One of the longest-running scientific studies of tropical forests has revealed rainforest trees are dying twice as fast as they were in the 1970s.
The paper, published in Nature, looked at hundreds of trees in plots across the Australian wet tropics.
From 1971 to 2019, researchers measured every plant in the plots greater than 10 centimetres in diameter, every two to five years.
Co-author, and Professor of Tropical Ecology at James Cook University, Susan Laurance said the data showed mortality rates started to increase in the 1980s.
She said they compared it with local climate conditions and found decreases in air moisture and rising temperatures were likely to be the main culprits, driven by climate change.
"It's always really hard to identify very specific climate signals, but by far the most likely response is going to be the increase in what we call vapour pressure deficit, or the power of the air to dry a surface," Professor Laurance said.
University of Technology Sydney Professor Alfredo Huete, who has also done extensive research on vegetation health and climate change, said it was extremely rare to see a study like this over such a long timescale.
He said the increase in mortality shown in the data was "significant" and "powerful".
He said even gaining consistent access to a variety of sites like these over decades was rare.
What does it mean?
Even with the mortality only increasing from about 1 per cent to 2 per cent, Professor Laurance said the deaths could still have a big impact.
"If you've got an increase in mortality, it means that the trees aren't living longer, and it means they're not sequestering all that carbon that we rely on," she said.
She said it could also mean a change in the composition of plants in rainforests, with more "pioneer" species taking over where older trees had died.
"Rather than sort of those later, mature phase rain forest species, which actually make up most of the tree diversity," Professor Laurance said.
"It's just a general increase in the turnover rates of trees, rather than being them being long lived and stable."
Professor Laurance said there could also be impacts on animal diversity in tropical rainforests.
"What we think of old growth rainforest species, they generally have really large seeds," she said.
"They produce a lot of fruit. They're a really important food source for vertebrate species.
"If there's an increase in other types of species, which don't tend to produce that sort of fruit, then we would expect kind of more probably less fruit production overall.
Professor Huete said it was further evidence of the chronic stress ecosystems are under, and the interaction with extreme weather events.
"When you compound any type of event, whether it's extreme or just a drought, or heat, it's going to yield different results when you subject a forest that's been subjected to long-term increases in the vapour pressure deficit versus a different area that may not have that," he said.
"It's sort of like saying that being a chronic smoker, or being subjected to air pollution all your life, is going to make you more vulnerable to any particular extreme or any kind of stress event."
Around the globe
Professor Laurance said she had conducted a similar study in the central Amazon over 25 years, with similar findings.
"The interesting thing is that all it happened at the same time," she said.
"I actually am really wondering if it is much more of a global phenomena than a local phenomena, but I don't know, that's just me speculating,
According to Professor Huete, Far North Queensland was an excellent sample environment to study what might be happening in tropical regions across the globe, and there was potentially still more to be learned from this data.
A future for the forest
Professor Laurance said if climate change and the associated decreases in air moisture and rising temperatures are not reversed, it could mean rainforests are reduced even further.
"The likely scenario is that there will be areas of the wet tropics, which I can envisage moving more towards a eucalypt woodland, than supporting rainforest," she said.
"Under this scenario, where there's a greater demand for water for trees, it could be that those areas start to shrink away as rainforest and get replaced by one of the types of eucalypt forests that we have here.
"They're much better adjusted for dealing with hotter, drier conditions."
Professor Laurance said this was a strong indication that what was happening on the reef, was also happening in rainforests, but more long-term research was needed to find out if these changes were happening elsewhere.
