Rogues of the Rainforest

Tropical vines are wandering, as they always have, but recent environmental changes are giving them an edge over other rainforest plants—a shift that could have enduring impacts on climates around the globe.

Boris Bernal Vargas often begins work with a 40-minute commute. Starting from his home in the village of Las Pavas, Panama, he walks through a pasture to a wooden dock on Gatun Lake. From there, he and a dozen other workers skim across a couple kilometers of water in a motorboat to Barro Colorado Island—a refuge of tropical forest that in some places has not been logged for hundreds of years. Beneath the canopy, workers follow a short path up a gentle slope, and at last, wade through ankle-deep leaves to a row of PVC pipes driven into the ground.

The forest is chaotic and tangled, seemingly impervious to the human impulse to catalogue and name. But past this row of markers, every tree larger than a person’s pinkie is tagged with a six- or seven-digit number. This 50-hectare swath contains roughly 350,000 trees—about three times the number of hairs on a person’s head. And for 40 years, Bernal Vargas and other researchers have measured the growth of each for the Smithsonian Tropical Research Institute (STRI), based in nearby Panama City.

Trees aren’t the only creatures of interest here. Starting in 2016, Bernal Vargas also investigated a long overlooked kind of plant: lianas, vines with thick, woody stems that climb tropical trees—often spiraling up their trunks—in search of sunlight. Within this plot, every liana above the same minimum pinkie thickness is also tagged. For 23 months, Bernal Vargas spent his days identifying each, gauging the diameter of its stem with calipers or tape measure, and penciling the results on a clipboard. The vines were so tangled that Bernal Vargas could often measure only a few dozen per day—an agonizingly precise, Sisyphean task, punctuated by moments of beauty and irritation: the sight of a golden puma passing silently; the searing stings of inch-long bullet ants that use the vines as highways; the droplets of urine spread by agitated howler monkeys whooping in the branches above.

But with persistence, Bernal Vargas and his coworkers eventually measured every liana on the site. It was the second time they had done this, and it confirmed a mysterious and troubling trend. During the past decade, the number of lianas had increased by 29 percent on Barro Colorado Island, reaching a record 117,100 individuals. Researchers have seen similar increases in dozens of other study plots scattered across the tropical forests of Central and South America.

Lianas seem like footnotes to the forest—scrawny weaklings dominated by the stout-trunked trees all around them. “When I started in the mid-90s, nobody cared about lianas. They’re just these things you trip over and cut with a machete,” says Stefan Schnitzer, an ecologist who employs Bernal Vargas and the other workers measuring these vines.

But viewed from above the canopy, lianas are the ones that dominate. A single vine can spread across dozens of trees, unfurling its leaves like parasols over those that grow more slowly. In the race to grab precious sunlight, lianas are literally outrunning and outmaneuvering trees, says José Medina-Vega, a former postdoc of Schnitzer’s who now works at the Smithsonian Global Earth Observatory Network in Washington, DC. “They have a guerrilla strategy.”

Scientists consider tropical forests critical allies in the struggle against climate change because they have the capacity to absorb and store billions of tons of anthropogenic CO2. But rising CO2 and temperatures are already causing rainforest trees to die younger, and lianas may compound the problem. They “have this really strong negative effect,” says Schnitzer, who has joint appointments at STRI in Panama and Marquette University in Milwaukee. “They prevent trees from taking up as much carbon.” They reduce tree growth. And they can send trees crashing prematurely to the ground.

Scientists have long thought that anthropogenic environmental changes trigger liana expansion: Perhaps rising levels of CO2, or atmospheric deposition of nitrogen and phosphorus have spurred the vines’ growth. But the exact mechanism has remained mysterious for 20 years. Some researchers even wonder whether it may be the misleading result of biases in the way vines are studied. Mounting evidence, though, suggests that the spread of lianas is not only a symptom of the ways that climate change is remaking tropical forests, but also, increasingly, a cause.

The competition among trees and lianas is not a battle between unrelated plants. Oddly, it grew out of an evolutionary struggle between tree species.

Coiling, woody vines, many lianas are actually descended from trees that forsook the Puritanical labor of building sturdy trunks and instead opted to slink up the trunks of other trees. This transformation to “structural parasite” has occurred many times, across a quarter of all plant families, sometimes pitting members of the same family against each other. And it truly is a transformation, because it involves much more than a slimmer trunk.

Because a liana’s stem does not need to support weight, the structure of its wood can maximize a different function: the transport of water from its roots to its leaves. Cutting through a liana stem often reveals a cross section ornately dotted, like the inside of a kiwi fruit. Those dots are water-sucking tubes, called xylem. Because they are wider than those in trees, they produce less friction with the water rising through them, vastly increasing the volume that can flow and allowing much more rapid photosynthesis and growth.

For over a century, researchers largely ignored lianas. But in the past few decades, they have finally begun to realize that lianas are important in their own right—powerful plants that can shred the social fabric of forests. It started in the late 1970s when a young graduate student named Francis Putz arrived for a fellowship at Barro Colorado Island, intent on studying how these resource thieves slither through the canopy, groping their way sightlessly from tree to tree. Using a modified .22 rifle, he would shoot fishing line over a branch 20 to 40 meters above, use that line to pull up a nylon cord, and use the cord to pull up a climbing rope. Donning a harness, he spent hundreds of hours dangling in the canopy—taking occasional afternoon naps—and familiarizing himself with the slow, bare-knuckle battles between climbing lianas and trees that did not want to be climbed.

Putz discovered that lianas are contagious. Once one managed to climb a tree, it sent shoots to grasp neighboring trees—not unlike Putz’s fishing lines. This contagion could spread surprisingly far. One liana, an entada pea vine with spiraling, meter-long pods, had spread across the tops of 49 trees.

Putz also saw how vigorously trees fought to avoid liana infestation. Some defended themselves with leaves and branches that easily peeled away, sending grasping vines crashing to the ground. Palms developed serrated sword leaves that could slice through young vines. Other trees had more violent means. In one frightening episode, Putz was 20 or 30 meters up a stout-trunked tabebuia tree when he saw a squall gathering in the distance. Having read John Muir’s account of riding out a storm high in a sequoia, he figured it was worth a try. It was, he says, “the kind of thing you do when you’re in your 20s.”

The winds arrived with the roar of an oncoming train and the screams of agitated monkeys. And for about 10 minutes, Putz jolted and swung on his ropes as the neighboring tree—a slender zanthoxylum with a thorny trunk and fern-like leaves, connected to his own by three different lianas—whiplashed, yanking his tree this way and that. When the wind subsided, he realized he had wetted himself. He also noticed that all three vines had broken. Putz came to see that the tree’s pliancy was defensive. In addition to breaking lianas, its mosh-pit thrashing stripped branches from neighboring trees—creating a buffer zone that the vines would find difficult to cross.

The stakes were clear: When lianas did manage to tether trees together, one falling individual could pull down the rest, killing them all. The lianas, though, could capitalize on the newly intense sunlight to grow rapidly over the clearing These rapidly-growing vines thrive wherever the sunlight is intense—whether at the top of the canopy, or in a clearing on the ground.

At the time Putz conducted this research, scientists generally assumed that these ecological battles were in equilibrium on a broad scale in forests that had little direct contact with industrial society. If lianas overran one spot, trees often balanced it out by dominating elsewhere. But another scientist’s work soon began to undercut this idea.

In the early 1990s, a PhD student at the Missouri Botanical Garden named Oliver Phillips was attempting to answer a longstanding question in ecology: Did forests with faster turnover—that is, faster growth, death, and replacement of trees—also have higher levels of biodiversity? To find out, Phillips had assembled records of tree growth and death from 40 tropical forest plots in Asia, Africa, Australia, and North and South America. Most had been surveyed only twice over 10 to 15 years, but collectively, they spanned from 1934 to 1993. And as Phillips tabulated the results, something strange emerged. The rate of tree turnover seemed to be increasing.

Up until 1960, about one percent of the trees in any given plot died and were replaced by saplings each year. But by the late 1980s, the rate had doubled to almost 2 percent. “I wasn’t looking for a pattern,” recalls Phillips. “The data were talking”—and they seemed to be describing a global change. Perhaps rising CO2, which provides the building blocks for trees to form carbohydrates during photosynthesis, was pushing trees to grow more quickly and die young. Or perhaps climate change’s increasing temperatures and droughts were killing trees early. Or stronger windstorms were simply felling more trees.

Whatever the cause, Phillips also noticed something else. Six of the plots, located in Peru, had tracked the growth of lianas since 1983, and in five, lianas seemed to be increasing. “It’s quite clear how lianas can and do accelerate tree mortality,” says Phillips, who is now at the University of Leeds in the United Kingdom. When he looked at dozens of other sites across the Amazon, he found a widespread trend: Lianas were growing more abundant at the roaring rate of 1 to 5 percent per year. Over 20 years, they had nearly doubled.

That 2002 discovery made a big splash. “No one believed it at first,” says Schnitzer, who leads the liana research team at the STRI in Panama. Some scientists wondered whether the researchers were simply getting better at counting lianas. But other studies found similar results.

Around that time, Schnitzer himself was drifting toward lianas. He had just finished his PhD dissertation at the University of Pittsburgh, showing that falling trees create gaps in tropical forests that boost biological diversity, by providing space where fast-growing “pioneer” tree species can root. Eventually, through a process called succession, those pioneers give way to slower-growing species that make up the bulk of the forest. But Schnitzer noticed (just as Putz had before him) that lianas also often thrive in these sunny gaps; and what’s more, he found that lianas can actually overwhelm the pioneer trees, shading them and slowing their growth. Gaps that would normally fill within 5 to 10 years often remained sparsely treed for 25 years or more. The lianas, he says, are “holding disturbed areas in a state of arrested succession.”

Over the next few years, the pervasive effects of lianas on forests became increasingly clear. In 2005 and 2006, Geertje van der Heijden, then a PhD student working under Phillips, turned to a set of forest plots in Peru to compare the growth rates of hundreds of trees that were or were not infested with lianas. She found that the trees with lianas seemed to grow more slowly and absorb less CO2, primarily because the vines blocked their sunlight. Van der Heijden estimated that across a single hectare (about 1.4 soccer fields), lianas probably prevented trees from absorbing about 2,000 pounds (920 kilograms) of CO2 per year.

The biomass of the lianas themselves compensated for 29% of that lost CO2 storage by trees—but they do a much worse job of storing carbon. If you look at the whole forest, says van der Heijden, “lianas are driving a shift toward more carbon [stored] in leaves and less carbon in stems.” And while woody stems can lock carbon away for many decades, leaves fall to the ground and decay, returning their carbon to the atmosphere within a few months.

In 2012, van der Heijden teamed up with Schnitzer to do an actual experiment, testing whether lianas really reduce the forest’s ability to absorb and store CO2. A kilometer south of Barro Colorado Island, on the shore of Lake Gatun, workers had used machetes and branch cutters to clear lianas from several swaths of forest. They then returned to record tree growth and death in areas with and without lianas. The results, published in 2015, were dramatic: Even when the biomass of lianas was added in, these vines reduced the forests’ overall absorption of CO2 by 76 percent—or about 8,920 kilograms (19,660 pounds) per hectare per year, nearly 10 times what her previous work suggested. “We were quite amazed,” says van der Heijden, who is now a forest ecologist at the University of Nottingham.

Despite these discoveries, the causes of liana expansion remained elusive. Schnitzer’s team found that higher CO2 levels boosted liana and tree seedling growth equally, ruling out that explanation. They found the same for nutrients deposited by fossil fuel air pollution. Schnitzer and others recognized that they would need to understand how trees are experiencing the warming climate on an intimate level to get closer to an explanation. A plant physiologist at STRI named Martijn Slot turned to the topic in earnest.

During the early months of 2016, Slot spent many days at the Parque Nacional San Lorenzo, a rainforest preserve on the Caribbean coast of Panama, about 15 kilometers northeast of Barro Colorado. In the dim morning light, Slot would step from the spongy forest floor onto the steel grating of a gondola attached to a 12-story crane. The contraption hoisted him 30 meters up, past layer after layer of dense foliage; past rainbow-billed toucans that croaked as they flitted away; past an occasional dangling sloth; until he emerged into sunlight among the trees’ crowns.

Speaking Spanish into his radio, Slot directed the crane operator through a series of delicate maneuvers that brought his gondola into the fragrant embrace of a leafy branch.

For the next few hours, Slot clamped a small plastic chamber over one leaf after another to measure CO2 uptake, in order to determine each leaf’s rate of photosynthesis. Sometimes an approaching thunderstorm forced him to descend. Other times, his gondola swung in a gust of wind and he accidentally snapped the leaf from its branch, forcing him to start over.

After dozens of crane sessions and 1,700 leaves, Slot found that warm temperatures push trees beyond their comfort zone, even on seemingly mild days. While the air temperature usually stayed below 32°C (91°F), sunlit leaves often surpassed 40°C (104°F). The leaves’ photosynthetic rates peaked at about 30°C (86°F) and then plummeted, falling by 90 to 98 percent when the leaf exceeded 40°C (104°F). Slot surmised that this happened because the trees had to triage their competing needs.

During photosynthesis, leaves absorb CO2 through tiny vents on their undersides called stomata, after the Greek word for “mouth,” because they resemble tiny lips. While a leaf’s stomata are open, “a huge amount of water is evaporating,” says Slot. “For every molecule of CO2 that’s being fixed, there’s 300 molecules of water being lost.” In this way, a single large tree can exhale around 150,000 liters (40,000 gallons) of water per year.

As the temperature climbs, so too does the rate of evaporation. At some point the leaf closes its stomata—its mouths—and holds its breath. This stops water loss, but also halts photosynthesis, because the leaf can no longer inhale CO2. Leaves regularly stop photosynthesis during the hottest hours of the day. But rising temperatures could force them to do this more frequently and for longer periods, reducing the trees’ food supply.

Many tropical trees “are already operating at their peak temperature,” says Kenneth Feeley, a tropical forest ecologist at the University of Miami in Florida. “As it gets hotter, they’re becoming less efficient and it’s going to lead to slower growth.” Amazonia has warmed by 0.9°C (1.6°F) since 1950, with another 1 to 4°C (1.8 to 7.2°F) of warming predicted by 2100, depending on the volume of greenhouse gasses that humans emit. Even if rising CO2 levels have caused forests to grow faster, as Phillips has found, rising temperatures could eventually erode and reverse the growth-enhancing effects of CO2. Meanwhile, changes in rainfall are already stressing the Amazon rainforest. The four-month dry season has lengthened by roughly 20 days since 1980, and the region experienced major droughts related to heat waves in 2005, 2010, and 2015-16, killing billions of trees.

This nexus between drought and heat could further boost lianas as trees falter, both now and in the future. Indeed, Schnitzer has found that lianas in Amazonia are more abundant in locales with longer and drier dry seasons. And he and van der Heijden have found that lianas on the south shore of Gatun Lake grow roughly three to four times as quickly as trees during the four-month dry season, even though they grow at similar rates in wet times. These differences especially stood out during the unusually hot and dry 2015-16 El Niño drought, when trees stopped growing entirely, but lianas continued to grow at normal rates. If that happens every five to seven years, whenever there’s an El Niño, says Schnitzer, it will give lianas “a huge advantage.”

The vines also have other abilities that may compound this one. In November 2015, Medina-Vega, Schnitzer’s former postdoc, began an experiment at the Parque Natural Metropolitano, a forest preserve bordering Panama City, which has a pronounced dry season, similar to Barro Colorado Island. There, he selected several dozen tree and liana branches, and meticulously labeled each of their 6,861 leaves with a sharpie. Over the next 17 months, he measured the growth of each branch and recorded every leaf that fell or sprouted. He found that at this site, lianas searched for sunlight more efficiently than trees did. The skinny liana branches lengthened by up to 38 meters—about 15 times more than any tree branch did. Liana leaves, like the stems, were also thinner than those of trees, making them “cheap” and “easily replaceable,” says Medina-Vega. When a liana’s leaf ended up in the shade, the liana simply shed it and replaced it with a new one, somewhere in sunlight. Having cheap leaves also allowed lianas to respond more nimbly to the seasons. Both lianas and trees lose their leaves during the dry season in this location, as in many places in the tropics, but Medina-Vega found that when the rains returned, lianas grew theirs back about a month earlier than the trees did.

So not only do lianas siphon water more quickly than trees, they also punch above their weight in the fight for sunlight. In forests with strong dry seasons, they account for less than 5 percent of the stem biomass—but produce 15 to 40 percent of the leaf mass. Climate change could amplify this advantage. “Let’s say these forests are getting drier and drier,” says Medina-Vega. “The performance of lianas will improve.” And tree growth could suffer.

Despite all the evidence of liana increase, it’s still uncertain what it means. Forest changes involve multiple complicated factors and can take centuries to play out, and the lives of human observers are short by comparison.

Tropical ecologist Flavia Costa of the Instituto Nacional de Pesquisas da Amazônia in Manaus believes that site history may have more to do with liana findings than any global phenomenon. Over a 10-year survey of the Adolpho Ducke Reserve in northern Brazil, she and her colleagues found that overall liana abundance remained the same, even though numbers at different plots fluctuated. “I think that data is not as strong as people try to picture,” she says.

Barro Colorado’s increase, for example, could have come from the fact that it didn’t become an island until 1914, when engineers building the Panama Canal dammed a nearby river. The rising waters led to the formation of Gatun Lake, and Barro Colorado, once a hill surrounded by swampland, became isolated at its center. This isolation could have triggered a longterm ecological cascade, if say, populations of large animals such as peccaries or howler monkeys declined as a result, limiting their dispersal of large tree seeds and giving small-seeded lianas a vacuum to fill.

Of course the forest plots that Phillips first used to demonstrate an increase in lianas, in 2002, were far more diverse, including dozens of sites across Peru, Bolivia, Ecuador, Venezuela, Brazil, Costa Rica, and other countries. But individual sites could still have biases. Forest changes occurring today might not be a response to present-day climate change, but rather the gradual drift of a forest back toward an equilibrium after some other event, like a severe windstorm that felled trees a century or two before. “It’s inherently hard to pull those apart,” admits Phillips. “None of us were around 100 years ago, 200 years ago, to see what the forest was like.” Researchers can only hope that working across a broad set of forest sites will cancel these local biases.

Perhaps the biggest mystery, though, is that there’s no consistent evidence that lianas are increasing in Africa or Asia. This might come down to a scarcity of long-term data, says Phillips, since scientists have established very few plots to track lianas on these other continents. He speculates that it might also reflect differences in tropical forests across the globe. The few remaining Asian forests often have higher canopies—up to 70 meters, versus 40 or 50 meters in the Amazon—and this might pose a bigger barrier for lianas trying to climb their way to sunlight. African forests, meanwhile, generally sit at higher elevations, making them cooler and damper than most of the Amazon. These forests have milder dry seasons and slower tree turnover, which might blunt lianas’ competitive advantages. But if those factors are indeed acting as buffers, they may not help for much longer.

The trees in African forests are starting to show wear and tear from climate change and the consequences could be widespread. Tropical forests play a critical role in helping to offset industrial society’s wild overproduction of CO2. The warning signs that they are increasingly unable to do so have been emerging for some time.

In the years after Phillips’s 1994 discoveries about forests turning over more quickly, the University of Leeds ecologist continued to follow the intimate lives of those trees as though they were human subjects in a long-term medical study. Feeding the girth of each tree’s trunk into a mathematical equation, his team estimated the total mass of trunk, branches, and leaves. From there, they calculated how much carbon it, and by extension, the surrounding forest, locked away as it grew. This led to a major finding in 2015. Phillips and his University of Leeds colleague Roel Brienen reported that the Amazon’s ability to store CO2 was gradually declining.

Analyzing 321 plots spread across the region, they found that the net amount of CO2 that the Amazon stored had peaked during the 1990s, at around 2 billion tons of CO2 per year—roughly the annual emissions of Russia from burning fossil fuels. By the 2000s, it had shrunk by nearly 30 percent.

Phillips sees increasing tree turnover as the biggest driver. “If you stimulate a mature forest, you can probably pack in more trees for a while,” he says. “But sooner or later the mortality rate will be going up.” He points out that trees which grow faster, or which grow tall at an earlier age, also die sooner. Tall trees suffer more during drought because it’s harder for them to suck water all the way up to their leaves. The trunks of faster-growing trees may also be also less dense, increasing the risk that wind or the fall of a neighboring tree will break them. And as rising temperatures increasingly interfere with photosynthesis in the future, this could also drive up mortality rates. Those dying trees—like fallen dryads yielding up their souls—will send their carbon wafting back into the air as they decay.

Perhaps most alarming of all, this pattern of reduced CO2 storage is starting to spread, with African tropical forests also showing early signs. The amount they locked away appears to have peaked around 2010, and is now declining. By 2040, African forests may lose around 50 percent of their annual CO2 absorption, and, as bioGraphic reported in 2017, the Amazon could cross over from absorbing CO2 to releasing it instead, a recent analysis published in Nature predicts. As warming and tree turnover continue to increase, Schnitzer believes that lianas could begin to boom in African forests, too. His hunch stems from his team’s 2021 analysis of the liana and tree data that Bernal Vargas and coworkers gathered at Barro Colorado Island from 2016 to 2017. In addition to showing a 29 percent increase in lianas, it also revealed a striking pattern across the landscape.

Nearly all of the new vines had sprung up in places where a falling tree had torn a hole in the canopy—creating an island of sunlight where its fallen lianas re-rooted and sent dozens of new stems slithering up neighboring trees. In other words, the increase in lianas appeared to be driven by the increased turnover in the trees themselves, though the study left open the possibility that the vines are also benefiting from longer dry seasons.

“No study has ever linked the liana increase to any clear mechanistic explanation, and here we do that,” says Schnitzer. “This is a massive finding.” It might be tempting to view lianas as passive beneficiaries of increased tree turnover, but Schnitzer says they play a more active role. “They’re killing the trees and then they’re exploiting the gaps that the trees leave,” he says. And as those gaps become more closely-spaced in the forest—as lianas overrun those gaps and slow the regrowth of trees—a positive feedback loop could take hold. Liana-filled gaps could become the beachheads from which hordes of rapidly growing vines pick and pry their way into swaths of unbroken forest that aren’t yet heavily infested.

The critical question is how the liana explosion will impact these forests’ ability to absorb CO2 at a much larger scale. To answer it, van der Heijden, the forest ecologist at Nottingham, plans to expand beyond the research plots and monitor liana growth from space. She and other researchers have spent several years analyzing satellite and aerial images of tropical forests from Central America, South America, and Asia, in hopes of finding ways to estimate the abundance of lianas remotely. If this approach works, then surveys that currently happen only once every few years could occur several times per year, and over thousands of square miles. They could also finally resolve questions about whether liana increase is widespread or simply the result of local biases. So far, the results are tantalizing.

When van der Heijden and her colleagues analyzed images from the Sentinel-2 satellite they found that a hectare of forest shines a bit more greenly when it’s infested with lianas, even when that entire hectare—hundreds of trees and lianas—is compressed into a few pixels. It’s early days, and van der Heijden and her colleagues are still trying to figure out what the satellites are actually measuring. It may be a true difference in leaf color, or the fact that liana leaves hang more horizontally than tree leaves, or the fact that most liana leaves are thinner than tree leaves, allowing more light to pass through.

Whatever the answer, reaching for satellites seems the most natural extension of the path that Putz blazed 40-plus years ago. Lianas cannot be fully understood from the ground; but viewing them from above, one can see how these vines venture far from their rooted stems, smothering one tree after another as they explore the canopy. Lianas “are the walking plants of the rainforest,” van der Heijden says. “They are the ones that can wander.”

Header video credit: Video by Blackbox Guild (Shutterstock),  Nancy Crowell, and TriumphRainbow (Shutterstock)

Douglas Fox

Douglas Fox

Douglas Fox is a freelance journalist who writes about earth, evolution, and polar sciences. His recent trip to Thwaites Ice Shelf was his sixth visit to Antarctica. Fox has written for Scientific American, National Geographic, The Atlantic, and other publications. He is a contributing author to The Science Writers’ Handbook.

Christian Ziegler

Lauren Owens Lambert

Christian Ziegler is a photojournalist & filmmaker specializing in natural history and science-related topics. He works for Max-Planck Institute for Animal Behaviour in Konstanz. He is a regular contributor to National Geographic, and has been widely published in other magazines like GEO. Christian’s aim is to highlight species and ecosystems under threat and share their beauty, and importance with a broad audience.

bioGraphic is powered by the California Academy of Sciences, a renowned scientific and educational institution dedicated to regenerating the natural world through science, learning, and collaboration.