Unexpected consequences from catastrophic mangrove dieback

Mangrove dead tree stemMeasuring methane emissions from a forest of dead mangroves in the Gulf of Carpentaria (credit: Luke Jeffrey).

When swathes of mangrove forests died along a 1000 kilometre stretch of coastline in northern Australia’s Gulf of Carpentaria, there was widespread shock.

Mangrove dead v living methane

Figure: Conceptual summary of dead vs living mangrove forest. Full details at end of article.

But the impacts of the catastrophic climate-induced mangrove dieback didn’t end there. In a world first, researchers from Southern Cross University have found that the dead trees released significant amounts of the potent greenhouse gas methane.

The results, published in a leading international journal New Phytologist, revealed that while living mangroves emit some methane, dead mangroves emit about eight times more.

Methane emissions from mangrove tree-stems have never been quantified until now.

“The findings were a surprise,” said lead author and PhD candidate Luke Jeffrey. “Currently very little is known about the role of tree-stem methane emissions globally and quantifying these from mangrove tree-stems has never been attempted.

“Due to the unique nature of the dieback event, we were able to compare methane tree-stem emissions from living and dead mangrove forests. This allowed us to understand what happens when climatic-change stressors result in forest mortality.

“What was concerning was that the dead mangrove forest emitted about eight times more methane than the living forest.”

The findings have implications for scientific understanding of how mangrove systems sequester ‘blue carbon’, which is carbon dioxide absorbed from the atmosphere and locked up in coastal wetlands such as mangroves.

“As the climate changes into the future, we may see events like the catastrophic dieback of mangroves in the Gulf of Carpentaria becoming the norm. This has significant implications for greenhouse gas emissions from these valuable coastal habitats,” Mr Jeffrey said.

This study completes the final chapter of Luke Jeffrey’s PhD thesis with Southern Cross University and was funded by the Australian Research Council. His thesis is entitled ‘The drivers and dynamics of methane and carbon dioxide in disturbed coastal wetlands’.

Figure: Conceptual summary of dead vs living mangrove forest, showing the up-scaled methane (CH4) emissions for mortality event area (Megagrams (Mg) CH4/ year), average tree CH4 emissions (μmol/tree/day), average tree stem CH4 emissions rate (μmol/m2 stem/day) and sediment CH4 emissions (μmol/m2/ day). Note: The ± standard error on the average fluxes and * indicates living mangrove tree-stem CH4 emissions up-scaled to the dieback area for healthy mangrove system comparison.

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