The health of mangrove trees of the Sundarbans has significantly declined over the past 30 years due to an increase in salinity, according to a study.
However, the number of trees did not decline during the period.
Researchers of the study said that the decline in health could critically hamper the forest's ability to spring back, making it vulnerable to climate related extreme weather events.
The authors of the study applied the Continuous Change Detection and Classification (CCDC) method to the Landsat data (satellite imagery). They wanted to find changes on the earth's surface using satellite data to pinpoint when a specific area changes from being occupied by mangroves to being bare soil or water.
This method should be considered for global application to mangrove monitoring, they said.
The study was published last month in a journal of MDPI, an organisational acronym used by two related organisations – the Molecular Diversity Preservation International and the Multidisciplinary Digital Publishing Institute – based in Basel, Switzerland.
Four researchers of Aberystwyth University, UK carried out the study titled "Using Continuous Change Detection and Classification of Landsat Data to Investigate Long-Term Mangrove Dynamics in the Sundarbans Region."
Katie Awty-Carroll, one of the co-authors of the study said there is evidence of a decline in the health of about 25 percent of the mangrove trees. Those weakened areas will be more exposed to harm in the future, especially if extreme events such as cyclones become more common.
Awty-Carroll and colleagues tapped into a 30-year time series of Landsat data (January 1988 to June 2018) covering the entire Sundarbans that spans southern coastal areas of Bangladesh and West Bengal, India.
The Landsat program carried out by NASA and the United States Geological Survey offers the longest continuous space-based record of the Earth's surface.
Though not sure what caused the decline in the health of the mangrove trees, the scientists speculate it may be linked to the impacts of increased salinity on Heritiera fomes, or the Sundari tree – the dominant mangrove species in the Sundarbans.
When contacted, Deputy Chief Conservator of Forests Amir Hossain Choudhuri said Sundari trees, which constitute almost 40 percent of the total vegetation of Sundarbans, is a species that cannot tolerate high salinity and is supposed to get weaker if salinity increases.
And the spike in salinity is obvious due to siltation of tributaries of Padma that are responsible for maintaining fresh water flow in the forest area, he added.
Amir explained that the vegetation of Sundarbans falls into three zones which hold different distribution of trees. The less saline zone, which covers the area of Bagherhat, predominantly hosts Sundari trees. The moderate salinity zone of Khulna is covered mostly with Geowa or River Poison Tree. Sathkhira zone, the high salinity area, has Goran trees as the dominant species.
Dilip Kumar Datta, an environment specialist at Khulna University, said South Asian coastal trees are naturally weak because their roots do not go down very deep in earth as the water table is pretty high.
The Sundari tree was previously identified as suffering from die-back – dying from the tip of its leaves or roots backwards – due to rising salinity.
Sundari trees species can grow up to 25 metres in height and is under the endangered category in the IUCN Red List. Die-back disease has been a large-scale problem since 1980 and was estimated to affect 5-6 percent of this species in 2010.
A previous study carried jointly by US-based nonprofit organisation Winrock International and Chattogram University in 2018 found that over the last 25 years, the Sundarbans has lost Sundari trees equivalent to 25 hectares of land area.