Australia’s mountain ash forests—among the most carbon-dense on Earth—are rapidly thinning under climate stress, threatening not only their role as vital carbon sinks but also regional water supplies. Now, researchers say a proactive management strategy known as selective thinning could help slow the decline, boost drought resilience, and safeguard both carbon storage and streamflow.
“Reducing stand density—thinning some trees to give others a better chance—could accelerate the natural self-thinning process,” said Dr Raphael Trouvé, lead author of a new study published in Nature Communications, funded by the Australian Research Council. “Decades of research show that thinned forests are more resilient to drought, and the trees grow faster and survive better during dry periods.”
Trouvé and colleagues from the University of Melbourne and the US-based University of New Hampshire analysed nearly five decades of data from forest monitoring plots across southeastern Australia. Their findings reveal that rising temperatures are intensifying competition for water and nutrients, with larger trees increasingly outcompeting smaller ones—leading to widespread tree loss.
According to the research, thinning has implications beyond just carbon storage. “Natural thinning of the mountain ash forests will likely impact Melbourne’s water supply,” Trouvé warned, citing data on streamflow and water yield.
Mountain ash (Eucalyptus regnans), native to southeastern Australia, can reach heights over 90 metres and stores more carbon per hectare than the Amazon rainforest. But as warming accelerates, the forests’ ability to retain carbon is diminishing. “Our study reveals how climate warming could turn them from carbon sinks into carbon emitters as excess tree deaths and decomposition release stored carbon,” Trouvé said.
The numbers are stark: for every degree of warming, tree density drops by 9%. With global temperatures projected to rise by three degrees by 2080, the forests could lose nearly a quarter of their trees. “As more trees die and decompose, they will emit carbon dioxide, with an impact equivalent to driving a million cars 10,000 kilometres per year for 75 years,” Trouvé explained. “And that doesn’t even account for the increasing threat of bushfires.”
The study highlights how warming intensifies natural thinning, undermining long-term carbon storage. “A growing tree needs space and resources to survive,” Trouvé noted. “Under resource-limited conditions, such as water stress, a big tree will outcompete smaller, surrounding trees, causing their deaths.” To counter this, researchers advocate for selective thinning as a climate adaptation tool. By reducing stand density, the research said, forest managers can help remaining trees thrive—preserving the forests’ critical role in carbon sequestration and water regulation.
For more information, see the official study in Nature Communications: Trouvé, R., Baker, P.J., Ducey, M.J. et al. Global warming reduces the carrying capacity of the tallest angiosperm species (Eucalyptus regnans). Nat Commun 16, 7440 (2025).
