Mangrove forests face a triple threat to their survival

A new study from the University of Exeter has identified a “triple threat” to mangrove forests, which are some of the most valuable ecosystems in the world. The researchers found that the long-term durability and survival of mangroves are threatened by rising sea levels, a lack of mud, and squeezed habitats.

Mangrove forests are extremely productive ecosystems that support high levels of biodiversity and provide coastal protection. In collaboration with experts from across the globe, the Exeter team describes how these coastal forests get pushed against their shores and suffer biodiversity loss.

The research shows that river dams decrease the supply of mud that could otherwise raise mangrove soils, while buildings and seawalls take up critical space that mangrove forests need for survival.

As a natural defense against tidal flooding, mangroves capture mud to raise their soils. However, the mangrove trees cannot protect themselves if they are submerged underwater for too long. This explains why the compounding effects of a dwindling mud supply and sea-level rise pose such a serious threat to mangroves.

The researchers used computer simulations to show how coastal forests retreat landward under sea-level rise, especially in areas along the coast where mud is declining in the water. The simulations accounted for multiple mangrove species, as well as interactions among tides and mud transport.

“Both mangrove coverage loss and diversity loss go hand in hand when that landward retreat is limited by expanding cities, agriculture or flood protection works,” said study co-author Dr. Barend van Maanen.

The model also revealed that mangrove trees with dense roots trap more mud, which can stop it from reaching forest areas further inland.

“This makes the more landward-located trees flood for longer periods of time, an effect that is intensified by sea-level rise,” said study lead author Dr. Danghan Xie. “Increasing landward flooding then seriously reduces biodiversity.”

“Human land use prevents the mangroves ‘escaping’ flooding by migrating inland, narrowing the mangrove zone and further endangering biodiversity.”

Narrow mangrove forests provide much less coastal protection. In some cases, they lose their protective properties altogether.

“The loss of mangrove species will have dramatic ecological and economic implications, but fortunately there are ways to help safeguarding these ecosystems,” explained study co-author Dr Christian Schwarz. 

“It is essential to secure or restore mud delivery to coasts to counter negative effects of sea-level rise. For coasts where mud supply remains limited, removal of barriers that obstruct inland migration is of utmost importance to avoid loss of mangrove forests and biodiversity.”

The study is published in the journal Environmental Research Letters.

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By Chrissy Sexton, Earth.com Staff Writer