The Amazon's Trees May Be Reaching a Dangerous Tipping Point

The forest in Peru’s Tambopata Nature Reserve was hot and noisy—filled with the buzz of cicadas and the raspy barks of white-throated toucans.

Botanists Riley Fortier and Rodolfo Vásquez were there to retrace the steps of the late American botanist Alwyn Gentry. It was Fortier’s first visit to this nook of the Amazon. But for Peruvian-based Vásquez, it was a strange homecoming, returning to the place where he had accompanied Gentry decades before.

Fortier and Vásquez tried to ignore the dozens of bees—a local species that loves to slurp up salt but fortunately doesn’t sting—crawling across their sweaty shirts and faces and into their ears and nostrils.

They sought out the moss-crusted, metal tags that Gentry had nailed to tree trunks 40 years prior. Many of the trees Fortier was looking for had died, fallen, or even vanished—decayed and erased by the fast-growing forest. But over the course of two months, they collected leaves from 151 of Gentry’s original trees, including 40 different species.

Back at his lab in the U.S., Fortier spent months analyzing the leaves.

His findings, recently published in the journal Global Change Biology, were concerning.

“The physiology of the tree itself has changed,” says Fortier, who works with the Missouri Botanical Garden in St. Louis.

The study suggests that while the leaves of Amazonian trees are adapting to drier conditions, they’re simultaneously becoming more vulnerable to hotter temperatures. Some trees are seeing their leaves overheat, warmed like dark asphalt baking in the summer sun.

For these reasons, scientists worry that Amazonian trees are approaching a dangerous tipping point in which healthy trees may struggle in a hotter climate.

The Amazon absorbs about a billion metric tons of the planet-warming carbon dioxide each year. The enormous forest locks away that absorbed carbon in wood, leaves, and soil.

But if trees thin out or die off, “it will be[come] a source of carbon, and that will amplify climate change,” says Jose Marengo, a climatologist at São Paulo State University in Brazil. A thinning Amazon forest would even change the patterns of rainfall, he says: “And of course that will change climate everywhere, not just in South America.”

Dryer, Hotter Air Hits the Amazon

Around 35 years ago, scientists began publishing research speculating that the Amazon would suffer as Earth warms. All told, the massive forest stores the equivalent of 500 to 700 billion tons of carbon dioxide—about equal to what all of humanity releases, worldwide, over 10 or 15 years.

Since 1951, the region’s dry season has lengthened by about 20 days—now lasting six months in some areas. The Amazon has also experienced a series of worsening droughts.

The 2005 drought “at the time was called the drought of the century,” says Marengo. But similarly severe droughts arrived in 2010, 2015, and 2023. “So we had many droughts of the century in less than 25 years,” he says.

During the most recent, in 2023 and 2024, late rains and above average temperatures caused millions of trees to drop their leaves, die, or vaporize in wildfires. More than 740 million tons of carbon dioxide billowed into the atmosphere.

Scientists had speculated, and hoped, that individual trees might somehow acclimate to dryer, warmer air—like a person acclimating to high altitude during a mountain vacation.

But, scientifically, it was hard to measure. Scientists monitor tree plots across the Amazon, counting and measuring tree trunks every few years to estimate how quickly they’re growing and capturing carbon dioxide. But no one has monitored individual trees to see if their physiology had evolved over decades.

“We can’t go back in time and test what these trees were doing in 1980,” says Kenneth Feeley, a tropical forest ecologist at the University of Miami in Florida, and a co-author on the new study.

So, he and Fortier came up with another way to see how trees were living and breathing in the past. They would use the samples that Alwyn Gentry had collected decades before, when he too was working for the Missouri Botanical Garden, to track down those same trees and measure how they had changed.

Studying the Amazon Rainforest in 1983

The Tambopata River meanders through a remote corner of Peru, 1,600 miles from the mouth of the Amazon River, not far from the Andes. In the summer of 1983, Gentry and Vásquez rode a motorboat along its brown, lazy curves, through unbroken forest, until they disembarked at a clearing where several palm-thatched huts stood over dirt floors.

The two men had known each other for nearly a decade at that point. Gentry was a mentor—“amazingly patient and generous” says Vásquez—who made frequent nocturnal forays into the forest so they could observe the moths that pollinated certain flowers. At the time of their 1983 trip, Gentry was trying to figure out why some parts of the forest were more diverse than others.

Gentry and Vásquez spent weeks surveying the forest, outlining several plots larger than football fields, and numbering each tree inside them with an aluminum tag. To help them, a local farmer named Nestor Jaramillo shimmied up hundreds of these trees, wrapping his legs tightly around the trunk, and collecting leafy twigs from as high as 100 feet. Gentry scribbled notes about their shapes, smells, vein patterns, and other traits.

The leaves they collected in Tambopata were then shipped home and filed away at the Missouri Botanical Garden's herbarium.

The surveys showed that these plots in Tambopata held some of the most diverse forests on Earth, with up to 75 tree species per acre.

Gentry didn’t appear to be thinking about climate change during that time, according to Oliver Phillips, who worked as a Ph.D. student with him between 1986 and 1993 and was also an author on Fortier’s recent study.

The idea that industrialized societies were somehow altering these forests from afar “just wasn’t on the horizon” back then, says Phillips.

But by the 1990s, these plots in Tambopata began to show that the world’s remote tropical forests were actually changing quite quickly—spurred on by a then-unknown influence.

Phillips published one of these early discoveries in 1994, showing that the world’s tropical forests were moving through generations at a faster pace. Old trees were dying and being replaced by new trees more quickly over time. In Tambopata, the rate of tree turnover more than doubled in just 12 years.

Gentry never got a chance to learn just how strongly the Amazon would respond to changes in the atmosphere. He died on August 3, 1993, at age 48, in a plane crash during a forest survey in Ecuador.

Returning to the Amazon to Find the Same Trees

While working on his Ph.D. in Feeley’s lab in the early 2020s, Fortier decided that he would try to use Gentry’s old herbarium samples to test whether individual trees were acclimating to climate change. To do this, he would need to find some of Gentry’s original trees and collect new leaves.

By browsing Gentry’s herbarium samples, and field notes from more recent expeditions, Fortier compiled a list of several hundred Amazonian trees that might still be alive. Then, in August 2023, Fortier flew to Peru and met up with Vásquez, who now works at the Herbario Selva Central in Oxapampa, Peru.

Vásquez had not walked beneath these trees since the 1990s; and as he returned, the local smell of smashed Brazil nut fruits—sharp and spicy—evoked strong memories.

“It was nostalgic,” he says, in Spanish. Touching the same trees that he’d visited with Gentry four decades before— “like what we call goosebumps.”

Sure enough, Fortier’s new leaves were different. They had smaller and fewer stomata, the mouthlike openings that leaves use to inhale carbon dioxide.

Fortier thinks this change is likely happening because these new leaves are growing in a world where atmospheric carbon dioxide is much higher than in 1983—allowing the new leaves to inhale it more efficiently.

This adaptation also helps the modern leaves deal with a dryer climate. When leaves open their little mouths to inhale carbon dioxide, they also lose water into the air—a process called evapotranspiration—so having fewer and smaller mouths reduces water loss.

“There was initially a benefit” from rising carbon dioxide, says Feeley. But over time, the leaf changes “might actually be pushing these plants in the wrong direction.”

That’s because evapotranspiration cools trees during hot parts of the day, like how sweet cools a human body.

“These trees are sweating less now than they used to,” says Feeley.

What do These Changing Leaves Mean for Trees?

Over time, hotter trees could become a big problem. Globally, the world has warmed by about 1.3°C since the industrial revolution, but certain pockets of the world are warming at different rates.

The Peruvian Amazon has warmed by 1.4°C since 1983; but because the leaves are evaporating less water, they’ve warmed up even more than the surrounding air. Fortier estimates the leaves have experienced 1.8°C of warming since 1983: “It has pretty big implications for the performance of these trees.”

At too-high temperatures, leaves can be permanently damaged—diminishing their future ability to photosynthesize and ultimately making it harder for trees to survive.

Martijn Slot, a plant physiologist at the Smithsonian Tropical Research Institute in Panama, has found that sunlit leaves can temporarily reach dangerous temperatures even in the current climate.

Factor in the leaves’ structural changes, and, in an increasingly hotter world, this could vastly increase the time that leaves spend at those dangerous temperatures.

The leaves “are going to suffer over time” with more and more “cumulative damage,” says Slot, who was not part of the new research project. It would force trees to replace their leaves more often.

In fact, Fortier’s measurements in Tambopata suggest this might already be happening. He found that since 1983, the trees have made their leaves progressively thinner and flimsier—like cheap paper cups, to be used briefly, then discarded.

This parallels the broader picture, pieced together by Phillips and other scientists, that Amazonian trees, riding on a carbon dioxide sugar high, are growing more quickly and getting larger than before. Yet they’re forming wood that is spongier and weaker, causing these same trees to break, fall, and die at younger ages.

The Amazon's Declining Carbon Power

Since 1990, the average time that carbon dioxide stays locked in the Amazon rainforest after being absorbed by a tree has dropped from 70 years, down to about 60 years—after which the decay of wood and leaves returns it to the atmosphere as carbon dioxide.

Meanwhile, the amount of carbon dioxide absorbed by the Amazon each year has fallen from 2.5 billion tons in the 1990s to around 1 billion now. Sometime after 2040, the Amazon could become a source of carbon dioxide. This transition into a carbon source might start slowly, but if large swaths of the Amazon thinned out, this could release 100 to 250 billion tons of carbon dioxide over a period of many decades.

Fortier believes that the changes he sees in tree physiology are contributing to these larger trends. “I think all of these things are connected,” he says.

As he and Feeley weave these pieces together, they see another threat on the horizon: an unusually warm Pacific Ocean in 2026—a powerful El Niño event—that may plunge the Amazon into another severe drought and heat wave—marked by dry riverbeds, dying trees, and potentially massive fires.

This punishment will descend onto an Amazon that still hasn’t recovered from the last drought, just two years ago. For some trees, the odds of survival might be slim, says Feeley:  “These trees may already be getting closer to their limits.”