Rainforest trees have been dying faster since the '80s

Wednesday, 22 June, 2022

Rainforest trees have been dying faster since the '80s

Tropical trees in Australia’s rainforests have been dying at double the previous rate since the 1980s, seemingly because of climate impacts, according to a long-term international study published in the journal Nature.

Susan Laurance, Professor of Tropical Ecology at James Cook University (JCU), was part of a study involving researchers from the Smithsonian Environmental Research Center, the University of Oxford 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.

“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,” Laurance said.

“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.” This indicates that the Earth’s natural systems may have been responding to changing climate for decades.

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

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.

“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,” said study co-author Professor Yadvinder Malhi, from the University of Oxford.

“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.”

Study co-author Associate Professor Lucas Cernusak, from JCU, referred to recent studies in Amazonia that have also suggested tropical tree death rates are increasing, thus weakening the carbon sink.

“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,” Cernusak said.

When the researchers crunched the numbers, they 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,” Cernusak said.

Image credit: ©stock.adobe.com/au/Greg Brave

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