Tree-ring study finds heat rise trims lifespan of tropical trees

The planet’s tropical forests are essential for a host of reasons but their ability to store carbon is key in the climate change fight. Now an international group of scientists say there is evidence that rising temperatures will further decrease the lifespan of tropical trees, likely affecting how much carbon the trees can actually hold.

The research work, led by Dr. Giuliano Locosselli at the Institute of Biosciences at Brazil’s University of São Paulo, is based on an examination of tree ring data to measure the age and speed of growth of more than 100,000 trees across the globe. The trees came from 438 different species, with the data collected across four years from 3,343 different sites.

“In the tropics, trees grow, on average, twice as fast as those in cooler regions of the world. But they also have a shorter average lifespan,” says Locosselli. “Our analysis suggests that the lifespans in the tropics will likely decrease further still. If tropical trees die earlier, this will affect how much carbon these forests can hold, raising concerns about the future potential of forests to offset CO2 emissions from fossil fuel burning.

“It could also cause changes in biodiversity and a decrease in the number of species on the planet.”

Average temperatures in tropical rainforests vary between 21°C and 30°C, according to the findings published in the Proceedings of the National Academy of Sciences (PNAS) in the United States. Paper co-authors Dr. Manuel Gloor and Dr. Roel Brienen of the University of Leeds explain that tree lifespans across the tropics will decrease with temperatures above 25.4°C, with increasing heat affecting South American rainforests but also those in West Africa. Dry conditions are expected to affect tree longevity too.

“Many regions in the tropics are heating up particularly rapidly and substantial areas will become warmer, on average, than approximately 25°C,” said Gloor. “Our findings – which are the first to demonstrate that there is a temperature threshold – suggest that for trees in these regions, their longevity is likely to be negatively affected.”

Locosselli, working in South America, notes that Amazon rainforests are already close to the heat threshold while the temperatures in the African rainforests are still lower. That’s likely to change with continued global warming.

“With this great increase in temperature, we might begin to see signs of increased tree mortality,” Locosselli said. “From this point of view, the scenario is quite bleak.”

It’s especially troubling because while tropical rainforests account for only 7 percent of all land, they are home to about half of all animal and plant species, and approximately 50 percent of the forest carbon stocks on Earth. Even small changes in the way tropical forests “work” can significantly change the atmospheric levels of CO2.

“Temperatures will keep rising in the near future even if we were to take drastic emissions reductions measures,” says Dr. Marcos Buckeridge, a colleague of Locosselli who directs the institute at the University of São Paulo.

“Thus it is unavoidable that the critical threshold for tree longevity will increasingly be exceeded in the tropics and thus it is even more important to protect tropical forests and curb greenhouse emissions.”