Mangroves provide the tropics with protection from the increasing incidence of storms and flooding caused by taiphoons and tsunamis. But the coastal forests are in decline. Efforts are underway to bring them back.
A green forest, rising out of the ocean, hugging the coastline like a green lifebelt - mangroves live only in tropical areas. They need warm conditions and a mixture of salt and fresh water. Their branches and web of aerial roots are home to numerous bird species. The waters around their base are alive with fish, the sediment crawling with crabs.
But since the 1980s, mangroves have declined by around 35 percent for various reasons. They often have to make way for harbors, airports or housing. Even more of the mangroves have been sacrificed for shrimp ponds that feed international demand for the small crustaceans, Ulrich Saint-Paul of the University of Bremen's Leibniz Center for Tropical Marine Ecology, told DW.
By destroying mangroves, tropical countries are paying a high price to consumers in other parts of the world who want cheap shrimp.
"[Mangroves] are not only important breeding areas for crabs and fish. They also protect the coast. They act as barriers against storms. And they play a major role in regulating the world's climate as a CO2 sink," explained Saint-Paul. Long-term objectives such as coastal protection, maintaining ecosystems or protecting the climate are being sacrificed for short-term profits, he added.
Building dams and diverting of rivers also poses a threat to mangroves.
"They need fresh water as well as salty," Rene Capote, who monitored mangroves in his native Cuba while working with the University of Bonn told DW. "Even if you are far inland and you divert or block a freshwater source, the water will stop reaching mangroves and kill them."
Mangroves also filter much of the sediment coming from the land side, keeping the sea water clean in many areas. "That is very important for tourism in beach areas, and for the health of related ecosystems like coral reefs," both important economic factors, said Capote.
In the case of extreme weather events, mangroves act as protective barriers in several different ways: by absorbing large amounts of water, reducing flooding after heavy rain and by breaking the power of wind and waves.
The aerial roots and numerous branches of the mangroves obstruct the flow of the water, explained Femke Tonneijck, a manager at Mangrove Capital, a partnership between Wetlands International, the Nature Conservancy and several other organizations in Indonesia.
During Typhoon Haiyan, which hit the Philippines in November, mangroves could have reduced the impact of the storm surge.
A study by Wetlands International shows that every kilometer of mangroves can reduce the height of the water by up to half a meter. Even the smallest abatement can save lives. Tonneijck stressed that the trees also provide firewood and building material in the aftermath of an extreme weather event.
"But to really reduce a whole storm surge, you would need very wide greenbelts," she cautioned. For that reason, she advocates a combination of natural and man-made measures in threatened tropical areas where the mangroves have disappeared. "For example you could have a dike with a mangrove belt in front of it to protect the dike."
Replanting the tidal forests can also help reclaim land that has eroded away. Tonneijck's group is working in an area of central Java where mangroves were replaced by aquaculture ponds for shrimp and fish, contributing to the erosion of hundreds of meters of coastline.
Restoration often fails in areas that suffer from erosion because the sediment balance is disturbed. Sediment can be trapped through "building with nature," using engineering techniques in combination with natural processes, says Tonneijck.
The project in Indonesia uses permeable dams made of local materials like bamboo and brushwood in front of the coastline. These structures mimic the function of mangrove roots and branches, as they dissipate waves and trap sediment. Once the sediment is stable, mangroves will naturally recolonize and form a new protective barrier. This method has been applied with salt marshes along the coasts of the Netherlands and Germany for centuries.
Bremen's Ulrich Saint-Paul says organizations like the World Bank are investing considerable amounts of money in projects to restore mangroves. "But in my view they always make one mistake: They plant the mangroves in monocultures, just like pine or fir plantations in Europe. This fails to take account of natural biodiversity, which lends a forest a far higher degree of ecological stability."
Capote stressed the need for careful long-term planning and continual monitoring. "More frequently than not, after a few years all the planting effort has been lost, because the survival rate of the mangroves is low due to a lack of proper initial assessment and follow-up," he added.
Sustainable shrimp culture
Capote argues for a sustainable approach to existing mangroves as well as restoration projects. "You can have sustainable aquaculture in the sense that only a portion of the mangrove area is used and restored afterward, on a rotational basis."
Reducing pollution from shrimp feed is another important factor. "There is a limit to how many shrimps can be nursed in a mangrove area," Capote said. "You can either have very high short-term profits that end when all the mangrove area has been killed. Or you can have a longer-term approach, with lower profits, when mangroves are given time to recover naturally."
Awareness of the value of mangroves rises after every catastrophic extreme weather event in tropical areas. But it doesn't last long, says Ulrich Saint-Paul.
"We need a long-term education program, especially in schools and adult education, to make it clear to people in mangrove regions how important the forests are and why we need to protect them."