The biggest trees in the Amazon are growing larger and more numerous, according to a new study that shows how an intact rainforest can help draw carbon dioxide out of the atmosphere and sequester it in bark, trunk, branch and root.
Scientists said the paper, published in Nature Plants on Thursday, was welcome confirmation that big trees are proving more climate resilient than previously believed, and undisturbed tropical vegetation continues to act as an effective carbon sink despite rising temperatures and strong droughts.
However, the authors warned this vital role was increasingly at risk from fires, fragmentation and land clearance caused by the expansion of roads and farms.
“It is good news but it is qualified good news,” said Prof Oliver Phillips from the University of Leeds. “Our results apply only to intact, mature forests, which is where we are watching closely. They suggest the Amazon forest is remarkably resilient to climate change. My fear is that may count for little, unless we can stop the deforestation itself.”
This caution comes as Brazil plans to pave a major road – the BR-319 – through the central Amazon, close to one of the last regions that still has large areas of pristine forest. Bolivia also continues to clear trees at an alarming rate for soy plantations and cattle ranching.
The study was conducted by almost 100 researchers from 60 universities in Brazil, the UK and beyond, who examined changes in the forest at 188 plots in the Amazon over the past 30 years.
They found the mean cross-section of trunks thickened by 3.3% per decade, with the greatest increase found in larger trees. This expansion was attributed to the rising amount of CO2 in the atmosphere as a result of the human-caused burning of gas, oil and coal.
Big trees such as castanheiras (Brazil nut trees), sumaúmas (kapoks) and Angelim vermelho can rise up above 30 metres so they dominate the canopy and lead the competition among plants for the light, water and nutrients needed to photosynthesise CO2.
The results highlight how the best-protected areas of the forest remain important allies in the struggle to stabilise the climate, though they alone are far from enough to deal with all of the extra carbon dioxide being pumped into the air globally by cars, factories and power stations.
They also show the importance of preventing further deforestation. The areas of the Amazon that have been heavily fragmented by agribusiness and infrastructure – mostly in the south-east – have already shifted from carbon sinks to carbon sources.
Big trees are more vulnerable to strong wind, fire and drought if they are isolated or left on the edge of a border between forest and cropland or pasture.
For decades, it had been assumed that huge trees – even in dense forest – would die more quickly as temperatures rose, because they were thought to have relatively shallow roots compared to their size. But recent studies show that many have very thin roots that can suck water from much greater depths.
It is not yet clear how mortality trends might have been affected by the severe drought of the past few years in much of the Amazon.
One of the lead authors of the new paper, Adriane Esquivel-Muelbert from the University of Cambridge, said the big trees were disproportionately important to the functioning of the biome. Although they account for just 1% of the trees in the forest, they were responsible for 50% of the carbon cycling and storage, and probably a similar proportion of the water cycling.
“Forests get much drier after big trees die. It alters the structure of the forest and it takes a long time to bounce back because the big trees take a long time to grow,” Esquivel-Muelbert said. “So it is a little bit hopeful that we find them increasing in size and number across the Amazon. There is some resilience there.”
