Mangrove coastal protection has dipped 25% in a decade: Report

The study also found that coasts next to deep water may be more exposed, than sheltered coasts, to threats from both ocean storm waves and cyclones making landfall from the deep sea. 

Mangroves are a necessary defense against rising sea levels and extreme weather events like cyclones and tidal surges, but human activities combined with climate change are affecting their growth. 

A report titled ‘Global distribution and decline of mangrove coastal protection extends far beyond area loss’ found that there is an “alarming, widespread decline in mangrove coastal protection index (MCPI), particularly along coasts exposed to deep water, possibly in concert with human destruction, cyclones, and intensifying oceanic boundary currents.”

The report’s authors used a process-based measure of mangrove coastal protection index (MCPI), incorporating cross-shore width, canopy height, and the normalized difference vegetation index (health index). They found that width or area of mangrove clusters generally declined at low rates but width increases in some regions reduced MCPI. 

Some mangroves more robust than others

Cluster exchange network analysis from 2007 to 2019 showed an 800% increase in mangrove forests with characteristically low height, width, and MCPI, cited the report. 

In a global context, this means that there has been a 25% decrease in MCPI from 2007 to 2019, primarily due to change in mangrove height or biomass, compared to a 2% decrease in area. 

The report stated that mangrove strands, less than 20 metres in height, and at relatively sheltered, low-latitudes and of high-MCPI, appeared to be resilient to destruction from cyclones.

It also identified key factors that determine mangrove growth and protectionist abilities: forest width, crown diameter, root density, tree density, forest vertical structure, species, and ecological conditions. 

A very important factor is also the distance a mangrove strand extends inland from the coastline, providing robust protection against tsunamis and hurricanes. The report found that a wider mangrove belt acts as a buffer zone, absorbing and dissipating the energy of incoming waves and storm surges before they reach populated coastal areas

Furthermore, variable factors like the height of pneumatophores (aerial roots of mangroves) and the kind of vegetation also affect the protectionist ability of a mangrove strand.

Local geographical factors

The report found that MCPI peaked in mangrove belts close to the equator, primarily along eastern Venezuela, northeastern Brazil, western Colombia, Gulf of Guinea, eastern Andaman Sea, northwestern Borneo, and western New Guinea. Alternatively, regions with low MCPI were primarily along the Gulf of California, Caribbean, Red Sea, Gulf of Oman, western India, southern China, and Australia.  

According to the report, the high MCPI mangroves are primarily distributed within latitudes of 5° N and 5° S, resulting from the combined influence of natural conditions like higher rainfall, warmer temperatures, and human factors. Furthermore, lower human activity in these regions have helped maintain wider coastal mangrove belts.

On the other hand, in higher latitudes like the northern Bay of Bengal, mangrove heights are shorter and they have less favorable growth conditions. However, the mangrove belts are extensive in the region due to a higher incidence of inland protection. 

The study also found that coasts next to deep water may be more exposed, than sheltered coasts, to threats from both ocean storm waves and cyclones making landfall from the deep sea. 

All these factors are important when building robust mangrove belts for coastal protection. By studying local factors, and with the help of such research projects, the world can put more informed protection strategies in place, which yield higher results against the threat of cyclones and rising seas.

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