ASPB’s 2005 AAAS Mass Media Fellow Sarah Nell Davidson is sending a series of “postcards” back to the ASPB News as she spends the current academic year abroad doing research for her PhD thesis.

Greetings from the Sundarbans—The Largest Contiguous Tract of Mangrove Forest on Earth

Bangladesh is roughly the size of England and Wales combined. Yet as much water flows through Bangladesh as through all of Europe. The river system is the ebb and flow of Bangladeshi life, as nearly all of its 140 million people are dependent on this vast network of river systems in carving out their livelihood.

Where the Ganges, the mighty Meghna, and the Brahmaputra Rivers pour into the Bay of Bengal, an 80,000-square-kilometer delta is formed, fringed by the largest contiguous tract of mangrove forest on earth—the Sundarbans. Although the Sundarbans may be most famous for its man-eating Royal Bengal Tiger population, their likely namesake is an endemic tree, the Sundari tree (Heritiera fomes)—the real king of this estuarine jungle.

My companion, a self-described GIS (geographic information system) geek from Redlands, California, and I venture south from the capital city of Dhaka on a 30-hour trip through Bangladesh’s river superhighways. Once we arrive in Mongla Port, the gateway to the Sundarbans, we are met by our guide, Bachchu, who’s aboard a more modest boat equipped for four. It is from this boat and the small rowboat we have in tow that we set out in search of tigers, Sundari trees, and the more than 270 bird species that inhabit the 601,700 hectares of forest reserve. The following morning, we pick up our own armed Bungalow Bills—two guys from the local forest service—who offer us protection as we venture on foot into the home range of the Royal Bengal Tiger, which has been known to attack humans. Although a run-in with an actual tiger is not in our cards, we do spot fresh tracks among the mat of the Sundaris’ aerial roots.

Like the Notre Dame cathedral, the Sundari tree, which slowly grows to 25 meters, relies on substantial buttresses for support in this brackish intertidal zone. Its aerial roots look like a sea of gnome’s caps protruding through the forest floor. Prized for their hardwood and straight growth, these trees have been plucked from their forest home for centuries and used in the production of boats, electricity poles, furniture, and charcoal. The overexploitation of the Sundari tree and other cohabitants has resulted in a 25% decrease in the productivity of this forest over the past 25 years. Although restrictions are now in place to prevent overharvesting of the Sundari tree, in a country where forest workers are paid very little and corruption prevails at every rung of the ladder, a small cut from a poacher’s bounty ensures a turn of the head.

A more serious threat to the Sundari tree originates upstream. Roughly 30% of Sundari trees have fallen victim to “top dying disease.” As suggested by its name, symptoms appear at the top of the main stem and progress downward, resulting in leafless trees that meet an early end. Although the disease was first reported in the Sundarbans in the 1930s, it was rare and manageable until it became widespread in the 1990s.

Over the past two decades, various studies have sought to ascertain the cause of top dying. Although many locals adhere to the theory that it is caused by a virus, the chief conservator of forests in the Sundarban region, Shaikh Mizanur Rahman, explains that it is in fact caused by an increase in salinity and siltation. Whereas the western region of the Sundarbans has always been more saline (and its Sundari trees less abundant and dwarfed), the eastern portion has enjoyed a greater fresh water flow, creating the unique habitat that typifies this part of the region. The more abundant plant life in the eastern half supports many more herbivores, such as the spotted deer, which in turn support the tiger population; both of these species are rarely found in the western half. A major reduction in the amount of fresh water flowing into the eastern region, however, has thrown the system out of balance.

It isn’t difficult to ascertain the cause of this decrease in fresh water flow. One has only to look 15 kilometers beyond Bangladesh’s borders to the Farakka Dam, which chokes the Ganges in neighboring West Bengal, India, diverting much of it to the Hooghly River, bound for Calcutta. Rahman reckons that during the dry season, the amount of water flow that makes its way to Bangladesh after passing through the Farakka Dam has been reduced by 75%, altering the delicate balance of sweet and salty in the river’s delta and causing major siltation problems as the trickle heads downstream.

Khan Alaluddin, a physicist at nearby Khulna Public College who studies the effect of increased salinization on the properties of Sundari wood, says that the normal level of salinity in the eastern region of the Sundarbans was about 3.5% to 4.0% on average. Areas of the west are up to 18.0%—a level that the Sundari tree cannot tolerate. As salinity increases in its eastern habitat, the Sundari tree loses its inherent ability to cope with its saline environment.

As our physiologists back at Cornell explained, it’s all about water potential. “Pure water enters a cell easily, because water potential outside is higher than inside the cell. If the soil has a lot of salt, it lowers the potential, and there is less drive for water to move into the leaf cells,” Bob Turgeon explained. Because gravity is also at work, it is hardest for leaves at the top of the tree to get water, which may be why the Sundari—the Sundarban’s local giant—is having the hardest time coping.

Roger Spanswick continued my refresher course in plant–water relations. “With an increase in salinity, if the cells in the leaves do not adjust to this decrease in water potential, they will lose turgor, and the stomates will close. To avoid wilting, the cells must accumulate more solutes to make their osmotic potential more negative. Aside from avoidance of wilting, the maintenance of a positive turgor pressure is important as the physical driving force for cell expansion and hence leaf expansion growth”—thus the balding of the Sundari.

But as I am reminded by another of my home-based physiologists, Tom Owens, “One cannot neglect the direct or indirect effects of higher salt concentrations on a wide variety of processes other than water transport. High solute concentrations in general, and specific ions more commonly, can screw up metabolism and transport at many different levels.” This metabolic mix-up might explain why in this new salty predicament, the Sundari tree is unable to fend off other diseases caused by viruses and fungi, resulting in a veritable field day for a plethora of pathogens.

Interestingly, Alaluddin’s studies on wood composition show that the increase in salinity has caused a major shift in wood composition, from its natural hardwood state to that more typical of soft woods; percent cellulose decreases, and percent lignin increases. He estimates that a 10% increase in salinity results in a 1% increase in wood softness.

Chief Conservator Rahman says he doesn’t subscribe to the widely accepted forecast that global climate change will cause ocean levels to rise significantly by the year 2050. “Some people say that due to greenhouse effect, the sea level will rise, and the Sundarbans will be under water,” he begins. “This may not happen, because siltation rate is higher than the rate of rise of sea water. That we will have a higher level of water—that is not true.” Fortunately, other local ecologists are more pragmatic. With the Ganges choked upstream, several rivers that contribute fresh water to the delta are already dead. An increase in sea level in the Bay of Bengal will exasperate the salt problem tremendously and may mean the end not only of the Sundari tree, but of the entire delicate ecosystem of the Sundarbans, tigers and all.

When I asked Rahman what the plan is to save the Sundari, his response was casual but for me bore much grimmer political undertones. “I am conservator of this forest,” he begins. “This sudden change in the environmental ecosystem is beyond my control,” he continues, blankly staring at a map of the Sundarbans before him. “If these trees are dying, we can take them out, because as a country we are short on wood.” And most conveniently for wood harvesters, it is the tallest trees that are most severely affected. Rahman continues, “So what is happening, there will be some changes in vegetation patterns. Big Sundari trees are dying, and this area is now covered by a new species, Gewa [Excoecaria agallocha]—so more Gewa are coming in.” Rahman seems strangely comfortable with this outcome.

Whether the Sundarbans will soon be renamed the Gewabans, or whether the Bay of Bengal will swallow up the forest in its entirety, this unique wonderland is in a critical state of threat. Bangladesh may not have the resources or political will to save the Sundarbans, but the people of Bangladesh are proud to be its primary stewards. Fortunately, an increasing number of nongovernmental organizations in the area are making the welfare of the Sundarbans their mission. For, as one tourism poster reminds them, “It is our Taj Mahal, it is our Everest.”

Sarah Nell Davidson
snd2@cornell.edu