Heat-stressed Australian forests are thinning fast, producing carbon emissions

(Press-News.org) Heat-stressed Victorian mountain ash forests are thinning fast, turning from carbon sinks to carbon sources, new research reveals.

Published in Nature Communications, the research shows forests will lose a quarter of their trees by 2080 due to global warning.

Mountain ash forests are currently one of Earth’s most effective ecosystems for storing carbon – they store more carbon per hectare than the Amazon.

But researchers say these forests will store less carbon in the future as warming causes more trees to die and decompose.

Scientists from the Universities of Melbourne and New Hampshire (USA) analysed almost 50 years’ data from Australian forest monitoring plots.

The researchers found that increasing temperatures are thinning mountain ash forests rapidly, threatening their long-term potential to store carbon and slow global warming.

Lead researcher, University of Melbourne Dr Raphael Trouve, explained that the forests’ natural thinning response to temperature stress means that the ability of large-scale tree-planting initiatives to reduce atmospheric carbon levels may decline over the coming decades.

“Australia’s mountain ash forests are one of the Earth's most carbon-dense ecosystems, but our study reveals how climate warming could turn them from carbon sinks into carbon emitters as excess tree deaths and decomposition release stored carbon,” Dr Trouve said.

“Data collected in forest studies since 1947 shows that warming is intensifying competition amongst trees for limited resources – mainly water – and causing around nine per cent tree loss in mountain ash forests for every degree of warming.”

A projected rise of three degrees Celsius by 2080 could reduce tree density in these forests by 24 per cent. Making up for this carbon loss would require establishing hundreds of thousands of hectares of new forests.

“As more trees die and decompose, they will emit carbon dioxide, with an impact equivalent to driving a million cars 10,000km per year for 75 years,” Dr Trouve said. “This predicted forest loss does not include the impact of bushfires, which is also increasing.

“A growing tree needs space and resources to survive. Under resource-limited conditions, such as water stress, a big tree will outcompete smaller, surrounding trees, causing their deaths.”

Dr Trouve said recent research has shown how natural thinning in forests changes streamflow and water yield.

“Natural thinning of the mountain ash forests will likely impact Melbourne’s water supply,” he said.

“One promising management option is reducing stand density: selectively thinning some trees to give others a better chance of survival. This would accelerate the natural self-thinning process and give the rest of the trees more water, nutrients, and space to grow.

“Decades of research around the globe has shown that thinned forests are more resilient to drought, and the trees in them grow faster and survive better during dry periods.”

Native to south-eastern Australia, soaring to over 90 metres, the mountain ash or Eucalyptus regnans is one of the tallest tree species in the world.

“The trend in natural forest thinning may depend on regional climate as well as tree species,” Dr Trouve said.

The Australian Research Council funded the research described in the paper as a Discovery Project entitled Is climate change altering the carrying capacity of the world's forests?

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