Average tree size across the Amazon has increased by 3.2% every decade consistent with a response to rising carbon dioxide levels, a new study suggests.
The study, co-led by Dr Adriane Esquivel-Muelbert, incoming Associate Professor of Ecology at the Department of Plant Sciences, University of Cambridge, found that big trees are increasing in size and number across the Amazon rainforest.
As the biggest 1% of trees account for half of the rainforest’s carbon cycling and storage, this indicates it is more resilient to climate change than previously thought.
The research, which was published in Nature Plants on 25 September, found that the average size of both large and small trees in Amazon forests has increased over recent decades, consistent with benefitting from fertilisation by increased atmospheric carbon dioxide.
Dr Esquivel-Muelbert commented: “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.
“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 study is the result of an international partnership of more than 60 universities in South America, the UK and beyond – including members of the RAINFOR Amazon Forest Inventory Network and other networks and individuals contributing to long-term forest science and monitoring in South American forests.
The team of almost a hundred researchers monitored the size of trees in 188 permanent plots and discovered that the increase has continued for at least 30 years.
Co-author of the study Professor Beatriz Marimon, from Universidade do Mato Grosso, who coordinated much of the Brazilian data collection in southern Amazonia, commented: “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.”
Dr Rebecca Banbury Morgan from the University of Bristol and joint lead author added: “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.”
According to previous research by the RAINFOR network, the Amazon forest plays a key role in taking up carbon which would otherwise be in the atmosphere.
“We knew that the total amount of carbon stored in the trees of intact Amazonian forests has increased. What this new study shows is that all sizes of tree have grown larger over the same period – the whole forest has changed,” added Professor Tim Baker from the University of Leeds, joint senior author of the study.
The study is the first of its kind to measure how increases in CO2 have systematically changed the tree size structure of Amazon forests. The team noted that as the biggest trees have grown larger, they have increasingly managed to dominate competition for resources.
The authors point out the new research has other implications too. According to Professor Oliver Phillips of the University of Leeds: “What happens to big trees – including how they deal with increasing climate threats and manage to disperse their seeds – is now mission-critical. The only way the giants will stay healthy is if the Amazon ecosystem stays connected. Deforestation is a huge threat-multiplier and will kill them if we let it.”
Reference: Esquivel-Muelbert, A., Banbury Morgan, R., Brienen, R. et al. ‘Increasing tree size across Amazonia’, Nature Plants (September 2025). DOI: 10.1038/s41477-025-02097-4
Image: View of the rainforest canopy. Photo credit: Adriane Esquivel-Muelbert.