Average tree size in the Amazon rainforest has been rising for decades — about 3.3% every ten years — and the most likely reason is extra carbon dioxide in the air.
The findings come from a study published on September 25 in Nature Plants. A global team tracked the forest using long-running plots and found that trees of many sizes are getting bigger, not just the giants.
“This is a good news story. We regularly hear how climate change and fragmentation is threatening Amazonian forests. But meanwhile the trees in intact forests have grown bigger; even the largest trees have continued to thrive despite these threats,” says the co-author of the study Professor Beatriz Marimon, from Universidade do Mato Grosso.
The researchers analyzed records from 188 permanent plots across the Amazon, representing about 30 years of measurements of tree trunks — a standard way to estimate tree size. They report that average tree size increased steadily across the region.
“Ahead of COP30 in Brazil later this year, these results underscore just how important tropical rainforests are in our ongoing efforts to mitigate against man-made climate change,” says the lead author of the RAINFOR paper Dr Adriane Esquivel-Muelbert, from the University of Cambridge. “Large trees are hugely beneficial for absorbing CO2 from the atmosphere and this study confirms that. Despite concerns that climate change may negatively impact trees in the Amazon and undermine the carbon sink effect, the effect of CO2 in stimulating growth is still there. This shows the remarkable resilience of these forests, at least for now.”
The growth pattern wasn’t limited to a few hotspots. Plots across different parts of the basin showed similar trends. And while the biggest trees grew the most in absolute terms, smaller and mid-sized trees also got larger.
The forest’s structure is also shifting. The number of large trees increased by about 6.6% per decade, while the smallest stems became less common, even as overall tree numbers stayed roughly level. This means more of the forest’s wood — and carbon — is now concentrated in big trees.
“Our paper also highlights how destructive Amazon deforestation really is. Large tropical trees are hundreds of years old. We can’t simply plant new trees and expect them to confer anything like the kinds of carbon or biodiversity benefits that the old, natural forest is providing,” says Rebecca Banbury Morgan from the University of Bristol.
The authors point to rising CO2 as the most likely driver of the size gains, noting that other possible factors, like nitrogen deposition, are weaker or too localized for the Amazon.
Journal reference: Nature Plants DOI: 10.1038/s41477-025-02097-4
