly infesting the planet’s tropical rain forests, he learned, robbing trees of water and nutrients at their roots, blocking sunlight at their crowns and causing them to topple.
Research on the woody vines had been limited, going back only to the mid-1980s. Intrigued by lianas and the ecology of tropical forests, Schnitzer soon found himself on a remote island in Panama, immersed in research. As part of a small team of scientists, Schnitzer tested conventional theories about the lianas’ role in species diversity and forest regeneration.
His fieldwork evolved into leading-edge experiments in Panama. By 2015, just after becoming Mellon Distinguished Professor of Biology at Marquette after a dozen years at the University of Wisconsin–Milwaukee, he had published studies that broke new ground in understanding how lianas cut into the carbon-reducing capacity of a rain-forest’s canopy. The implications drew a straight line to climate change, garnering headlines in The New York Times and Smithsonian, and drawing major grant funding.
“The reason why this is a big problem is that the capacity of tropical forests to continue taking up carbon would change dramatically if vines increased in abundance relative to trees,” he says. “All the data suggest an increase.”
Last May, Schnitzer received a four-year $900,000 continuing grant from the National Science Foundation that will allow him to maintain a team of graduate and postdoctoral students in Panama and at Marquette, who will help collect and analyze data on the relationship between woody vines and tropical forests. When he’s not teaching or writing grant proposals, Schnitzer spends a third of the year on Barro Colorado Island in the Panama Canal waterway, his home away from home and living laboratory for the past 20 years. The fieldwork is his favorite part of the job.
Going back 15 years, Schnitzer’s early studies of the island’s ecosystem focused on the relationship between increasingly abundant lianas, the replacement of trees that die and fall to the ground, and the rain forest’s famed but perhaps threatened reputation for harboring diverse plant species.
Previous studies on the island had found that liana infestation had increased from 32 percent in 1968 to 47 percent in 1979. By 2007 infestation had grown to nearly 75 percent.
The liana blight had wide-reaching ramifications. As trees died and fell, the vines seemed to crowd out and stifle tree regeneration by rapidly dominating the resulting gaps in the forest. But because the thin vines had a much smaller biomass than the trees they replaced, he suspected they would far from compensate for the trees’ lost carbon uptake.
To test the hypothesis, in 2008 Schnitzer and his team began staking out several lush test sites in the rain forest, measuring and identifying all rooted plants, nearly 50,000 in all, to establish a comprehensive database. On another site, the team sectioned off 16 plots of equal size. On eight of the plots, lianas were cut and removed, from the ground to the canopy. On the remaining eight plots, lianas remained intact. The 16 plots were surveyed in 2011 after the liana removal, and again in 2014.
For three years, researchers measured the growth of the lianas and trees. On plots where lianas were absent, trees thrived, growing in treefall gaps. On plots where lianas remained, researchers collected and weighed the leaves and branch litter that fell from the canopy. By the third year, the forest biomass had been reduced by 76 percent per year on plots where lianas were present compared to where lianas had been removed.
In other words, “you have about 76 percent more carbon-capture per year when lianas are absent,” says Schnitzer, who notes the confirmation of similar infestations in tropical forests and subtropical forests in South and North America. “This carbon figure is alarming. If lianas increase even by a little bit, they’re going to lower the capacity of tropical forests to uptake carbon. That’s the pattern we see.”
Armed with the NSF grant, new questions will be explored as Schnitzer’s research moves forward. This year, another full survey of the vines and trees will begin. Of the various vine species that have increased in mass and number, scientists will look for a shared set of traits that signal an advantage. They will then grow these hardy vine species and subject them to various tests — exposing them to drought conditions (which could be a factor in the liana’s rise) or high levels of carbon dioxide or nitrogen — to see precisely what might trigger the advantage.
Schnitzer frequently labels himself an ecologist, a scientist who studies the diversity, distribution, biomass and population of organisms, as well as competition and cooperation. Through that lens, he hesitates to offer solutions to the liana problem. Removing them is not feasible, nor sensible. Vines connect the canopy and allow arboreal animals to travel through the forest. The fruit, flowers and leaves provide food to certain species. “We would not know what the effects of losing the vines across large areas in the forest would be.”
Science eventually will answer the question of how to manage the infestation. Meanwhile, “I’m just here to figure out what’s happening,” Schnitzer says, at once concerned and fascinated by the liana vine.