Sunderbans losing its dense mangrove cover: Govt report

Updated on Jan 14, 2022 12:36 AM IST

This is worrying because the mangrove forests acts a green buffer against cyclones, protecting Kolkata from bearing the direct brunt of storms that originate in the Bay of Bengal.

A file photo of the Sunderbans river delta. (Getty Images/iStockphoto)
A file photo of the Sunderbans river delta. (Getty Images/iStockphoto)

The Sunderbans, the world’s largest mangrove forest, and home to the Royal Bengal Tiger, is gradually losing its very dense tree cover, data released by the Forest Survey of India on Thursday has revealed.

Experts say rising salinity levels are taking a toll on the mangroves. This is worrying because the mangrove forests acts a green buffer against cyclones, protecting Kolkata from bearing the direct brunt of storms that originate in the Bay of Bengal.

The India State of Forest Report 2021 released Thursday revealed that the very dense mangrove cover in the Sunderbans has shrunk by two square kilometres – from 996 sq km to 994 sq km - between 2019 and 2021. The delta had a mangrove cover of 999 sq km in 2017 which had declined by three square kilometres in 2019. And it was 1038 sq km in 2011. A 4.23% decrease over a decade is worrying.

“We have to check exactly where the very dense forest cover has gone down. Progressive salinization is one of the reasons behind decline in density of mangrove. Very severe cyclone Amphan which hit the delta in May 2020 has definitely taken a toll” said Debal Roy, chief wildlife warden of West Bengal.

The delta was hit by two very severe cyclones – Amphan in May 2020 and Yaas in May 2021 – which took a toll on the vegetation. Nearly one-third of the delta was severely damaged by Amphan. The cyclones also push sea water inland, increasing salinity levels in the delta.

The report claims that total mangrove cover, which includes very dense mangrove, moderately dense mangrove and open mangrove cover, has increased from 2112 sq km to 2114 sq km in 2021. But this increase is because of a rise in open mangrove cover, primarily on account of plantations. Moderately dense mangrove cover has remained unchanged during these two years.

The report mentions three kind of mangrove forest cover -- very dense, moderately dense and open. While very dense mangrove is defined as those which have canopy density more than 70%, those with canopy density 40-70% are defined as moderately dense, while those in the 10-40% range are considered as open mangroves (these are generally plantations done during afforestation programmes and not natural forests).

“Rising salinity level in the delta is taking a toll on several mangrove species such as Sonneratia apetala, Nypa fruticans and Bruguiera gymnorhiza. Their growth is becoming stunted as they can’t withstand salinity above 20 psu (practical salinity unit). The average salinity of sea water in Bay of Bengal is 33 psu (which is equal to 33 gram of salt in one litre of water). Because of reduced sweet water flow and intrusion of salt water from the sea these trees are either dying or suffering from stunted growth,” said Abhijit Mitra, former head of the marine science department of Calcutta University.

The Sunderbans forest sprawls over 10,000 sq km across India and Bangladesh, with 40%of it in India, and is home to many rare and globally threatened wildlife species such as the Royal Bengal Tiger and the estuarine crocodile. In India it is confined to the southern tip of West Bengal and spans two districts , South 24 Parganas and North 24 Parganas.

Experts pointed out that the Sunderbans has been shrinking over the past few years and warned that if the trend continues it could have a disastrous effect on both Kolkata and the livelihood of local villagers.

“The Sunderbans’ thick mangrove acts as a bio-shield and saves Kolkata from the direct onslaught of cyclones that originate over the Bay of Bengal. The mangrove also acts as a spawning ground for several fish and other aquatic animals such as crabs and prawns on which the locals depend for their livelihood,” Tuhin Ghosh, director of School of Oceanographic Studies in Jadavpur University, had said in an earlier interview.

While very severe cyclone Amphan hit the Sunderban on May 20, 2020 with wind speeds up to 185 kmph, by the time it crossed the mangrove forest and reached Kolkata its wind speed had come down to around 130 kmph.

The state government planted around 50 million mangrove saplings after cyclone Amphan. Again in May 2021 after cyclone Yaas hit the area, chief minister Mamata Banerjee said another 150 million mangrove saplings would be planted in the three coastal districts of West Bengal.

Among all states West Bengal has the largest mangrove cover in the country followed by Gujarat and the Andaman and Nicobar Islands.

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  • ABOUT THE AUTHOR

    Joydeep Thakur is a Special Correspondent based in Kolkata. He focuses on science, environment, wildlife, agriculture and other related issues.

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