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What Killed Northern Australia’s Mangroves?

Last year’s massive die-off was the largest ever observed

This image, taken from space last summer, shows a long swath of dead mangroves on Australia's northern coast. (NASA Earth Observatory images by Joshua Stevens, using Landsat data from the U.S. Geological Survey)
smithsonian.com

Australia’s mangroves are tangled trees and shrubs that thrive along the coastline in places other plants can’t survive. But last year, something started happening to huge swaths along the north coast of Australia: They died en masse in such huge numbers that their desiccated roots could be seen from space. What made them die? As the AFP writes, new research has the answer: Thirst.

In a new study in the journal Marine and Freshwater Research, scientists reveal that the mangroves were hit by a triple whammy of below-average rainfall, high temperatures and low sea levels. It all added up to the worst mangrove die-off recorded—an event that affected over 18,000 acres across 621 miles of coastline in Australia’s Gulf of Carpentaria.

The die-off occurred in a remote region that’s sparsely inhabited. It’s also known for being a great place for mangroves. The trees and shrubs are common in relatively hostile coastal areas, and can survive the buffeting of ocean tides, saltwater and hurricanes. The hardy plants filter salt water, which allows them to survive conditions other plants simply couldn’t tolerate.

That filtration helps other species, too. Norm Duke, a mangrove expert who co-authored the paper, told The Guardian’s Michael Slezak last summer that they act like kidneys, filtering and purifying water for marine life like corals and seagrasses. Seagrass, in turn, feeds species like turtles.

Mangroves protect life in other ways. Their root structures keep coastlines from disintegrating during storms, preventing erosions. They also sequester huge amounts of carbon—but scientists warn that, as mangrove forests decline, they will emit a disproportionately large amount of carbon back into the atmosphere.

In a commentary in The Conversation, Duke and co-author Penny van Oosterzee say that they used satellite imagery to confirm the dieback. “The mangroves appear to have died of thirst,” they say. And the fact that the dieback coincided with a huge coral bleaching event in the nearby Great Barrier Reef and extreme climate events makes them suspect that human-caused climate change was at play.

In a release, Duke pleads for better shoreline monitoring in Australia, even in places that are sparsely populated. Australian scientists will meet to discuss the issue at a conference next week. It’s uncertain whether the mangroves will bounce back—but now that scientists know what happened, they can work on ways to respond.

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