As more trees die in the Amazon Basin, the forest’s capacity to absorb carbon dioxide weakens. However, the main drivers of tree death across the region have been largely unknown, until now.
A newly published study in the journal Nature Communications provides insight into why trees die in the Amazon, and why the rate of tree death may be increasing.
According to the study, the main risk factor explaining tree death is the mean growth rate of species. Faster-growing tree species tend to have shorter life spans, and thus record more deaths over a given period.
As climate change progresses, these fast-growing species are faring better. But because they die sooner, they absorb less carbon than a slow-growing, long-lived species. This could spell trouble for the climate, as the Amazon accounts for 12% of the world’s land-based carbon sink.
“The capacity of Amazon to absorb carbon is declining over time. And the main reason is because [tree] mortality is increasing. So, we need to understand mortality. And then that becomes the very pressing question,” Adriane Esquivel-Muelbert, the lead author of the study from the Birmingham Institute of Forest Research, U.K., told Mongabay.
The Amazon grape tree (Pourouma cecropiifoliais) is one of many fast-growing pioneer species. Image courtesy of RAINFOR.
Because most trees live comparatively longer than other living things in the forest, understanding why, how, and where trees die requires a lot of data collected over a long time. For this massive analysis on tree death, more than 124,500 trees and 23,600 tree deaths were monitored and analyzed over a 30-year time span.
“Understanding the main drivers of tree death allows us to better predict and plan for future trends — but this is a huge undertaking as there are more than 15,000 different tree species in the Amazon,” Esquivel-Muelbert said in a statement.
The data used in this study come from RAINFOR, a long-term Amazon monitoring effort, which assembled the research of more than 100 scientists for this analysis. The scientists monitored 189 plots of 1 hectare (2.5 acres) each, and roughly every three years measured all of the trees greater than 10 centimeters (4 inches) in diameter, taking note of the health and appearance of each tree.
Tree monitoring in action on a RAINFOR plot in Peru. Photo courtesy of Roel Brienen.
Dead trees underwent a “forensic exam,” following a protocol that looks at whether the tree died broken or standing, and if there are any visible marks from disease. A fallen, uprooted tree, for instance, likely died in a windstorm. A tree that died standing likely succumbed to physiological factors such as drought or pathogen stress.
“This involves detailed, forensic work and amounts to a massive ‘CSI Amazon’ effort conducted by skilled investigators from a dozen nations,” Oliver Phillips, a professor at the University of Leeds and co-author of the paper, said in a statement.
More than half, 51%, of tree deaths in the study were attributed to structural damage, mostly from windstorms, but different regions of the Amazon showed different risk factors for trees. Overall, the southern and western Amazon had higher mortality rates. Wind seemed to do more damage in the western Amazon, whereas the southern Amazon had more tree deaths due to water stress and drought.
“Across the whole basin there is nowhere where wind is not really important,” Esquivel-Muelbert said, “even in the center of the Amazon, where wind is not the main thing … it’s still a big proportion of the deaths.”
The driest and warmest region of the Amazon is in the south, so the researchers expected the trees there would largely be adapted to dry and hot conditions, Esquivel-Muelbert said. However, the southern Amazon was the only place with an increased risk of tree death related to drought and the drought tolerance of trees, meaning some species here are being pushed to their adaptive limits.
A standing dead tree in the Amazon. Photo courtesy of Adriane Esquivel-Muelbert / RAINFOR.
“It’s worrying that the south is potentially already at risk,” Esquivel-Muelbert said. “But it’s also good to know that the rest of the [Amazon] basin seems to be quite protected … or not yet affected. We couldn’t detect the drought signal yet.”
“Now that we can see more clearly what is going on across the whole forest, there are clear opportunities for action,” Beatriz Marimon, from Mato Grosso State University, who coordinated several research plots in central Brazil for the study, said in a statement. “We find that drought is also driving tree death, but so far only in the South of the Amazon. What is happening here should serve as an early warning system as we need to prevent the same fate overtaking trees elsewhere.”
Esquivel-Muelbert, A., Phillips, O. L., Brienen, R. J. W., Fauset, S., Sullivan, M. J. P., Baker, T. R., … Galbraith, D. (2020). Tree mode of death and mortality risk factors across Amazon forests. Nature Communications, 11(1), 5515. doi:10.1038/s41467-020-18996-3
Banner image of Amazon forest canopy in Peru by Rhett A. Butler/Mongabay.
Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough
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This article was originally published on Mongabay