On some Australian islands, sea level rise may be helping mangroves thrive

The geologic history of the Howick Islands might benefit mangroves as oceans rise

An aerial photo of an island with mangroves visible in a bright blue body of water.

Australia's Howick Islands (one pictured) were built on top of ancient coral reef debris and are now home to a diverse mangrove forest ecosystem.

Sarah Hamylton

Some mangrove forests off Australia have flourished in the last several decades, a new study reports. And, counterintuitively, rising sea levels may be responsible.

Off Australia’s northern coast, the skeletal remains of ancient coral reefs form the bedrock of numerous wooded islands. These low-lying tropical oases are home to diverse animals and plants, including mangrove forests that pepper their coasts and serve as vital habitat and carbon storers. A recent survey of one cluster of those islands — the first in 50 years — shows that swelling seas might have led to a massive mangrove expansion, researchers report November 1 in Proceedings of the Royal Society B.

In other parts of the world, rising seas have put mangroves at risk (SN: 6/4/20). But at the Howick Islands in the Great Barrier Reef, the story is different because of its unique geologic history.

“We usually are focusing on areas of mangrove loss,” says Temilola Fatoyinbo, a forest ecologist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not involved with the study. “So it’s always encouraging to see areas where there’s mangrove gain.”

Mangroves — groupings of different plants that have adapted to thrive along coastlinessoak up carbon dioxide and store it as “blue carbon,” a term for carbon that is sequestered in marine environments (SN: 9/14/22; SN: 11/18/21).

“There’s a lot of interest in using mangrove blue carbon to mitigate climate change,” says Kerrylee Rogers, an environmental scientist at the University of Wollongong in Australia. “But there remains a lot of questions around their capacity to adapt to sea level rise.”

In 2021, a team led by Wollongong environmental scientist Sarah Hamylton visited the Howick Islands to see how the mangroves there are coping with sea level rise. Hamylton flew a drone over the mangroves to capture aerial imagery, while others walked through the brine to assess the plant diversity and measure individual trees. Using the measured widths and heights of several mangroves, the team extrapolated tree widths for the rest of the forest from the drone’s height data to estimate the total mangrove biomass.

The islands host nearly 54,000 metric tons of mangroves, the team estimates, which is roughly 10,000 more metric tons than was there in 1973. The forest’s reach has also expanded on many islands. While about 25 hectares of every 100 on Newton Island were covered by mangroves in 1973, for example, the forest now blankets close to 40 hectares out of 100.

A photo of several mangroves growing in the foreground while white ships are visible in the background.
Mangrove cover on Bewick Island (pictured) has grown by 50 percent since 1973. Researchers attribute this expansion to rising sea levels, which opens new habitat for the coast-loving plants.Oxana Repina

The Howick Islands are uniquely suited to supporting mangroves as the ocean rises. At the end of the Last Glacial Maximum, around 12,000 years ago, water levels rose around northern Australia, and coral reefs grew upward to fill the space that had opened for them. When sea levels fell thousands of years later, the exposed reefs eroded into sediment. With sea levels now rising again, the researchers suspect the mixture of saltwater and sediment makes a perfect home for the salt-tolerant mangroves (SN: 9/29/22).

“As sea level came back down, it wasn’t suitable mangrove habitat,” Rogers says. With those parts of the island now submerged, “it now is suitable again, and it’s largely because it was set up for those conditions [2,000] to 4,000 years ago.”

The new findings highlight the need for mangrove research on a local scale, Rogers says. “In a global model, this would get lost.” Fatoyinbo agrees. “Studies on the local scale are really useful,” she says, “to better understand big patterns.”

Rogers and Hamylton are now working on a bigger effort to study mangroves around Australia. “If we’re going to invest in mangroves … to provide us blue carbon and to protect shorelines, we need to understand how dynamic they are,” Rogers says. “And then get some more understanding of how they’re going to adapt to sea level in the future.”

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