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Australian tropical rainforest trees have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, due to rising heat extremes and drier conditions.

Critical Change Identified

This crucial shift, which impacts the stems and limbs of the trees but excludes the underground roots, began approximately 25 years ago, according to recent research.

Forests typically absorb carbon as they develop and release it upon decay and death. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to grow with higher CO2 levels.

However, nearly 50 years of data gathered from tropical forests across Queensland has revealed that this essential carbon sink could be under threat.

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests turned into a carbon source, with increased tree mortality and insufficient new growth, according to the research.

“It’s the first tropical forest of its kind to display this sign of change,” commented the lead author.

“It is understood that the moist tropics in Australia exist in a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests worldwide, and further research are needed.

But should that be the case, the results could have significant implications for international climate projections, carbon budgets, and environmental regulations.

“This paper is the first time that this critical threshold of a switch from a carbon sink to a carbon source in tropical rainforests has been definitively spotted – not just for one year, but for two decades,” remarked an authority on climate science.

On a global scale, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the coming years. “This is concerning,” it was noted.

Ongoing Role

Even though the balance between gains and losses had changed, these forests were still serving a vital function in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an even more rapid transition away from fossil fuels.

Research Approach

The analysis utilized a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across 20 forest sites. It focused on the carbon stored above ground, but not the changes in soil and roots.

Another researcher emphasized the value of gathering and preserving long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But looking at these decades of recorded information, we find that is not the case – it allows us to confront the theory with reality and improve comprehension of how these ecosystems work.”