'They died of thirst': Extreme conditions wipe out 1000km of Australian mangrove forest

The death of so much mangrove forest in one hit is "unprecedented", a researcher says.
NORMAN DUKE

The death of so much mangrove forest in one hit is "unprecedented", a researcher says.

The death of mangrove forests stretched over 1000 kilometres of Australia's northern coast a year ago has been blamed on extreme conditions including record temperatures.

About 7400 hectares of mangroves strung along the Gulf of Carpentaria died in early 2016 because of the unusual warmth, a prolonged drought and an El Nino that reduced local sea levels by about 20 centimetres, said Dr Norman Duke, head of the Mangrove Research hub at James Cook University.

"Essentially, they died of thirst," Duke said, adding that the sea-level drop triggered a "highly significant loss of tidal waters".

Before and after photograph of the massive dieback along the Gulf of Carpentaria.
NORMAN DUKE

Before and after photograph of the massive dieback along the Gulf of Carpentaria.

El Nino events are marked by a stalling or reversal of the easterly equatorial winds that would typically build up waters in the western Pacific. Still, previous El Ninos had not produced the huge death rate of mangroves as seen last year.

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The mangrove wipeout could have multiple impacts, including the loss of fisheries worth hundreds of millions of dollars ...
NORMAN DUKE

The mangrove wipeout could have multiple impacts, including the loss of fisheries worth hundreds of millions of dollars and more coastal erosion because of the loss of forest protection.

Duke said scientists now know that mangroves, much like coral reefs, are vulnerable to a warming climate and extreme weather events. Until now, Australian mangroves were considered to be in relatively good condition.

The mangrove wipeout could have multiple impacts, including the loss of fisheries worth hundreds of millions of dollars, more coastal erosion because of the loss of forest protection, and poorer water quality given the filtering role the trees play, he said.

Scientists examined the dead trees for signs of a plant pathogen but found the impacts to be widespread across the 20-odd mangrove species. They were also not confined to pockets of plants that might point to a culprit other than extreme weather.

The dieback of trees took four to five months to become apparent, and even then the damage gained little attention given the region's remoteness from population centres. The collapse of the important kelp forests off the Tasmanian coast in recent years is another instance of rapid ecological change largely out of the public view.

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The Gulf dieback has been a wake-up call for action on shoreline monitoring," Duke said. "We urgently need a national shoreline monitoring program commensurate with our global standing."

Leading specialists and managers will hold a workshop during next week's Australian Mangrove and Saltmarsh Network annual conference in Hobart to press for such monitoring to be set up.

Duke's research in the 2016 dieback will be published in the Journal of Marine and Freshwater Research on Tuesday.

 - SMH

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