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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to becoming a source of emissions, due to rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which affects the stems and limbs of the trees but does not include the underground roots, started around a quarter-century back, according to recent research.

Forests typically absorb carbon during growth and release it upon decay and death. Generally, tropical forests are considered carbon sinks – absorbing more CO2 than they emit – and this uptake is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland has shown that this essential carbon sink could be under threat.

Research Findings

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to show this symptom of change,” stated the lead author.

“We know that the humid tropical regions in Australia occupy a somewhat hotter, arid environment than tropical forests on different landmasses, and therefore it might serve as a future analog for what tropical forests will experience in global regions.”

Worldwide Consequences

One co-author noted that it is yet unclear whether Australia’s tropical forests are a harbinger for other tropical forests globally, and additional studies are required.

But if so, the results could have significant implications for international climate projections, carbon budgets, and environmental regulations.

“This paper is the initial instance that this critical threshold of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for two decades,” remarked an expert in climate change science.

On a global scale, the portion of carbon dioxide taken in by forests, trees, and plants has been quite stable over the past few decades, which was assumed to continue under many climate models and strategies.

But should comparable changes – from absorber to emitter – were observed in other rainforests, climate projections may understate heating trends in the future. “This is concerning,” it was noted.

Ongoing Role

Even though the equilibrium between growth and decline had changed, these forests were still playing an important role in absorbing carbon dioxide. But their diminished ability to absorb extra carbon would make emissions cuts “more challenging”, and necessitate an accelerated shift from carbon-based energy.

Data and Methodology

This study utilized a distinct collection of forest data dating back to 1971, including records tracking roughly 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but excluded the gains and losses below ground.

An additional expert highlighted the importance of collecting and maintaining extended datasets.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But examining these decades of recorded information, we discover that is not the case – it enables researchers to confront the theory with reality and better understand how these systems work.”