'They Died Of Thirst': Extreme Conditions Wipe Out Forest Over 1000 Kilometres

Fairfax - Peter Hannam

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 Norman Duke, head of the Mangrove Research hub at James Cook University.

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

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

Before and after photograph of the massive dieback along the Gulf of Carpentaria. Photo: Norman Duke

Dr 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, and there had never been such dieback recorded.
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.

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. Photo: Norman Duke

"The Gulf dieback has been a wake-up call for action on shoreline monitoring," Dr 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.

Dr Duke's research in the 2016 dieback is published in the Journal of Marine and Freshwater Research.

Links

• Neglecting the environment is a bad investment
• Remote mangrove ecosystem suffers 7000 hectare dieback
• Humanity mows down grass with high carbon capture potential
• Mangroves: the unsung star of coastal ecosystems
• Why we need to talk about mangroves