For decades, researchers anticipated that climate change would stunt the Amazon’s tallest trees. But a gargantuan new study says the opposite: the giants of the rainforest are thriving, and even the smaller trees are getting bigger. Over the last 30 years, the forest as a whole has grown larger, remaking one of the most vital ecosystems on the planet.

An international team of nearly 100 scientists who monitored 188 forest plots in the Amazon. The plots together cover more than 225 hectares and are among the most pristine tropical places left on Earth. Measuring trunks as thick as oil drums and as narrow as broomsticks, the scientists monitored growth patterns spanning decades. And what they learned surprised them.

A Rainforest Seething with Big Trees

The numbers show that tree size in the Amazon has become increasingly bigger by a little over 3 percent every decade. Median size fared nearly 2 percent better per decade, and the biggest trees grew nearly 6 percent bigger. The frequency of big stems — those measuring more than 40 centimeters in diameter — rose by 6.6 percent per decade. At the same time, forest’s lowest-order trees were on the decline, falling more than 1 percent per decade, as deep-shaded understory stems declined even more rapidly.

Scientists in Colombia measuring a giant Ceiba tree. (CREDIT: Pauline Kindler)

The result is that the rainforest comes to be more and more dominated by big trees with big canopies. “Small trees persist, but fewer in number, and their share of the forest’s total mass of wood is declining. This is a positive news story,” said Professor Beatriz Marimon of Universidade do Mato Grosso, coordinating data collection in south Amazonia. “We hear all the time how fragmentation and climate change is threatening Amazonian forests. But meanwhile the trees in intact forests are getting bigger; even the big ones are still doing well in spite of these threats.”

Testing the Hypotheses

To get a sense of the change, researchers weighed four possible scenarios. If added carbon dioxide in the atmosphere simply fertilized growth, all the trees would get an equal boost. If short plants received the biggest boost, they’d be up with the tall ones surrounding them. If giants were penalized by heat and storm stress, large trees would fall behind. And if resources favored the canopy elite only, then the biggest trees would cement their grip on the forest.

The results were a mix of impacts. The smaller trees did grow faster compared to the bigger ones, showing that they are using the extra carbon dioxide. But it was the giants that grew ahead in the absolute sense, dominating space and resources.

“Large trees are well worth their weight in gold when it comes to pulling CO₂ out of the atmosphere and this study confirms that,” explained Dr. Adriane Esquivel-Muelbert of Cambridge University, one of the paper’s lead authors. “Despite concerns that climate change would have a negative impact on Amazonian trees and ruin the carbon sink advantage, the effect of CO₂ to stimulate growth endures.”. This reveals the amazing toughness of these woods, at least temporarily.

View of the rainforest canopy. (CREDIT: Adriane Esquivel Muelbert) Why Size Matters

Tree size matters because it influences how much carbon forests can store. Thick, old trunks store immense amounts of carbon for centuries, while younger trees store less and release more easily when they die. Standards Amazonian forests intact have been taking up more carbon than they release for the last few decades, acting to buffer us from human-driven climate change. This new study reveals the mechanism behind that trend: not only are there more giant trees, but size classes across the board are growing.

The implications are worldwide. “Before COP30 in Brazil later this year, these findings highlight just how crucial tropical rainforests are in our continued efforts to counteract man-made climate change,” Esquivel-Muelbert said. “The forest is paying us a debt by removing carbon from the atmosphere, but it can’t keep doing so indefinitely without assistance.”

Secret Risks Below the Canopy

The decline of small trees and understory may reflect poorer regeneration of the forest. Fewer developing shoots may mean fewer recruits to replace old giants when they eventually die. “Our study also shows just how destructive Amazon deforestation is,” added Dr. Rebecca Banbury Morgan from the University of Bristol, a second lead author. “Giant tropical trees are centuries old. We cannot simply attempt to plant new trees and expect them to deliver anything remotely like the kind of biodiversity or carbon value that the original, natural forest is producing.”

Potential impacts of growth stimulation and climate change on forest structure. (CREDIT: Nature Plants)

In addition to carbon, deforestation also diminishes biodiversity by fragmenting habitats, inhibiting the dispersal of seeds, and increasing the risk of fire. University of Leeds Professor Oliver Phillips, a senior author, warned that these pressures jeopardize the future of the Amazon.

“How large trees end up – how they deal with accumulating climate danger and spread their seeds – is mission-critical today,” he said. “The only way the giants survive is if the Amazon world stays connected. Deforestation is a vast threat-multiplier and will kill them if we let it happen.”

A Benchmark for the Future

The long-term monitoring program behind this study, known as RAINFOR, has been monitoring Amazonian trees since the 1980s. It is one of the largest and longest ecological data collections in the world. By monitoring the same plots every few years, researchers can detect subtle changes that would be impossible to measure in short-term censuses.

“We were aware that the total quantity of carbon found in the trees within intact Amazonian forests has increased,” said Professor Tim Baker of the University of Leeds. “What this new work suggests is that all tree sizes have increased over the same period – the whole forest has changed.”

Spatial trends of mean tree size and the scale parameter across Amazonian forests. (CREDIT: Nature Plants)

The researchers caution that what their study shows is a snapshot of the present trends, not a guarantee of tomorrow’s stability. Rising carbon dioxide appears to be driving growth, but climate extremes might yet reverse the gains. Hard droughts, intense storms, and more lightning flashes all pose a threat, particularly big trees whose wide crowns are more exposed. If the giants begin to fall in greater numbers, the carbon sink will rapidly weaken.

Practical Implications of the Study

The study is highlighting resilience and vulnerability in the Amazon. To the positive, the forest is still growing, its giant trees thriving in a time of rising carbon dioxide. That further enhances the Amazon’s role as one of the world’s most powerful natural barriers against climate change.

To the negative, the disappearance of the small stems and the perpetual threat of deforestation raise questions about renewal over the long term. Intact forest protection matters. You can’t replace the carbon-sequestering capacity of a tree that lives for centuries by growing saplings.

The research sets a benchmark that will allow scientists to predict how tropical rainforests respond to an age of global warming, guiding climate mitigation, conservation, and international policy over the next few years.

Research findings are available online in the journal Nature Plants.

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