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Australian tropical rainforest trees have achieved a global first by shifting from serving as a CO2 absorber to turning into a carbon emitter, due to rising heat extremes and drier conditions.

Critical Change Discovered

This crucial shift, which impacts the stems and limbs of the trees but excludes the root systems, 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 regarded as carbon sinks – taking in more carbon dioxide than they emit – and this uptake is assumed to increase with rising atmospheric concentrations.

However, close to five decades of data collected from tropical forests across Queensland has shown that this vital carbon sink may be at risk.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with increased tree mortality and inadequate regeneration, according to the research.

“This marks the initial rainforest of its kind to show this symptom of transformation,” 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 coming example for what tropical forests will experience in other parts of the world.”

Global Implications

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests worldwide, and further research are required.

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

“This research is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not merely temporarily, but for 20 years,” remarked an authority on climate 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 expected to persist under many climate models and policies.

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

Continued Function

Although the balance between growth and decline had shifted, these forests were still serving a vital function in soaking up CO2. But their diminished ability to take in additional CO2 would make emissions cuts “more challenging”, and necessitate an even more rapid transition away from fossil fuels.

Data and Methodology

The analysis drew on a unique set 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 below ground.

Another researcher highlighted the importance of collecting and maintaining long term data.

“It was believed the forest would be able to absorb additional CO2 because [CO2] is rising. But examining these long term empirical datasets, we find that is incorrect – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”