Mangroves are commonly described as the ‘forests of the sea’, owed to their roots being submerged in water, whilst adapted to thrive in both freshwater and saline conditions. Due to their ability to withstand challenging conditions such as storm surges and tidal influxes, mangroves are the second most biodiverse ecosystem after the coral reef system; the intricate root system supports an incredible range of unique ecological communities. As high surviving as mangroves are, they are being threatened by environmental change, specifically sea level rise; this poses concern as to whether they will be able to survive the effects of future climate change.
Act as coastal defences
Mangrove forests are sensitive to cold conditions, so restricted to the tropics and subtropics. The two most abundant areas are: The Atlantic East Pacific, along coastlines of Central America including the Everglades in Florida, Mexico and Belize, as well as the Indo West Pacific, such as coastlines of Madagascar, Sri Lanka and Indonesia.
Overwhelmingly, there are 80 different species of mangrove trees, all of which are grown in areas of low-oxygen soil where shallow waters allow fine sediments to accumulate. As you can see in the picture above, the tangled roots are substantially long, appearing above the water, where they have been adapted to handle the rise and fall of tides twice a day. This adaptive advantage is merely a natural coastal defence mechanism; they endeavour natural flood control, acting as erosion buffers by functioning to reduce wave energy and slow the flow of water over the soil surface. These flood storage and erosion buffering functions are important, specifically during storm events in hurricane season, where sediment movement can be reduced by the complex network of roots.
Tropical storms such as hurricanes and cyclones are widespread in mangrove locations; larger waves and high sea levels sound like bad news, but in actual fact, mangroves contribute to reducing damage. Even during severe storms, the branches of forest canopy will act to lessen wave energy. Even if water depth is reduced by a small reduction of 5-50cm per km of mangrove, this is enough to decrease the extent of flooding in low lying areas. Meanwhile, the complex network of roots can serve to trap large sediments and moving objects.
The 2017 Atlantic Hurricane season was by the far the most destructive for Central America, with 17 named storms ranking the highest number of consecutive hurricanes in history. Although the costliest on record, for some parts of the USA, mangroves were a saviour. The Everglades National Park in Florida is home to 144,000 hectares of mangroves, occupying 1,300 species and the most valuable nursery ground for fish, molluscs and birds. In response to Hurricane Irma, research shows that the 2.7 mile stretch of mangroves reduced storm surge by over a foot, protecting many lives and settlements. It’s no surprise that they are described as ‘nature’s unmatched shock absorbers!’
Climate change and mangrove threat
It wasn’t long before climate change crept up on this ecosystem, as it has done on most already. A serious climatic impact that will, if not already, has threatened mangroves is sea level rise, as it is the most certain outcome of climate change and the greatest threat to mangroves. The Intergovernmental Panel on Climate Change (IPCC) have predicted a 0.26-0.9m increase in sea level by 2100, assuming serious implications to low island mangroves. Though, the key response to such rise in sea levels relates to salinity perturbations.
Although mangroves are ‘facultative halophytes’, where saltwater is an ecological requirement and freshwater is a physiological requirement, there’s so much they can take before their resilience to salinity fails. Once salinity exceeds a certain threshold, it will delay root initiation and seedling establishment. Many studies have simulated different salinity concentrations on mangrove plants and mangrove growth was disrupted in most scenarios. One species of mangrove is the Agallocha plant, which is considered to be the most sensitive to changing salinity levels. In a study at the Futian Mangrove Reserve in China, salinity beyond 15 PSU (Practical Salinity Unit) delayed growth, where 25 PSU was deadly. Unfortunately, it seems probable that a 25PSU will occur by 2100, indicative of major mortality of this mangrove plant.
In response to sea level rise, we have already seen landward migration of mangroves in low lying Pacific Islands such as Bermuda and American Samoa; this is going to be a substantial problem in all mangrove areas as the rate of sea level rise exceeds the rate of sediment build up. The setback faced is that there are obstructions to mangrove migration such as the invasion and displacement of freshwater marshes and swamp habitats, coastal management and human settlement. Such obstruction obviously limits migration and results in the loss of mangrove forest instead of changing distribution. The greater the rate of sea level rise, the faster mangroves will encroach into other habitats and cause a biodiverse catastrophe. This is why mangrove management and restoration should not be ignored given the potential for saltwater intrusion and mangrove migration.
What’s been done to save them
In terms of conservation value, mangroves are important to water quality through their natural filtering properties and nutrient recycling, removing pollutants through geochemical mechanisms. Above all, mangrove ecosystems are the biggest carbon storage, emphasising the need for management; if carbon stocks are disturbed, carbon emissions will be high, exacerbating climate change further. The question into whether mangroves can withstand climate change depends on the level of protection for the ecosystem.
There have been both local and international initiatives to save mangroves. Local initiatives are focused on stabilising and restoring sediment balance to limit erosion processes. The use of hybrid engineering approaches such as grids of permeable dam structures placed in front of the mangrove coastline can trap sediments and allow settling, thus dampening erosive waves and limiting saltwater intrusion. Permeable structures could be made from natural material such as bamboo or brushwood, merely mimicking the structure of a mangroves natural root system. Such technique allows natural colonisation of mangroves as erosion processes are minimised, further sustaining the biodiversity.
On a global scale, there are international initiatives, one being the Bonn Challenge which is run by The International Union for Conservation of Nature (IUCN). It’s target is to bring 150 million hectares of the world’s degraded land into restoration by 2020. The initiative aims to restore ecological integrity, and to increase mangrove coverage by 20% by 2030. It is an ambitious goal that requires a lot of global collaboration.
Something that shouldn’t be forgotten is that a healthy ecosystem is a resilient one too, so the more people involved in global mangrove conservation, the more achievement we see in battling environmental change.