Rainforest trees have been dying faster since the 1980s

Susan Laurance, Professor of Tropical Ecology at James Cook University, was part of a study involving researchers from the Smithsonian Environmental Research Center, Oxford University, and the French National Research Institute for Sustainable Development (IRD).

She said the researchers found the average death rates of tropical trees in Australia have doubled in the last 35 years, as global warming increases the drying power of the atmosphere.

“We found trees are living around half as long, which is a pattern consistent across species and sites across the region. And the impacts can be seen as far back as the 1980s,” said Professor Laurance.

Dr David Bauman is a tropical forest ecologist at the Smithsonian, Oxford and the IRD, and lead author of the study.

“It was a shock to detect such a marked increase in tree mortality, let alone a trend consistent across the diversity of species and sites we studied. A sustained doubling of mortality risk would imply the carbon stored in trees returns twice as fast to the atmosphere,” said Dr Bauman.

Dr Sean McMahon, Senior Research Scientist at Smithsonian and senior author of the study said “Many decades of data are needed to detect long-term changes in long-lived organisms, and the signal of a change can be overwhelmed by the noise of many processes.”

The scientists said that one remarkable result of the study is that the increase in mortality seems to have started in the 1980s, indicating the Earth’s natural systems may have been responding to changing climate for decades.

Oxford Professor Yadvinder Malhi, a study co-author said“In recent years the effects of climate change on the corals of the Great Barrier Reef have become well known.

“Our work shows if you look shoreward from the Reef, Australia’s famous rainforests are also changing rapidly. Moreover, the likely driving factor we identify, the increasing drying power of the atmosphere caused by global warming, suggests similar increases in tree death rates may be occurring across the world’s tropical forests.

“If that is the case, tropical forests may soon become carbon sources, and the challenge of limiting global warming well below 2 °C becomes both more urgent and more difficult,” said Professor Malhi.

“Long-term datasets like this one, which looked at more than 8300 trees over 50 years’ of data in 24 forests, are very rare and very important for studying forest changes in response to climate change. This is because rainforest trees can have such long lives and also that tree death is not always immediate,” said Professor Laurance.

Associate Professor Lucas Cernusak from James Cook University was a co-author of the study. He said recent studies in Amazonia have also suggested tropical tree death rates are increasing, thus weakening the carbon sink, but the reason is unclear.

“Intact tropical rainforests are major stores of carbon and until now have been ‘carbon sinks’, acting as moderate brakes on the rate of climate change by absorbing around 12% of human- caused carbon dioxide emissions,” said Dr Cernusak.

The team believe the main climate driver is the increasing drying power of the atmosphere. As the atmosphere warms, it draws more moisture from plants, resulting in increased water stress in trees and ultimately increased risk of death.

When the researchers crunched the numbers, it further showed the loss of biomass from this mortality increase over the past decades has not been offset by biomass gains from tree growth and recruitment of new trees.

“This implies the mortality increase has translated into a net decrease in the potential of these forests to offset carbon emissions,” said Dr Cernusak.

A link to the paper can be found at: https://www.nature.com/articles/s41586-022-04737-7