Australia’s tropical rainforest trees have being dying at double the previous rate since the 1980s, seemingly because of global heating, according to new research that raises concerns tropical forests could start to release more carbon dioxide than they absorb.
The study, published in the journal Nature, found the average life of tropical trees in north Queensland had been reduced by about half over the past 35 years . The finding was consistent across different species and rainforests.
Scientists said it indicated natural systems such as rainforests may have already been responding to the climate crisis for decades, and suggested other tropical forests across the globe may be experiencing a similar rise in death rate.
David Bauman, a tropical forest ecologist at the University of Oxford and and the study’s lead author, said it was a shock to detect such a marked increase in tree mortality.
Oxford professor Yadvinder Malhi, a study co-author, compared the changes in Australia’s rainforests to those in corals in the Great Barrier Reef, which have suffered four mass bleaching events over the past seven years.
“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,” he said.
“If that is the case, tropical forests may soon become carbon sources, and the challenge of limiting global warming well below 2C becomes both more urgent and more difficult.”
The study examined data from more than 8,300 trees in 24 north Queensland forests. Much of the data came from a CSIRO lab in Atherton. The lab is focused on tropical forest research and is closing down.
Prof Susan Laurance, a tropical ecology expert at James Cook University and a co-author of the study, said the CSIRO had been monitoring tree plots used for the study since 1971.
“The beauty of this research is that it’s one of a few long-term studies and it’s so hard to get funding to do that,” she said. “It’s a little bit sad because CSIRO was probably the only organisation in the country that was funded long-term to be able to do [that research].”
Russell Barrett, a senior research scientist at the Australian Institute of Botanical Science, said the findings of the study were significant, and should serve as a climate warning “as clear and stark as mass coral bleaching events on the Great Barrier Reef”.
“It is just much harder to see and document,” he said.
He said it could cause a re-think of the potential for forests to store carbon. “A doubling of tree death risk dramatically changes our calculations for the quantity of carbon stored in our forests, and how long it is likely to stay there” he said.
A study in 2020 found tropical forests were taking less carbon from the atmosphere, increasing the likelihood of an accelerated climate breakdown. It pointed to the need to cut carbon-producing activities faster to counteract the loss of carbon sinks.
Barrett said while the study focussed on tropical forests in North Queensland, the drying atmosphere affected all Australian plant communities. It highlighted the need for more studies in a range of habitats, he said.
“This need is especially great for plant communities that are already at the edge of their climatic windows, such as alpine vegetation and wet rainforests,” he said.
Laurence said she would seek funding from the Australian Research Council to continue the research. She hoped to analyse the age of the trees affected and the implications for the ecosystem.
She said if old-growth tropical trees were the most at risk it could affect rainfall patterns.