As salinity builds up in Sundarbans, one mangrove species dies a slow death, giving way to another

Analysis of satellite images from the past three decades shows a diminishing number of Sundari trees and they are as elusive as the most celebrated resident here - the Royal Bengal Tiger

Moinul Hoque Chowdhurybdnews24.com
Published : 16 Sept 2022, 08:05 PM
Updated : 16 Sept 2022, 08:05 PM

The Sundarbans were once thick with Sundari trees across thousands of hectares of land but its diminishing number over the past three decades has made way for Kankra trees in the world’s largest mangrove forest.

The Sundari has become as elusive as the most celebrated resident here - the Royal Bengal Tiger.

An analysis of satellite images revealed that the area of Sundari and Gewa trees bristling the Chandpai range shrunk from 16,000 hectares to 12,500 hectares in the 34 years from 1988-2022.

In the same period, Kankra trees, which are naturally resilient to the salinity of the waters, expanded from 165 hectares to well over 2,000 hectares. Experts say environmental changes brought about the shift in the ecology of the region and other species like Sundari will struggle to survive if the salinity keeps rising.

Experts say environmental changes brought about the shift in the ecology of the region and other species like Sundari will struggle to survive if the salinity keeps rising.

From July 2021 to June 2022, Chief Scientific Officer Md Mahmudur Rahman and Senior Scientific Officer Md Ziaul Islam of Bangladesh Space Research and Remote Sending Organization, or SPARRSO, studied satellite sensor data on the forest ecosystem to identify changes over the last three decades.

The data, which included images, clearly showed that Kankra trees grew where Sundari trees were across 272 sqkm in the northeast Chandpai range.

Mahmudur said: “We found some changes in our study. It’s tough to say whether this was caused by climate change or some other reasons. The Sundari tree was afflicted by diseases and that might be the reason for them perishing.”

The surface images for 1988, 989, 2000, 2010 and 2022 were from orbital satellites Landsat 5, Landsat 7, Landsat 8 while those for 2004-2005 were drawn from QuickBird-2 and the WorldView-2 delivered photographs for 2021, he said.

“The Sundarbans is facing different challenges due to climate change. One of the goals of this study is to observe what impact it is having on the ecology.”

Mahmudur added that the number of Sundari trees is on the decline, though not at a rapid pace. “

“The area covered with Kankra trees is now reaching 2,500 hectares where it stretched around 165 hectares 30 years ago and that’s obviously due to a big change in the ecosystem.”

KANKRA TREES AND SALINITY

In a separate study, Prof Md Kamruzzaman from Khulna University’s Forestry and Wood Technology Discipline found the expansion in Kankra trees' vegetation is due to the increasing salinity of the waters.

“One species of trees is on the decline while another is on the rise is obviously due to environmental changes. Sundari trees had been dying off due to the top dying disease for some time,” he said.

“The disease’s been afflicting this species of trees there since the 1980s and has become more widespread now. Another study by a group in my department ascertained that Kankra trees are spreading.”

“These trees are growing well due to being tolerant to salt water. This is also a threat to Sundari trees, who have not seen any growth over the past three-four decades and are afflicted by one disease or another.”

“The environment is now unsuitable for Sundari trees. There will be no open space if these trees keep dying and are replaced by another. Changes must be brought to nature,” Kamruzzaman insisted.

The multitude of Sundari trees is the reason the forest is called the Sundarbans. But these trees are absent where the waters are too salty. Sundari trees thrive in freshwater. As there is less fresh water than before, salt water has entered the country and higher salinity in human habitats prevents crops from growing.


Mentioning that all trees in the Sundarbans are not resilient to salinity, he said: “All over the world, the biodiversity is different wherever there’s salinity in the water and the number of trees is less. This is a change,” he said, stressing that the satellite images help accurately discern the number of Kankra trees.

“The multitude of Sundari trees is the reason the forest is called the Sundarbans. But these trees are absent where the waters are too salty. Sundari trees thrive in freshwater. As there is less fresh water than before, salt water has entered the country and higher salinity in human habitats prevents crops from growing.”

“The Sundarbans will continue to be here, but it will lose different species. Fresh water is the root of much diversity. The number of trees is declining in freshwater zones of the Sundarbans. The size and growth of trees have dropped in the Satkhira range. Trees other than Gewa and Goran are rare.”

He said the smaller sampling of the study reflects the overall situation of the Sundarbans.

THE SUNDARBANS

The Sundarbans stretch across 10,000 sq km in Bangladesh and India, while 6,017 sq km are located in Bangladesh, spanning the Khulna, Bagerhat and Satkhira districts. The rest of it lies in North 24 Parganas district in India.

It is the world's largest uninterrupted tidally washed mangrove forest, featuring upstream water flow, salty oceanic currents and muddy chars or shoals. Most of the trees there have aerial roots to absorb oxygen directly from the air. Apart from the most widely spread Sundari and Gewa trees, the region is also home to Pashur, Dhundal, Goran, Bain, Kankra and Kewra species of trees.

Scottish botanist David Praine wrote a book on the flora of the Sundarbans and nearby areas and mentioned 334 species of plants, 165 species of algae and 13 types of orchids. He observed that most mangrove plants are evergreen, short shrubs or tall trees.

In the Sundarbans, there are 289 species of land creatures - 42 mammals, 35 reptiles, eight amphibians and 219 others including fishes. Aside from being the home of the Royal Bengal Tiger, the region contains spotted deer, muntjac or barking deer, rhesus monkeys, wild cats, hedgehogs, otters and wild boars, among others.

Among the reptiles, estuarine crocodiles are the largest, while king cobras, monocled cobras, pythons and several other species of sea snakes also roam the forest.

There are also 24 known species of shrimps, 14 types of crabs, and some species of snails and mussels. Around 320 species of birds nest in the Sundarbans - along with 50 species of migratory birds these include egrets, cranes, storks, snipes, red-wattled lapwing and a multitude of others.

The Sundarbans is also a huge source of honeybees.

SUNDARI TREES

Sundari trees, which have the scientific name heritiera fomes, comprise almost 70 percent of the land area of the forest. These trees flourish under low salinity conditions and grow from 15 to 25 metres from the ground.

They have straight trunk leaves, elliptic leaves, roots with pneumatophores and blind root suckers, and grow bell-shaped flowers and micronutrient-rich fruit.

The roots of these trees strongly hold the soil beneath them, prevent erosion and protect the coastal areas. They are also a huge source of lumber.

The combination of woodcutting and top-dying disease are seen as the chief reasons for their diminishing number.The top-dying disease, as the name suggests, starts from the peak of the trunk and gradually makes its way down. It also affects the tree’s growth.

KANKRA TREES

These salinity-resilient trees, scientific name bruguiera gymnorhiza, grow seven to 20 metres in height. They are found in saline mangrove swamps like the Sundarbans.

Also a source of wood, these trees have a glabrous, smoothish, trunk with reddish brown bark, short prop-roots rather than long stilt roots and help soil stability and protect the coast. Bees and other wild animals find homes in these trees.

Toufique Imrose Khalidi
Editor-in-Chief and Publisher