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When hunting for the world’s largest tropical trees, watch your step. In the Panamanian jungle, vigilance is key: To venture into the thick, teeming understory is to duck under the webs of orb-weaver spiders, squeeze between tangles of fallen liana vines and try desperately not to disturb the universe that is the forest floor.
Then, after watching the ground so attentively, it’s time to look up. There, the sight of a giant tree could knock anyone off their feet—and put a cramp in their neck. The largest of them can grow to be 10 feet wide and over 300 feet tall. For a frame of reference, picture the Statue of Liberty, who lifts her torch to a height of 305 feet. These trees are living monuments in their own right.
Even after spending years among these giants, Evan Gora and Adriane Esquivel-Muelbert haven’t grown tired of their magnificence. Gora is both a forest ecology staff scientist at the Cary Institute in New York and a research fellow at the Smithsonian Tropical Research Institute in Panama, and Esquivel-Muelbert is a global forest ecologist at the University of Birmingham in England and the Mato Grosso State University in Brazil.
“These giant trees have witnessed so much,” Esquivel-Muelbert says. Preserving them isn’t just a matter of forest conservation, but of historical and cultural conservation as well.
The two of them spend entire days journeying through the jungle in search of giant trees, dead or alive. Their work began on Panama’s Barro Colorado Island, home to a deeply studied tropical forest.
But even there, where researchers have conducted and published more than 13,000 studies over the last century, the nature of these giant trees remains mysterious.
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Last October on the island, Gora, Esquivel-Muelbert and their research team embarked on the first expeditions of the Gigante Project, an ambitious effort to find out what’s killing giant tropical trees around the world. And the island was a great place to start. The largest trees in the research forest are about 1,000 times bigger than the median tree size, Gora says.
Some of the dead or dying giants look like towering skeletons—sunlight streams through the gaping hole in the canopy where their crowns once flourished. Now, they’re leafless and snarled in lianas. At ground level, beetles have moved in, eating away at the trunks and leaving behind sawdust-like piles of frass.
Learning what kills giant trees is a question of increasing urgency. In 2022, a Nature study conducted in Australia suggested that the death rates of tropical trees there have doubled since the 1980s.
The research gives tropical forest ecologists reason to believe that the giant trees are also being affected by this trend. But they don’t know for sure. Existing mortality data is not based on giant tree deaths. In fact, research on tropical trees has historically overlooked giants because their enormous size, long life spans and rarity make it difficult to study them. Therefore, scientists don’t have enough data to understand giant tropical tree mortality, or how it’s shifting due to climate change.
“We have no confidence about what’s going on for these big trees,” Esquivel-Muelbert says.
The Gigante Project seeks to fill that knowledge gap. Esquivel-Muelbert, Gora and their colleagues have launched an international effort to investigate drivers of giant tropical tree mortality, including lightning, wind, drought and disease. But their deaths aren’t the only mystery Gigante hopes to solve.
“If you had to guess, how old do you think the oldest trees in tropical forests are?” Gora asks.
“We don’t know,” he says. “The number of trees that are dated are in the dozens. We have no concept. We don’t even have a ballpark.”
How long does it take for a sapling to become a giant? How long can they live after they reach full size? Which giant tree species appear where? These are just a few of the seemingly endless questions that remain unanswered about giant tropical trees. The Gigante Project will inform the answers to these queries as well. But studying mortality is especially important at this time in Earth’s history, at the precipice of catastrophic warming and biodiversity loss.
A climate feedback loop
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Research suggests human-driven climate change is playing an outsized role in increasing the mortality of tropical trees. The 2022 Nature study that showed a doubling of tropical tree mortality since the 1980s identified atmospheric drying triggered by global warming as a likely cause. Growing frequency and intensity of storms, drought and heat are just some of the other mechanisms by which climate change stresses and kills tropical trees.
Because the giant trees have survived for so long through many different pressures, one may assume that they would be more resilient than smaller trees. But while they are more tolerant to stress, their exposure to stress is greater. For example, giant trees are more vulnerable to wind and lightning due to their height, and some evidence suggests their high demand for water may mean they’re more vulnerable to drought, too.
Climate change is likely altering the mortality rates of tropical trees, and this added complexity makes understanding how and why giant ones die more pressing than ever before.
“We know that these temperature increases are occurring everywhere,” says David Bauman, a tropical forest ecologist not involved in the Gigante Project, and lead author on the Nature study.
But just as climate change has a big impact on tropical tree mortality, tropical tree mortality has a big impact on the climate. Tropical forests are responsible for a major share of terrestrial carbon storage.
“They’re really important because of that massive stock,” says Tom Pugh, a Gigante Project collaborator and scientist who studies interactions between the terrestrial biosphere and the global climate. “If you look at the total stock of carbon just in wood in the tropics, it’s about 250 petagrams. And in the rest of Earth’s forests, it’s about 150 petagrams altogether.”
When trees die, they release their stored carbon back into the atmosphere. The largest one percent of trees—the giants—store half of the above-ground biomass carbon in tropical forests. When they die and decay, it’s like setting off a carbon bomb in slow motion, which directly contributes to atmospheric warming. At the same time, losing these trees significantly degrades the carbon storage capacity of the entire ecosystem.
In looking at the relationship between giant tropical tree death and the climate, a feedback loop begins to take shape. As rising global temperatures kill more giant tropical trees, climate change is exacerbated through stored carbon release and the degradation of tropical forest carbon sinks.
Before scientists can even begin to understand how this theoretical feedback loop manifests in tropical forests, they need to build a basic understanding of what kills these giants. Only after establishing this baseline knowledge can they begin to ask questions about how climate change is influencing giant tropical tree mortality. And that work comes with difficulties.
“Trying to think about what can kill something that’s survived everything that’s affected it for a thousand years, that’s a pretty remarkable challenge,” says Gora.
Drone-powered research
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When forming the Gigante Project, Gora and Esquivel-Muelbert had to answer a significant question: How do we study life that is, in every sense, larger than us? These trees can be hundreds of feet tall and survive for millennia. They are also exceedingly rare, which makes studying their deaths all the more difficult. Capturing enough deaths to reveal mortality trends requires huge survey areas.
The project will span five sites across four different countries: Panama, Brazil, Cameroon and Malaysia. At each site, the researchers will collaborate with scientists and technicians based in each region to monitor giant tree death using on-the-ground observations and drone surveys.
When a giant tree dies, the evidence is in the canopy. Some fall, leaving holes behind, while those that remain standing lose leaves and branches. From above, a giant dead tree is easy to spot: Its bare, brown crown sticks out amid a sea of green. Drone imaging will allow Gigante researchers to look for canopy holes and crown damage to pinpoint the timing and location of giant tree deaths.
“Drones are what make the Gigante Project possible,” says Pugh.
These aerial assessments could create a data “gold mine” that reveals trends in mortality drivers across entire landscapes, says Helene Muller-Landau, a Gigante collaborator and tropical forest scientist at the Smithsonian Tropical Research Institute. Her lab created the digital workflows that the team will use to track giant tree deaths in real time. Then, they’ll use monthly data on mortality drivers like storms and lighting to begin narrowing down potential causes of death.
Equally as important, however, are the GPS coordinates that the drone images will assign to each giant dead tree, allowing the Gigante team to navigate to these sites in the field. Once they arrive, the real detective work begins.
An international initiative
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In the Smithsonian research forest, the team members move around a standing dead giant like forensic investigators surveying a murder scene, gathering evidence that would help them determine a cause of death.
While Esquivel-Muelbert documents crown and trunk damage, Gora bounds around the site looking for the signature marks of a lightning strike. Identifying a lightning strike in the field requires a trained eye that can recognize the distinct pattern of damage caused by flashover, when the strike’s electrical current jumps from the giant tree to surrounding trees.
Developing their field protocol has been challenging in part because some mortality drivers are more difficult to diagnose than others. When it comes to wind, fresh structural damage to the living crown is a good indication. Lightning strikes cause crown damage, too, but they leave behind a fingerprint: that signature flashover pattern. But drivers such as drought and pathogens don’t cause that sort of obvious damage—at least not right away.
To assess water stress, the Gigante team will have to measure soil water content and atmospheric conditions. They will look for lianas, beetle infestations, internal rot and other biotic factors that could have degraded the tree’s health. Combining these measurements and observations with aerial assessments of the canopy will allow the team to approximate the most probable pathway of mortality for each giant.
The team will replicate this at all five of their research sites: Barro Colorado Island in Panama, Fazenda Vera Cruz and Reserva Ducke in Brazil, Danum Valley in Malaysia and the Dja Reserve in Cameroon. In each region, they’ll work with researchers who call these places home and have dedicated their careers to studying them.
“We have really great partners across our sites who have a lot of data about these forests,” Gora says. “Because we don’t know that much about these big trees, it’s a lot easier to add that part into a forest we know a lot about already.”
They have partnered with several institutions around the world, connecting researchers among more than ten countries. Early career scientists and PhD students will also work directly with Gigante and receive training on how to implement cutting-edge technology and methodology to study giant tropical trees.
Gora and Esquivel-Muelbert hope that working with in-country partners and young scientists will empower more researchers to conduct studies on these trees. They also hope that it promotes equity in their field.
“When we do tropical science, it’s very easy for someone from the global north to come with loads of money and do their science and go. We are hoping that changes here,” Esquivel-Muelbert says. “We have a really collaborative project that makes the most of having this rich network of collaborators and creates great science because of that diversity.”
Whether these massive, old, carbon-storing organisms persist or disappear will drastically alter our planet. This three-year project is just the beginning of what Esquivel-Muelbert and Gora hope will become a renewed global effort to study them.
“We have so many questions, and there’s so much potential,” Esquivel-Muelbert says.