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The average size of trees in the Amazon rainforest has been steadily increasing as carbon dioxide levels have risen, meaning these larger trees play a more important role in determining whether the forest can remain a carbon sink.

How forests will react to a changing climate is an open question. For example, one hypothesis is that larger trees will decrease in abundance because they are more susceptible to climate-linked phenomena such as drought or high winds. Understanding how it will play out is crucial for models of the future climate because forests take up huge amounts of CO₂ from the atmosphere, locking it away to slow global warming.

at the University of Cambridge and her colleagues at the have been measuring the diameter of trees in 188 plots with an average area of 12,000 square metres across the Amazon basin. The monitoring periods varied, but some were as long as 30 years. During that time, CO₂ concentrations in the atmosphere have risen by nearly a fifth.

“What we’re following is some space in the forest and in that space the average tree size is bigger, meaning that the trees can pack more carbon in that space than they could in the past,” says Esquivel-Muelbert. The researchers have found that, on average, trees have increased in diameter by about 3.3 per cent each decade.

“The structure of the Amazon Forest is changing quite consistently across the whole basin,” says team member at the University of Bristol, UK. “We have more bigger trees and fewer smaller trees, so the average size has shifted up towards those bigger trees.”

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Normally the average diameter of trees in an area of undisturbed old-growth forest would stay roughly the same, she says, as saplings take the place of fallen big trees and grow. The researchers think the Amazon trees are responding to the increase in atmospheric CO₂ levels by growing more, and accumulating more biomass. “The winners are the big trees that compete better for light and for water,” says Esquivel-Muelbert.

This means the big trees are disproportionately important to the amount of carbon the forest can hold, and the consequences of losing them would be disproportionately big, she says.

“The important finding is that CO₂ has been acting as a fertiliser, increasing tree growth, and in many ways that is reassuring, because wood is a globally significant carbon sink,” says at Durham University, UK. “However, will this continue to be the case as the climate continues to change, potentially shifting the balance between growth, nutrients, temperature and CO₂?”