When we look at the towering trees of old-growth forest patches in the Amazon, we might think these ancient beings have reached their maximum size and width.
It turns out they have not, a new study suggests. It shows that even the largest and oldest Amazonian trees still capture carbon dioxide (CO2)—and keep getting bigger, albeit at a slow pace.
Led by Adriane Esquivel-Muelbert, an ecologist at the University of Cambridge, the researchers analyzed 3 decades of tree measurements from 188 primary forest plots spread across nine Amazonian countries. Each plot was around 1 hectare, about the size of a city block, and was measured by teams using tapes and notebooks, often under harsh conditions.
The plots were selected from the Amazon Forest Inventory Network (RAINFOR), which has become one of the most important monitoring efforts in tropical ecology, according to Esquivel-Muelbert. The monitoring period varied between 1971 and 2015.
“We already knew the Amazon works as a carbon sink,” she said. “But we wanted to understand what’s happening inside the forest—what kinds of trees are changing, and how.”
The study, published in Nature Plants, found that the average tree size had increased by 3.3% per decade over the past 30 years. Large-canopy trees—those with trunks wider than 40 centimeters—grew even faster in diameter. Smaller trees shaded by larger ones also grew, while the size of medium-sized trees remained relatively stable.
The consistency across the Amazon basin suggests the increasing amount of CO2 in the atmosphere is the ingredient fattening up the trees. “Carbon is an extra resource,” Esquivel-Muelbert explained. “With the same amount of light, a plant can photosynthesize more efficiently when there’s more CO2 available.”
In other words, as humans release more carbon into the atmosphere, Amazonian trees seem to be using some of it to grow. The researchers interpreted the pattern as a mix of two effects: a winner-takes-all response, in which the tallest trees gain even more advantage, and a carbon-limited benefit response, in which smaller, shaded trees find it easier to survive in low light. Both effects can occur at the same time, leading to more biomass for both groups at the extremes of the size scale.
The study also found no sign that large trees are dying faster, contradicting earlier hypotheses that canopy giants would be the first casualties of heat and drought. The resilience of these ancient trees—some of them centuries old—is important because they sequester a disproportionate share of the forest’s carbon.
“The largest 1% of trees account for about half of all the carbon stored and absorbed by the forest,” Esquivel-Muelbert said. Losing them would mean losing much of the Amazon’s buffering power against climate change.
Not Exactly Good News
“It doesn’t mean carbon dioxide is good for the forest. What we’re seeing is resilience, not relief.”
The findings might sound like good news, but “it doesn’t mean carbon dioxide is good for the forest,” Esquivel-Muelbert said. “What we’re seeing is resilience, not relief.”
Carbon dioxide might be fattening up old trees, but its consequences for the global climate totally offset what might look like an advantage or a good thing at first sight, she emphasized.
To Tomás Domingues, a forest ecologist at the Universidade de São Paulo in Ribeirão Preto, the new results offer valuable real-world confirmation of what experimental models have long proposed. “The study shows that the community as a whole is gaining biomass, presumably due to higher CO2,” he said. “That aligns perfectly with what we’re testing at AmazonFACE.”
AmazonFACE—a large-scale open-air experiment near Manaus in the Brazilian state of Amazonas—exposes forest patches to elevated concentrations of atmospheric carbon to simulate future conditions. One of its main goals is to see how long the carbon fertilization effect can last before the forest runs into another limitation: the lack of nutrients such as phosphorus, calcium, magnesium, and potassium.
“The CO2 effect has a short life,” Domingues explained. “Trees can only turn extra carbon into growth if they have enough nutrients. In the Amazon, everybody—trees, microbes, fungi, insects—is competing for the same scarce resources.” If nutrients become limited, he added, growth could plateau or even reverse, regardless of the CO2 supply.
Still Holding On
The new findings highlight how complex the Amazon’s responses to human-driven change can be. While extra carbon has acted as a growth stimulus so far, climate stressors, especially heat, drought, and windstorms, are also intensifying.
Previous studies suggested that the Amazon’s overall carbon storage capacity is starting to weaken. Changes in species composition, repeated droughts, and the spread of degradation along the southern and eastern edges of the basin are already weakening parts of the system. “The forest is still resisting,” Esquivel-Muelbert said, “but that doesn’t mean it will resist forever.”
“These forests are resilient, but they’re irreplaceable. If we lose them, they don’t come back in our lifetime.”
Domingues noted that 30 years of observations, though impressive for tropical fieldwork, still capture only a short moment in ecological time. “For the forest, 30 years is nothing,” he said. “These trees live for centuries. We need to keep watching.”
Despite the unknowns, both researchers are clear: Protecting mature, intact forests is crucial if we want to fight climate change. Reforestation won’t replace the carbon storage capacity of old-growth trees. “These forests are resilient, but they’re irreplaceable,” Esquivel-Muelbert said. “If we lose them, they don’t come back in our lifetime.”
The study’s main message, Esquivel-Muelbert added, is not that the Amazon is thriving under climate change. It’s that the forest is still holding on, at least for now.
—Meghie Rodrigues (@meghier.bsky.social), Science Writer