Extract from ABC News
More trees, including in the cool temperate forests of Tasmania, are dying naturally as global temperatures rise from climate change. (Getty Images: Leisa Tyler)
In short:
The mortality rate of trees in forests from Tasmania to the Top End is increasing as the world warms, according to a new study.
Scientists suspect this rising death rate is part of a global trend but are unsure exactly how heat kills trees in cold and tropical climates.
What's next?
Ongoing research could be hampered as state government agencies pull back from on-ground monitoring at long-term study sites.
Australia's trees must contend with many lethal factors, from intense megafires to introduced diseases and invasive species.
But beyond these specific pressures, new research indicates the underlying natural death rate of trees in major forests across the country is rising.
This increase in tree deaths is due to higher average temperatures from climate change, according to a study published in the journal Nature Plants, and it has scientists concerned that forests will sequester less carbon dioxide in years to come.
The world's average temperature has warmed about 1.3 degrees Celsius compared to an 1850–1900 pre-industrial baseline.
Senior study author and plant physiological ecologist Belinda Medlyn, from Western Sydney University, said the research team was "startled" to see tree death rates, from cool temperate forests in southern Tasmania up to the savannas of the tropical north, steadily increase over the past six decades.
"We [in Australia] have a much more variable climate than many other parts of the world," she said.
"We get regular long droughts, so the forests are adapted to those tough conditions.
"Seeing this increase in mortality over time … suggests that it is really a global phenomenon, that we are seeing changes to forest function."
Pattern repeated across forest types
Every year, a percentage of trees in a forest die from natural causes. This is known as the mortality rate, and it can differ between forest types.
To establish mortality rates for Australian forests, Professor Medlyn and her colleagues examined 83 years' worth of state government and independent data from 203,721 individual trees, representing 958 species across 2,724 sites in the eastern states, the NT, ACT, and northern WA.
Monitoring sites in South Australia were not large enough to be included in the study, and the researchers could not access WA government data for south-western forests.
A new study has found tree mortality rising across major Australian forests. (Getty Images: Mark Evans)
Sites that were included were broken down into four environment types:
- tropical savannas in the north
- Queensland's tropical rainforests
- warm temperate forests in Queensland and NSW
- cool temperate forests from Tasmania to NSW
Professor Medlyn said typically trees in each site would be visited every five years or so by whoever was monitoring that plot.
Data would then be collected about how much a tree had grown or if it had died.
The new analysis looked at 26,227 documented tree deaths and excluded trees killed by factors like dieback or bushfire.
The researchers found that between 1963 and 2020, the tree mortality rate more than doubled in tropical rainforests — from 0.5 per cent to 1.3 per cent.
Tropical savannas saw a jump in annual tree deaths from about 1.5 per cent in 1996 to 2.7 per cent in 2017.
Warm temperate forest mortality rates more than tripled from 0.2 per cent in 1943 to 0.7 per cent in 2018, while the tree death rate for the cool temperate zone increased from 0.4 per cent in 1966 to 0.7 per cent in 2019.
Small change, big effects
Professor Medlyn said the rises in mortality rates might seem small, but they show a fundamental part of forest function has been changing.
"The implication will be that you've got a reduction in the amount of carbon that will be stored because we're turning over trees faster than we used to," she said.
"We're not going to lose all of our forests by any means, but there's clearly really quite significant change underway."
Australian National University forest ecologist David Lindenmayer, who was not involved in the study, said temperatures were rising rapidly in Australia, and climate change was a clear and present danger right now.
"The accelerated rates of mortality will have major implications for the integrity of ecosystems, levels of carbon storage, and suitability of ecosystems for many elements of biodiversity," Professor Lindenmayer said.
"The changes in temperature are driving profound changes in forest structure via accelerated mortality — the addition of altered fire regimes will only make it worse."
Tropical rainforests have seen a 153 per cent increase in natural tree deaths per year compared to 1963. (Getty Images: Mark Kolbe)
Professor Medlyn said higher tree mortality could also mean we've been overestimating the amount of carbon stored by forests in revegetation or carbon credit schemes.
University of Melbourne political geographer Kate Dooley, who was not involved in the study, said forests could not be relied on to offset ongoing fossil fuel emissions.
"If Australia's climate targets are to genuinely contribute to limiting warming, carbon removals from forest restoration need to be accounted for separately from reductions in fossil fuel emissions."
Dr Dooley said forest management in Australia needed to monitor for a range of ecological traits to inform its biodiversity and climate goals "rather than the current reliance on modelled carbon sequestration rates [that] encourage reforestation over forest protection".
How heat kills trees is unclear
The study links temperature rises to more tree deaths, but exactly how heat kills trees is not known.
Professor Medlyn said several hypotheses need to be tested.
These included increased water stress, or whether frequent heatwaves cause cumulative damage that eventually kills trees.
"If we can understand what that [death] mechanism is, then that does give us the scope to start thinking about ... the ways that we could potentially adapt forests," Professor Medlyn said.
But some avenues for ongoing research into how our trees are affected by climate change may soon be closed off.
Trees in tropical savannas, like WA's Kimberley region, have some of the highest natural mortality rates found in Australia. (Getty Images: Auscape)
Much of the data used in the mortality study came from state governments, which originally set up monitoring sites as part of native forestry operations — an industry which has either been banned or is shutting down in several states.
Professor Medlyn said many research sites had now been abandoned, and remote sensing technology such as satellites can't capture tree deaths yet.
"There's been a radical reduction in the number of plots that have been monitored," she said.
"The warm temperate forests are the worst. We've gone from 900 plots down to less than 100.
"And that really concerns me because this is the type of data that really allows us to see what's happening in forests."
There are concerns from scientists that too many monitoring sites for trees are being abandoned in warm temperate forests. (Getty Images: Jakub Maculewicz)
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