By Sahana Ghosh
Mangrove forests, nature’s buffer against disasters, are imperiled by unregulated coastal development, shrinking of deltas and climate change linked extreme events.
The degrading ecological health of mangroves affects their resilience and recovery potential against climate change consequences like sea-level rise.
Enhancing the adaptive potential of mangroves is the need of the hour, experts said.
Cyclone Bulbul, the seventh cyclone to hit India in 2019, battered the country and its neighbour Bangladesh’s coasts, killing at least 20 and displacing millions. But the damage could have been much worse had not the world’s largest continuous block of mangroves – the Sundarbans – stood on the frontline, breaking the cyclone’s force, according to officials and experts.
Spanning more than 10,000 square kilometres, the Sundarbans region of Bangladesh and India is the biggest mangrove forest in the world and also the most critical area for Bengal tiger survival. Sprawled out across 6,017 square kilometers, the Bangladesh side of the forest covers nearly 60 percent of the total area of the Sundarbans. The rest is in India in the state of West Bengal.
The Sundarbans mangroves in West Bengal, on the east coast of India, account for almost half of the total area under mangroves in the country. Apart from the east and west coasts of the mainland, mangroves are found and on the islands of Andaman and Nicobar and Lakshadweep. Indian mangroves represent 3.3 percent of global mangroves and about 56 percent of global mangrove species.
As per Forest Survey of India estimates (2017), mangrove cover in the country is 4,921 square km, which is 0.15 percent of the total geographic area of the country. There has been a net increase of 181 square km in the country’s mangrove cover as compared to the 2015 assessment, according to the FSI report, though the number is debatable as conflicting data for certain stretches of mangroves is reported by non-governmental organisations. After West Bengal, Gujarat and Andaman and the Nicobar Islands have the maximum mangrove cover.
Considering the growth and diversity, mangroves of Andaman and Nicobar are best in the country, said P. Ragavan of Council of Scientific and Industrial Research (CSIR), National Botanical Research Institute, Lucknow.
Mangrove forests are unique coastal wetland ecosystems of tropical coasts and provide invaluable and irreplaceable services, making their protection critical, said Ragavan, co-author of the book chapter ‘Current Understanding of the Mangrove Forests of India’.
Important ecological functions offered by mangroves include water filtration, prevention of coastal erosion, coastal protection from storms and floods, carbon storage, buffering of ocean acidification, food, timber, and livelihood provision, and biodiversity protection. The ecological health of mangroves globally, as well as in India, remains degraded, and implicit species loss has been witnessed despite mangrove expansion in many regions, the book chapter underscores.
The protective role of mangroves as a disaster reduction tool has widely been recognised in recent times, Ragavan said.
Mangroves prevent coastal erosion by reducing the height and energy of wind and swell waves passing through them and minimise the impact of natural hazards such as typhoons, cyclones, hurricanes, and tsunamis, helping to shelter lives and assets of coastal communities.
For example, during the 1999 Odisha supercyclone, mangroves significantly reduced the number of human deaths in Kendarapara, one of the worst-hit coastal districts in Odisha. Villages with wider mangroves between them and the coast experienced significantly fewer deaths than the ones with narrower or no mangroves.
Hundreds of meters of mangroves are needed to significantly reduce waves.
“Wave height is reduced by 13-66 percent every 100 metres of mangroves and for storm surges, surge height is lessened by 5-50 cm every km. But thousands of meters of mangroves are required to reduce flooding impact,” explained Ragavan.
Though some researchers are skeptical about the ability of mangroves to shield against tsunamis, there is growing evidence, particularly from the devastating Indian Ocean tsunami of 2004, that mangroves reduced tsunami impacts by reducing the destructive energy of water flowing inland.
Mangrove belts of several hundred meters wide have been shown to reduce tsunami height by between 5 and 30 percent. Wider mangrove forests are more effective at reducing tsunami height, as well as the speed of the water and the area flooded by the tsunami.
Dense forest vegetation also helps to reduce tsunami depth and area of flooding. However, large tsunamis (more than four metres in depth) can damage mangroves, eventually destroying them and making them less effective at reducing tsunami flows.
“Most scientific studies conclude that the protective role of mangroves depends on the location-specific attributes of the mangrove forest like the tree density and width of the forest and the diameter of trunks and roots, along with an array of other information (floor shape, bathymetry, and spectral features of waves). So precise site-specific information is imperative for making efficient use of mangroves’ protective capacity,” said Ragavan.
Mangroves have high resilience to damage caused by storms. “But increasing frequency of tropical cyclones has threatened the resilience capacity of mangroves. They have the capacity regenerate naturally but severe tropical cyclones trump this natural potential,” said Ragavan.
According to an IPCC report it is likely that there will be fewer or the same number of tropical cyclones but more intense tropical cyclones (including tropical storms, hurricanes, and typhoons) in the future.
Mangroves also adjust vertically and move horizontally to cope with sea-level rise. “However, these processes are greatly influenced by local abiotic (sediment inputs and local geomorphic settings) and biotic (plant productivity, peat development, and the accumulation of refractory mangrove roots and benthic mat materials) factors.
The quality and quantity of mangroves also determine their adaptive potential. But land reclamation for developmental activities, uncontrolled upstream developmental activities and climate change-induced extreme events are key drivers of mangrove degradation, said Ragavan.
According to the Maharashtra forest department, sea-level rise could wipe out 95 percent of mangroves in the west Indian metropolis of Mumbai, which has the largest mangrove cover (6,600 hectares) for any urban area in the world.
The health of mangroves influences their capacity to withstand disasters.
“In the Sundarbans, Avicennia plants are dominant but they don’t have as dense root systems as Rhizophora. Rhizophora can enhance the potential to tackle larger cyclones. In Pichavaram mangroves in Tamil Nadu, Avicennia is dominant now, you can see Rhizophora only in the banks of the creeks. The main reason is deltas are shrinking. In the Sundarbans, freshwater supply is reducing but it is compensated by glacier melt. But in the case of Pichavaram, the freshwater supply is mainly dependent on water-starved Kaveri,” said Ragavan.
Coastal developmental activities have also drastically altered hydrological connectivity and linked mangrove landscapes that are needed for vertical and horizontal mangrove processes.
“Hydrological connectivity or the amount, quality, quantity, and timing of freshwater and sediment delivery to estuaries and mangrove forests determine the vertical adjustment of mangroves, whereas spaces like tidal flats and other adjacent landscapes such as salt marshes can act as natural corridors and are needed for horizontal movement,” said Ragavan.
Climate change adaptation expert Anjal Prakash said mangroves are being slowly but systematically done away with, in the name of development. For instance, the bullet train project that is estimated to speed through the Mumbai-Ahmedabad corridor will affect at least 30000 mangroves.
Prakash said cities will have to come around to a fresh understanding of what resilience means and incorporate environmental planning so as to not disturb natural resources such as mangroves and connected mudflats and water bodies.
“One has to understand how they are connected and how this connectivity can be preserved for existing natural resources, such as mangroves. They can build around it and for that planners need to have an understanding of the water flows and other links,” Prakash, Associate Professor and Associate Dean at TERI School of Advanced Studies, told Mongabay-India.
Enhancing the adaptive potential of mangroves is the need of the hour, emphasised Ragavan, spelling out the potential strategies such as facilitating the connectivity between adjacent ecosystems, preserving the hydrological connectivity of existing mangroves, and protection of natural corridors.
“There is an immediate need for an effective site-specific, long-term, and integrated ecosystem-based protection, management, and rehabilitation strategy with sound scientific knowledge and drastic legislative measures to limit/regularise the coastal developmental activities,” he said.
In addition to strictly regularising developmental activities in the vicinity of mangroves, species-specific restoration for threatened species, long-term monitoring of ecological health of mangroves, Ragavan and co-authors of the book chapter advocate change in the legislative definition of mangroves.
Mangroves are often regulated under legal frameworks created originally for forests in general, environment, water, land, or marine fisheries. In many nations that host mangroves, the legislative definition of mangroves covers only the wooded component (mangrove forests) of the ecosystem.
“But in nature, mangroves are closely interconnected to adjacent ecosystems, both seawards and landwards. Hence, the legislative definition of mangroves should be changed to ‘mangroves as an integrated system’ to safeguard hydrological connectivity and adjacent natural corridors,” added Ragavan.
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