Wetlands International

Mangroves can cope with sea level rise by increasing soil height

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Source: Wetlands International

A new report by The Nature Conservancy and Wetlands International shows that mangroves can adapt to rising sea levels by building up soils in some locations, allaying fears that mangroves may be lost as sea levels rise. This is important because mangroves provide risk reduction services against coastal hazards such as waves and storm surges.

Mangroves can protect human lives and property by reducing the impacts of storm surges and waves. However, a major concern has been that mangroves may be lost as sea levels rise, leaving communities more exposed to coastal hazards. This review conducted by The Nature Conservancy and Wetlands International shows that under some circumstances, mangrove soils can build up at similar rates to local rises in sea level, allowing mangroves to survive in situ.

“We need to understand how mangrove soils build up, so that we can maintain suitable conditions for them to do so into the future,” says Nyoman Suryadiputra, Director of Wetlands International Indonesia. “Protecting mangroves is vital for many coastal communities, who rely on them for their livelihoods as well as the coastal defence benefits they provide.”

Past evidence suggests that some mangrove soils have built up at rates between 1 and 10 mm per year. Currently global mean sea level is rising at a rate of 3 mm per year. This suggests that in some places, mangrove soil surfaces will be able to keep up with rising sea levels. This is key, as mangrove trees cannot survive if their aerial roots are submerged for a long period of time.

This latest report highlights the need to maintain, restore or enhance sediment supplies to mangrove areas. The sediments contribute to the build-up of soils, but the supply of sediment to many mangrove areas has been reduced because of dams built on rivers. Mangroves also need protecting from pollution and the felling of trees: the underground roots of healthy trees can push the soil up, while the roots of trees weakened by habitat degradation are less able to hold soils together, potentially leading to erosion and loss of surface soils. Restoring mangrove areas and safeguarding the health of trees can help mangrove soils to build up and so keep pace with sea level rise.

“In some areas, however, mangrove soils may not be able to build up fast enough to keep pace with sea level rise”, alerts report lead author Dr Anna McIvor of The Nature Conservancy. “In these areas, local planners should allow space for mangroves to colonise landward areas as sea levels rise. This will help to ensure that mangroves continue to reduce risks from coastal hazards into the future, benefiting local communities”.

Some mangrove forests have survived in the same location for thousands of years by building up soils beneath them as sea levels rose. In Twin Cays in Belize, mangroves have created a layer of old roots and sediments that is up to eight metres thick in some places. By building up soils, mangroves also help to lock up greenhouse gases such as carbon dioxide, and this provides another reason for protecting mangroves and their soils from degradation and loss.

Dr Mark Spalding, senior marine scientist at The Nature Conservancy said, “It is essential that we protect mangrove forests as they provide many vital services, not just coastal defence, but also fisheries and carbon storage.”

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