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Australian tropical rainforest trees have achieved a global first by transitioning from acting as a carbon sink to becoming a source of emissions, driven by increasingly extreme temperatures and drier conditions.

Critical Change Discovered

This significant change, which affects the trunks and branches of the trees but excludes the root systems, started around a quarter-century back, according to new studies.

Forests typically absorb carbon as they develop and release it when they decompose. Generally, tropical forests are regarded as carbon sinks – taking in more carbon dioxide than they release – and this uptake is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across northern Australia has revealed that this essential carbon sink could be under threat.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests turned into a carbon source, with more trees dying and inadequate regeneration, as the study indicates.

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

“We know that the moist tropics in Australia exist in a somewhat hotter, arid environment than tropical forests on other continents, and therefore it might serve as a future analog for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

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

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

“This research is the initial instance that this tipping point 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,” stated an authority on climate science.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under numerous projections and strategies.

But if similar shifts – from absorber to emitter – were detected in other rainforests, climate forecasts may understate heating trends in the coming years. “Which is bad news,” he added.

Ongoing Role

Even though the equilibrium between growth and decline had shifted, these forests were still playing an important role in absorbing carbon dioxide. But their reduced capacity to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

This study drew on a unique set of forest data dating back to 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots.

An additional expert emphasized the importance of gathering and preserving extended datasets.

“We thought 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 enables researchers to confront the theory with reality and better understand how these systems work.”