Mangroves: The Natural Defence for Coastlines

Figure 1 - A map highlighting the distribution of mangrove forests (in black) throughout the globe (Romañach, 2017)

Across the intertidal zone are a collection of mangrove forests located along the coastlines of equatorial countries (figure 1), the largest being The Sundarbans in Bangladesh covering a total of 140,000 ha. These forests are often referred to as 'the roots of the sea' due to the complex aerial root system that spend the majority of the time above sea level enabling the plant to survive in the anaerobic soil, making these ecosystems tolerant to a hypersaline environment. With 40% of mangrove species labelled threatened by the IUCN, these coastal areas could be at greater risk of coastal erosion, storm surges and sea level rise. It is important these issues are understood to aid conservation of global coastlines as well as these beautiful unique habitats.

Figure 2 - A photograph showing the dense root systems of mangrove forests that reduce erosion 

Erosion is defined as “the process of eroding or being eroded by wind, water, or other natural agents”  and the complex, uneven aerial root system (figure 2) of mangroves reduces erosion of the shore. Mangroves have reportedly reduced the height waves up to 66% and slow the flow of seawater as the vegetation acts as a buffer between the land and sea. This enables 70-80% of incoming sediment to settle resulting, increasing biomass. This helps to increase the fertility in these harsh environments, increasing biodiversity, as mangrove soils have a high nutrient content. This gradual accumulation of biomass, over a long-time period, allows peat to form which is a long-term carbon sink helping to reduce carbon in the atmosphere. The dense roots bind soil particles together meaning an increase in mangrove trees means that less soil particles will become dislodged by incoming waves. This reduces the risk of flooding and damage to infrastructure in coastal communities. This risk increased in Guyana as after the removal of mangroves, coastal erosion increased 3 m annually.

The dense vegetation helps to reduce the impact of hurricanes and tsunamis inland by reducing the force of the storm surge. Figure 3 shows a storm surge is a large increase in sea water due to extreme weather, that often carries debris, that causes damage to infrastructure and flooding. The complex structure of mangrove forests increases the frictional resistance resulting in a drop in the force of the storm surge and slowing the flow rate of the wave. Storm surges are the biggest cause of damage during storms affecting causing large scale damage to properties, roads, biodiversity and coastal erosion. Studies have reported that mangroves can reduce the amplitude of a storm surge by 6 to 10 cm/km resulting in a decline in inland flooding and may fewer individuals at risk. However, the force from the waves can damage the mangroves often resulting in uprooting and damaging trunks so the health of mangrove forests should be monitored to ensure future storm surges have a reduced impact.

Figure 3 - A graphic showing the large increase in wave size during a storm surge 

Not only do mangroves provide a habitat to endangered species, including the Bengal tiger, but there are also major benefits to the human population as they reduce the risk to coastal settlements. Despite these major benefits, deforestation of these unique habitats is still on the rise due to the growth in aquaculture and the high value of mangrove wood potentially putting coastlines at risk.

Bibliography

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