Catastrophic mangrove dieback in Australia

Unexpected consequences from catastrophic mangrove dieback – “What was concerning was that the dead mangrove forest emitted about eight times more methane than the living forest”

Desdemona Despair,

9 July, 2019

4 July 2019 (Southern Cross University) – When swathes of mangrove forests died along a 1000 kilometre stretch of coastline in northern Australia’s Gulf of Carpentaria, there was widespread shock.

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’.

Contact

• Jessica Nelson, 0417 288 794, jessica.nelson@scu.edu.au

Unexpected consequences from catastrophic mangrove dieback

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Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality

ABSTRACT: Growing evidence indicates tree‐stem methane emissions may be an important and unaccounted for component of local, regional and global carbon budgets. Studies to date have focussed on upland and freshwater swamp‐forests; however, no data on tree‐stem fluxes from estuarine species currently exist. Here we provide the first‐ever mangrove tree‐stem methane flux measurements from >50 trees (n=230 measurements), in both standing dead and living forest, from a region suffering a recent large‐scale climate‐driven dieback event (Gulf of Carpentaria, Australia). Average methane emissions from standing dead mangrove tree‐stems were 249.2 ± 41.0 μmol m−2 d−1 and eight‐fold higher than from living mangrove tree‐stems (37.5 ± 5.8 μmol m−2 d−1). The average methane flux from tree‐stem bases (~10cm above ground) was 1071.1 ± 210.4 μmol m−2 d−1 and 96.8 ± 27.7 μmol m−2 d−1 from dead and living stands respectively. Sediment methane fluxes and redox potentials did not differ significantly between living and dead stands. 

Our results suggest both dead and living tree‐stems act as methane conduits to the atmosphere, bypassing potential sedimentary oxidation processes. Although large uncertainties exist when up‐scaling data from small‐scale temporal measurements, we estimated that dead mangrove tree‐stem emissions may account for ~26% of the net ecosystem methane flux.

Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality