Queensland Rainforest Trees Switch from CO2 Absorber to Emissions Source in World First

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

The Tipping Point Discovered

This significant change, which impacts the trunks and branches of the trees but does not include the underground roots, started around a quarter-century back, according to new studies.

Trees naturally store carbon during growth and emit it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this absorption is expected 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 may be at risk.

Research Findings

Roughly 25 years ago, tree stems and limbs in these forests became a net emitter, with more trees dying and inadequate regeneration, as the study indicates.

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

“We know that the humid tropical regions in Australia exist in 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 other parts of the world.”

Worldwide Consequences

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

But should that be the case, the findings could have significant implications for global climate models, carbon budgets, and climate policies.

“This research 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 just for one year, but for two decades,” remarked an expert in climate change science.

Worldwide, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was expected to persist under many climate models and policies.

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,” it was noted.

Continued Function

Even though the equilibrium between growth and decline had shifted, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an even more rapid shift from carbon-based energy.

Research Approach

This study drew on a distinct collection of forest data dating back to 1971, including records tracking roughly 11,000 trees across 20 forest sites. It considered the carbon stored above ground, but excluded the gains and losses in soil and roots.

Another researcher highlighted the importance of gathering and preserving long term data.

“We thought the forest would be able to store more carbon because [CO2] is rising. But examining these long term empirical datasets, we find that is incorrect – it enables researchers to confront the theory with reality and better understand how these systems work.”