Report Finds That Mangroves Can Cope With Sea Level Rise

A new report by The Nature Conservancy and Wetlands Intl. 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 Intl. 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.

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,” said report lead author Dr. Anna McIvor of The Nature Conservancy. “In these areas, local planners should allow space for mangroves to colonize 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 8 meters 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 defense, but also fisheries and carbon storage.”

Source: The Nature Conservancy, Wetlands Intl.