Amazon rainforest trees are resisting climate change by getting fatter from CO2 in the atmosphere

Trees of all sizes across the Amazon rainforest are getting fatter due to climate change, a new study shows.

Rising carbon dioxide (CO₂) concentrations in the atmosphere have created a more resource-rich environment for plants in the Amazon, leading to an average 3.3% increase in the circumference of trees at their base every decade since the 1970s, researchers have found.

"We knew that the total amount of carbon stored in the trees of intact Amazonian forests has increased," study co-author Tim Baker, a professor of tropical ecology and conservation at the University of Leeds in the U.K., said in a statement. "What this new study shows is that all sizes of trees have grown larger over the same period — the whole forest has changed."

This fattening is "good news," because it suggests Amazonian trees are more resilient to global warming than previously thought, study co-author Beatriz Marimon, a professor and tropical plant ecologist at Mato Grosso State University in Brazil, said in the statement.

Previous studies indicate that rising temperatures and CO₂ levels are pushing the Amazon rainforest ever-closer to a tipping point that could transform the ecosystem into a savanna in the next 100 years — but in the meantime, trees are making the most of the climate by locking away massive amounts of carbon and bulking up, the new research finds.

For the study, the researchers collected data from 188 plots across the Amazon rainforest, measuring what is known as the trees' basal area, or how much space their trunks occupy on the forest floor. Monitoring began in 1971 and ended in 2015, but different plots were observed for varying lengths of time during this period, with the longest continuous plot monitoring time being 30 years.

The team, made up of almost 100 tropical plant scientists, designed the study with several possible outcomes in mind. One of these outcomes, known as the "winners-take-all" response, described a scenario where only larger trees benefit from rising CO₂ levels. Large trees have more access to light and nutrients than smaller trees do, meaning they are more resilient to changing conditions, according to the study.

Another outcome, dubbed the "carbon-limited benefit" response, described a case where smaller trees benefit more from rising CO₂ because they are so resource-limited to begin with that any increase would have a stronger effect overall than in larger trees.

A combination of these outcomes, dubbed the "benefits shared" response, was also possible, the scientists wrote in the study.

The results, published Thursday (Sept. 25) in the journal Nature Plants, suggest the "benefits shared" response prevails — for now. "The trees in intact forests have grown bigger," Marimon said, adding that even the largest trees, which are typically more vulnerable to climate-related events such as drought and lightning, are thriving in places without deforestation.

However, the researchers noted that over time, increases in basal area could become more pronounced in large trees, which would then dominate the ecosystem at the expense of small trees.

"Large trees are hugely beneficial for absorbing CO₂ from the atmosphere and this study confirms that," study joint-lead author Adriane Esquivel Muelbert, an associate professor of tropical plant ecology at the University of Cambridge, said in the statement. "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," she said.

None of the studied plots showed declines in basal area, indicating that negative climate effects have so far been outweighed by rising CO₂ availability. But this could change soon, the researchers warned in the study, with a slowing of tree growth and a bump in mortality expected in the coming decades.

Slowing growth and higher mortality may arise from a combination of factors — including heat stress, water stress, wildfires and storms — which are already increasing in frequency and severity. Other than diminishing our carbon emissions, the best way to buffer the Amazon rainforest against these factors is to leave it intact, the researchers said.

"These results underscore just how important tropical rainforests are in our ongoing efforts to mitigate against man-made climate change," Esquivel Muelbert said.