Victoria’s mountain ash forests are thinning rapidly as the globe heats up, and could lose a quarter of their “giant” trees that grow up to 80m tall in the coming decades, research has found.
Forests of Eucalyptus regnans – one of the tallest tree species in the world – lose about 9% of their trees for every degree of warming, according to a University of Melbourne-led study published in Nature Communications.
“These are giants,” said lead author Dr Raphael Trouve. “They are the tallest flowering plant on Earth – that means they regularly reach 60 to 80m tall.”
The researchers analysed data collected from mountain ash forests over more than 50 years to determine tree mortality rates and forest carrying capacity – the maximum number of trees of a given size that the forest can sustain.
They found that forests growing in the warmest conditions had the lowest carrying capacity, which further decreased with rising temperatures.
Eucalyptus regnans lose about 9% of their trees for every degree of warming, research has found. Photograph: University of Melbourne
“We found that for each extra degree of temperature, the number of trees that the forest can sustain drops by 9%,” Trouve said. “By 2080 – with three extra degrees, as we expect – that tallies up to around a quarter of the trees gone.”
The estimated forest loss did not include the additional impact of bushfires, which are expected to grow in severity as the planet warms up.
The forest thinning was thought to be the result of increased competition for limited resources. “A growing tree needs space and resources to survive,” Trouve said. “Under resource-limited conditions, such as water stress, a big tree will outcompete smaller, surrounding trees, causing their deaths.”
Changes in forest carrying capacity would probably have knock-on effects. “Its not just about trees, it’s about the carbon they store, the planet we need, the water we drink,” he said.
Mountain ash forests are considered one of the Earth’s most carbon-dense ecosystems, storing more carbon per hectare than the Amazon. But as more trees died and decomposed, the forests would eventually shift from carbon sinks into sources of emissions, the study said.
skip past newsletter promotion
Sign up to Clear Air Australia
Adam Morton brings you incisive analysis about the politics and impact of the climate crisis
Privacy Notice: Newsletters may contain info about charities, online ads, and content funded by outside parties. For more information see our Privacy Policy. We use Google reCaptcha to protect our website and the Google Privacy Policy and Terms of Service apply.
after newsletter promotion
“Climate change is stressing forests all over the world, and that can be exacerbating the rate of thinning” said Dr David Bowman, a professor of fire science at the University of Tasmania with a background in eucalypt ecology, who was not involved in the study.
Bowman said climate change – and the combination of heatwaves, rising temperatures and chronic droughts – was placing forests under enormous stress globally, and particularly affected those in mid-latitude, temperate environments.
Giant trees were particularly vulnerable. “They’re getting exhausted,” he said.
Then, as forests were dying and thinning, they became more open and more flammable, as fuel – in the form of leaf matter, litter, fine branches and stems – accumulated in the forest.
The risk of bushfire then became a “nightmare scenario” for large trees – already struggling to survive – which would struggle to recover.
