Response of Pioneer Mangrove Tree Species of the Sundarbans to Increased Salinity

Abstract:

Sundarbans is the largest single block of tidal halophytic mangrove forest in the World. She is a UNESCO World Heritage as well. Probably the name ‘Sundarbans’ was derived from the pioneer mangrove tree species Sundari (Heritiera fomes). Sundarbans is experiencing both predictable and unpredictable climate change impacts. Climate change is affecting many regions of the World but Sundarbans is one of the worst hits due to high level of salinity, sedimentation, cyclones, storm surges, and land erosion. Brackish water from the Bay of Bengal is invading freshwater bodies percolating deep into the soil. The knowledge of the response of tidal freshwater ecosystems to sea level rise and salinification is necessary to predict the consequences of changes in mangrove ecosystem structure and function due to global change, including potential impacts on estuarine eutrophication. A plot of 500m² areas at the base of each canal along the eastern side of the Passur River was established. From the last point of the human settlement (Nandabala Canal) to the Bay of Bengal we counted 40 canals and collected data from 40 plots. We observed water salinity, the abundance of tree species and the incidence of ‘Top dying’ disease at each plot. The salinity level was highly correlated with the abundance of Sundari tree (Heritiera fomes) and the incidence of ‘Top Dying’ disease, where negatively correlated with the abundance of Keora tree (Sonneratia apetala). Among five forest types, the fresh swamp has disappeared due to increased salinity. Mixed fresh-brackish swamp has been converted into brackish swamp. Most of the brackish swamps have become mangrove scrubs. Sundari has been suffering from 'Top dying' disease in the present mixed fresh-brackish and brackish swamps. It has disappeared from the past brackish swamp and mangrove scrub. Sundari is being replaced by Keora (Sonneratia apetala) and consequently the name of Sundarbans will be replaced by Keorabans.