On a hot, buggy morning in January, Stefan Schnitzer, a Marquette University biologist, spritzed insect repellant on his forearms, tucked khakis into knee-high rubber boots, and hiked three miles from his lab at The Smithsonian Tropical Research Institute to a study plot in the Panamanian jungle. Ticks and, on occasion, dengue fever-carrying mosquitos infest Schnitzer's parcel, on Barro Colorado Island in Panama's Lake Gatun. The site also harbors bullet ants that inflict one of the world's most painful stings, and a host of palm-size golden orb spiders. Schnitzer picked up a PVC pipe the length of a walking stick for swatting spider webs strung across his route.
Schnitzer strode nimbly up a steep slope, where workers had carved steps into the hillside and laid cement pavers to ease access to the forest. The path leveled, the stepping-stones petered out, and the track narrowed. A howler monkey in the distance barked a warning to would-be intruders.
Reaching his study site after two hours of hiking, Schnitzer, one of the world's leading experts on the ecology of rainforest vines, paused and studied a tangle of knotted liana vines six feet high and twice as wide, composed mostly of loops of the species Coccoloba excelsa. These are the most abundant lianas on the island, and they promiscuously sprout shoots that root in the soil, sending up a profusion of new stems. Dozens of dancing blaze-orange streamers flagged the thicket's coils.
This was all part of Schnitzer's research into a little-understood but important phenomenon that could impact the pace of climate change: In the jungles of Central and South America, vines are becoming more common, and as they proliferate, they are impeding the ability of tropical forests to soak up carbon dioxide and sequester it as wood.
For reasons that are not entirely clear, the abundance of liana vines has doubled in recent decades, according to research by Schnitzer and an earlier study. As a result, these rapidly expanding woody vines are increasingly shading and choking rainforest trees, reducing the amount of carbon they sequester from the atmosphere. In his jungle study plot, Schnitzer is endeavoring to understand why vines proliferate faster than the trees that abut them, and how much carbon liana vines store compared to trees. Trees and other plants absorb about 25 percent of the CO2 that humans release from tailpipes and smokestacks.
The Smithsonian has managed the Barro Colorado forest, halfway across the Isthmus of Panama, since the 1920s. The tropical jungle there is among the most intensively studied rainforests on earth. In 1980, two ecology pioneers, Stephen Hubbell and Robin Foster, laid out what was then the world's most ambitious tropical forest plot for intensive, long-term research. It contains roughly 125 acres and includes about 250,000 trees, each marked with a numbered aluminum tag. Several thousand are too broad for a pair of adults holding hands to reach around and touch. With a team of helpers, Hubbell and Foster have measured the diameter of each tree. They re-census the site every five years, adding "recruits" — trees that have grown big enough to be included in their study — and removing trees that have fallen.
For years, Hubble and Robbins did not bother surveying lianas. "Lianas are such a small part of what you see when you look around," says Schnitzer, noting that they contain less than 5 percent of a tropical forest's standing carbon.
But ecologists have begun to wonder if lianas might play an outsized role in the forest. Schnitzer is particularly interested in how vines colonize gaps in the canopy left by tree falls. In 2007, he and Hubbell, by then his mentor, began a liana census of the Barro Colorado plot. It became, and remains, the largest survey of lianas in a single tract. It took 14 people working full-time for 12 months to tag, measure, and record 65,000 vines. A botanist identified each specimen, encompassing 162 species.
Foresters have traditionally calculated the number of planks they can obtain from trees using allometry, the science of how an object's dimensions, such as linear size and cubic volume, relate to each other. Biologists adopted the same technique for calculating the carbon stored in trees and lianas. The process is straightforward for trees. Researchers plug a single measurement — by convention, the trunk diameter at roughly four feet above the ground — into a simple formula worked out through years of research.
But quantifying the volume of carbon stored in liana vines is more challenging. In his study plot, Schnitzer approached a tree-sized growth with waxy leaves that would pass as a tree for all but trained experts. It fact it was a sapling-like vine, Connarus panamensis. "It grows like a tree until it's quite tall," before grabbing branches and trunks for support as a normal vine does, Schnitzer says. He notes that unlike trees, which tend to grow straight up into the canopy, lianas follow a more erratic and looping path around the understory. Such factors, along with the relative paucity of studies of liana measurements, have compounded the difficulty of vine research and carbon storage. Nevertheless, Schnitzer has developed protocols — now widely adopted — for measuring lianas.
In 2015, Schnitzer published a paper in the Proceedings of the National Academy of Sciences revealing new evidence that vines could reduce the tropical forest's ability to absorb CO2 and slow global warming. Wielding machetes, he and a crew of five had completely removed every liana from eight forest tracts on a spit of land near Barro Colorado. It took a week. They tallied up how much carbon was contained in wood and leaves in the vine-free jungle and in a corresponding set of normal, viney areas.
Schnitzer had predicted that, freed from shading and strangling vines, trees would grow more vigorously. Because trees support their crowns with sturdy, carbon-rich trunks that lianas don't need, Schnitzer hypothesized that the vine-free jungle might contain far more carbon than the control forest. After three years, a re-census confirmed that. The pruned tracts had absorbed 75 percent more carbon per year than the control areas where lianas and other vines were allowed to grow freely. "It was stunning," says Schnitzer.
In a response to Schnitzer's paper reporting these findings, Hans Verbeeck, a biologist at Ghent University in Belgium, wrote in a subsequent issue of the Proceedings of the National Academy of Sciences that that "liana proliferation has a potential high impact on the future carbon cycle of tropical forests." He called for climate researchers to include the changing prevalence of lianas in future climate models, a task he has begun himself.
Despite solid evidence that vines have become more common, Schnitzer says he's unsure if vine proliferation will continue to increase in the world's tropical forests. A forecast would require an explanation — so far lacking — for what's behind the increase in vines that he and others have observed. Schnitzer suspects that one culprit might be increased treefall frequency and mortality due to such changes as enhanced storm intensity. Some scientists believe that the increased CO2 in the atmosphere might favor lianas over trees. But at least one study, by David Marvin, an ecologist at The Nature Conservancy and a former student of Schnitzer's, suggests otherwise. Marvin raised tree and liana seedlings together in chambers with air containing twice the normal concentration of CO2. Lianas grew faster than controls — but so did trees. There was virtually no difference. However, the experiment tested only a small number of species and lasted less than a year. A longer test, with a larger group of varieties, might have turned out differently.
Schnitzer's 125-acre plot contains so many lianas that, after the re-census is completed, he should be able to determine which particular species are most responsible for the observed increase in liana biomass. Then, he plans to investigate what characteristics give these vines their advantage. But even with his experiments, he says, making such determinations is difficult. In tropical forests, the huge diversity of species and the boundless variation in how they interact can frustrate even the most robust scientific experiments.
What if his study comes up empty-handed?
"We'll have the most rigorous non-finding in ecological history," he replies.