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This “accelerated mortality” has profound impacts for climate change, as the rainforests could release more carbon dioxide into the air then they absorb.
Lead author of the study, Dr David Bauman, a tropical forest ecologist, said he was “shocked” by the findings.
“A sustained doubling of mortality risk would imply the carbon stored in trees returns twice as fast to the atmosphere.”
Co-author of the study, Associate Professor Lucas Cernusak from James Cook University, explained the rainforests act as “carbon sinks”.
“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 per cent of human-caused carbon dioxide emissions,” Cernusak said.
“This implies the mortality increase has translated into a net decrease in the potential of these forests to offset carbon emissions.”
The study examined 8300 trees across 24 North Queensland forests and found the trees could have been responding to climate change for decades.
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Professor of Tropical Ecology at James Cook University Susan Laurance was involved in the study and said “the impacts can be seen as far back as the 1980s”.
The CSIRO has been monitoring the tree plots for the study since 1971.
Others included the Great Barrier Reef in Queensland and alpine regions across Victoria, NSW and Tasmania.
Distinguished Professor Bill Laurance, the director of the Centre for Tropical Environmental and Sustainability Science at James Cook University, said similar trends are being seen in rainforests overseas.
“We’ve found similar trends in the Amazon basin, where rates of tree death have also risen markedly in recent decades.
“Sadly, it’s really not that hard to kill a rainforest tree – just warm things up a bit and quite a few species will just drop their leaves and die standing.”