Mangroves are a group of trees and shrubs that live in the coastal intertidal zone. Mangrove forests only grow at tropical and subtropical latitudes near the equator.
There are about 80 different species of mangroves. They range from low shrubs to 200-foot-high timber trees. Each mangrove has a filtration system to keep much of the salt out and a complex root system that allows it to survive in the intertidal zone. Some have snorkel-like roots called pneumatophores that stick out of the mud to help them take in air; others use prop roots or buttresses to keep their trunks upright in the soft sediments at tide's edge. This tangle of roots allows the trees to handle the daily rise and fall of tides. The roots also slow the movement of tidal waters, causing sediments to settle out of the water and build up the muddy bottom.
Mangrove forests stabilize the coastline, reducing erosion from storm surges, currents, waves, and tides. The plants' interlocking roots stop riverborne sediments from flowing out to sea, and their trunks and branches serve as a fence-like barrier that diminishes the erosive power of waves. Calls for mangrove conservation increased following the 2004 Indian Ocean tsunami. Where mangrove forests were intact, they served as natural breakwaters, dissipating the energy of the waves, mitigating property damage, and perhaps saving lives. There is evidence that mangroves saved lives and reduced property damage during Typhoon Haiyan in the Philippines in 2013.
The forests mangroves form are among the most productive and biologically complex ecosystems on Earth. Birds roost in the canopy, shellfish attach themselves to the roots, and snakes and crocodiles come to hunt. Mangroves provide nursery grounds for fish; a food source for monkeys, deer, tree-climbing crabs, even kangaroos; and a nectar source for bats and honeybees.
And mangroves are adaptable. A 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, despite rising sea levels. This 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. 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 other 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. In Louisiana, a pilot program has indicated that aerial spraying of mangrove seeds has been effective in establishing or re-establishing mangroves along the coast.
Despite their multifaceted importance, mangroves are under threat worldwide. They are sacrificed for salt pans, aquaculture ponds, housing developments, roads, port facilities, hotels, golf courses, and farms. Even for an International Boat Show in Miami. And they die from oil spills, chemical pollution, sediment overload, and disruption of their sensitive water and salinity balance. In 2016, a combination of unusually low rainfall and warmer ocean temperatures caused perhaps the largest mangrove die-off seen anywhere in the world, stretching along 700km of Australia’s Gulf of Carpentaria.
One of the greatest threats to mangrove survival comes from shrimp farming. A prime location for shrimp ponds happens to be the shore zone occupied by mangroves, an unhappy conflict of interests that has a predictable outcome - mangroves ripped out to allow room for shrimp ponds. To compound matters, shrimp farmers typically abandon their ponds after a few crop cycles (to avoid disease outbreaks and declining productivity) and move to new sites, destroying more mangroves as they go.
Mangrove forests are also highly effective carbon sinks. They absorb carbon dioxide, taking carbon out of circulation and reducing the amount of greenhouse gas. Measurements by scientists suggest that mangroves may have the highest net productivity of carbon of any natural ecosystem (about a hundred pounds per acre per day) and that as much as a third of this may be exported in the form of organic compounds to mudflats. Research has also shown that a significant portion of the carbon ends up in forest sediments, remaining sequestered there for thousands of years. Conversion of a mangrove forest to a shrimp pond changes a carbon sink into a carbon source, liberating the accumulated carbon back into the atmosphere—but 50 times faster than it was sequestered.
If mangroves were to become recognized as carbon-storage assets, that could radically alter the way these forests are valued and could help slow, or even reverse, the loss of highly beneficial mangroves along tropical and subtropical coastlines around the world.
Mangroves (NOAA National Ocean Service)
Mangroves (National Geographic)
The Response of Mangrove Soil Surface Elevation to Sea Level Rise, Anna McIvor, Tom Spencer, Iris Möller and Mark Spalding. Published by The Nature Conservancy and Wetlands International. 2013.
Tsunami-Blocking Mangroves Lure Carbon Investors: Southeast Asia (Ocean Health Index)