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

Critical Change Identified

This significant change, which affects the trunks and branches of the trees but does not include the root systems, began approximately a quarter-century back, according to recent research.

Trees naturally store carbon during growth and emit it when they decompose. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to grow with rising atmospheric concentrations.

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

Study Insights

Approximately 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to show this symptom of change,” stated the principal researcher.

“We know that the moist tropics in Australia occupy a somewhat hotter, arid environment than tropical forests on other continents, and therefore it could act as a coming example for what tropical forests will encounter in other parts of the world.”

Worldwide Consequences

One co-author mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are required.

But if so, the results could have significant implications for international climate projections, CO2 accounting, and climate policies.

“This research is the first time 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 share of carbon dioxide absorbed by forests, trees, and plants has been quite stable over the last 20 to 30 years, which was assumed to continue under many climate models and strategies.

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 equilibrium between growth and decline had shifted, these forests were still serving a vital function in absorbing carbon dioxide. But their reduced capacity to absorb extra carbon would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

The analysis drew on a distinct collection of forest data starting from 1971, including records tracking approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots.

An additional expert emphasized the value of collecting and maintaining extended datasets.

“It was believed the forest would be able to store more carbon because [CO2] is rising. But examining these long term empirical datasets, we discover that is incorrect – it allows us to compare models with actual data and better understand how these systems work.”