Reduced freshwater flow through the Bangladesh part of Sundarbans since the commissioning of Farakka Barrage on Ganges river in India and resulting increase in salinity are causing the forest to transform itself.
According to a new study, Sundarbans' trees are losing height and leaf area while wood density is increasing. Also, smaller trees like Goran and Gewa are replacing Sundari, which has been taking the most severe blow. The overall 'productivity' of the forest is also on the decline.
How this is happening
Being located in the low-lying coastal area, Sundarbans is dominated by the sea, where freshwater rivers help to maintain salinity level suitable for mangrove trees.
Freshwater supply from the distributaries of Ganges into the Sundarbans has considerably declined since the construction of the Farakka barrage in the 1970s.
The Bangladesh Sundarbans receives a mean minimum monthly discharge of zero to 170 cubic metre per second (cumec) through Gorai river during the dry period when a minimum of 194.4 cumec freshwater discharge is needed to maintain a low saline condition inside the forests, reveals a study titled 'Bangladesh Sundarbans: Present status of the environment and biota.'
The zero or very low freshwater discharge through the forest allows seawater to intrude deep inside the mangrove forest, increasing salinity of water and soil. Average soil salinity level has already increased by 60% as a result, says the study which was jointly conducted by Professor Abdul Aziz from the department of Botany, University of Dhaka, and Ashit Ranjan Paul from Bangladesh Forest Department.
Researchers fear that sea-level rise (SLR) could further aggravate the degradation of the forest. According to another study, the SLR rate along the Bangladesh coast in the 20th century, 5.93 mm/year, was substantially higher than the global average of 1.0 to 2.0 mm/year.
Impact on flora
Increased salinity is affecting growth and distribution of the mangroves in the Sundarbans.
A study published this year confirms that the height and leaf area of the trees are reducing, while wood density and leaf succulence are also impacted by increased salinity.
The study titled 'Solving the fourth-corner problem: Forecasting ecosystem primary production from spatial multispecies trait-based models' shows that the aforementioned changes, especially the decrease in tree-height lead to an overall productivity loss.
The study, conducted by Swapan Kumar Sarker, Richard Reeve and Jason Matthiopoulos, predicts a substantial productivity loss over the entire ecosystem. In the worst stress scenario – i.e. a 50% rise in salinity and siltation –the ecosystem is predicted to lose 29.5% of its current total productivity by 2050.
The tallest mangrove trees are highly likely to be turned into dwarf communities.
"Our study determines that, although every species of the ecosystem will lose height under the future stress scenarios, the loss will be substantially higher for the Sundari trees – a whopping 47%," said Swapan Kumar Sarker, the lead researcher who is a professor of the Department of Forestry & Environmental Science at Shahjalal University of Science and Technology. Loss of height in the trees is set to reduce the forest's carbon sequestration capacity as the biomass decreases.
The study created environmental and species distribution maps for five future stress scenarios for 10% to 50% increase in salinity and siltation. It says productivity in some tree communities may also increase, particularly where the salinity increase is relatively low.
Regarding other changes in the mangroves, Professor Sarker added, "Gewa is expected to undergo ecosystem-wide range expansion by 2050, and it loses the least amount of height and leaf area compared to others. The increase in wood density (9%) is highest for Sundari."
The most productive tree communities are currently located in the freshwater-dominated eastern and northern Sundarbans. For decades, increased salinity has been taking its toll on the Sundari trees, wiping it off from Satkhira and Khulna ranges.
A comparison of the forest types observed nine years after the commissioning of Farakka Barrage revealed that coverage of Sundari decreased by 10.4%, while total tree cover reduced by 4% compared to 1959. Gewa, however, proved to be more tolerant to salinity.
The earlier study by Professor Aziz and Mr Paul revealed that increase in salinity also affected growing stock (the number of trees per hectare) of the mangrove. A total of 180 growing stock in 1983 reduced to 144 by 1996, as recorded by Bangladesh Forest Department.
Trend analysis at that time showed that by 2020, the total number of plants would have been reduced to 109 per ha of forestland. The number of Sundari would be reduced to 80 per ha, which is 37.9% compared to 1959. Increased salinity also affects growth of Golpata (Nipa Palm).
Reduction of forestland
Sundarbans, or the whole Bangladesh for that matter, was formed by the deposition of sediments from the Himalayas. However, satellite data shows that from the 1970s to 2000s, Sundarbans, instead of gaining any new land, actually decreased by 1.1%, which is equivalent to 66 sq km. Another report from around the same time, estimated the total forestland loss to be approximately 127 sq km.
Because of low flow of freshwater during dry periods, sediments pile up in the Ganges and Gorai instead of depositing into the southern edge of Sundarbans. This retards the growth of forestlands.
"For Sundarbans, the problem is two-pronged. Salinity increases on one hand, and erosion of land on the other," Professor Abdul Aziz told the Business Standard.
Impact on fauna
There is no disagreement over the impact of increased salinity in the surface water on the wildlife as they need freshwater for drinking. However, expert opinion is divided on the impact of changes in vegetation on Wildlife.
Some researchers and wildlife experts suggest that the changing forest might negatively impact biodiversity as well.
"Deer are a bit sensitive, and its population is affected by increased salinity. Wild boar – an omnivore – on the other hand, is more adaptive to changes. Both belong to prey-species for tigers, but deer constitute 70% of a tiger's food," Dr M A Aziz, professor at the Department of Zoology, Jahangirnagar University told TBS.
However, experts, including Dr M A Aziz pointed out that deer love Keora fruit and leaves, which are not affected by salinity increase.
Since birds prefer certain canopy layers, some researchers suggest that loss of tree-height will negatively impact bird species. Others opined that the impact might be insignificant as the birds will find shelter in other shorter trees in the absence of humans around.