With no research yet conducted, the Forest Department says it cannot prescribe how to save the Sundari tree
On the ragged edge of the Sundarbans, where the forest meets the brackish tides, Islam Sana from Tetultala Char in Maheshwaripur Union of Khulna's Koyra Upazila, counts change by what is missing. "Five or six years ago this was dense Sundari," he says, pointing into a thinning stand. "Now it's almost bare."
For 40 years, the Koyra fisherman-forager's life has orbited the mangrove's rhythms, flood and ebb, blossom and bee, but the rhythm is off. "Many Sundari have dried up," he says.
The irony is painful.
The Sundarbans, the world's largest mangrove forest, owes its name to the abundance of the Sundari tree. But rising soil salinity and an abnormal spread of parasitic vines have put the species under existential threat.
Forest dwellers and officials say Sundari has long suffered from "top die-back" -- crowns collapse and branches wither. As salinity increases in the forest's water and soil, saplings fail to survive, reducing natural regeneration. On top of that, a recent surge of vine-type parasitic plants has pushed the species into acute risk.
According to the Forest Department, these parasites slowly "asphyxiate" Sundari by siphoning sap from host trees, weakening them until leaves yellow and drop. Eventually, the trees die standing.
Md Rezaul Karim Chowdhury, divisional forest officer (DFO) of the Sundarbans East Forest Division, said parasitic growth has taken hold in about 25-30 percent of Sundari trees, especially older ones, across various areas of the eastern division.
"Previously, parasites stayed confined to branches. Now they've spread down to the trunk. Trees are weakening and eventually drying out and dying," he said.
The situation has become a shared concern for forest officials and everyone connected to the Sundarbans.
Mominul Islam Nahid, senior research officer at the Bangladesh Forest Research Institute (BFRI), said they have catalogued multiple parasitic species here -- herbs, shrubs, vines, ferns --but admits the specific culprits targeting Sundari have not yet been isolated.
Nor has there been the kind of structured research that could translate observation into a rescue plan. The Forest Department says bluntly it cannot yet prescribe how to save the Sundari trees.
Scale raises the stakes. Skirting the Bay of Bengal, the Sundarbans spans about 10,000 sq km across Khulna, Bagerhat and Satkhira districts in Bangladesh and South 24 Parganas in India's West Bengal.
Bangladesh's portion covers 6,517sqkm, or 66 percent of the total area. The latest 2024 survey identified 184 plant species in the forest, including trees, vines, shrubs, grasses, and parasitic plants, with parasitic pressure now most visible in certain areas.
DFO Rezaul noted the Bangladesh side is administratively split into East and West divisions: West covers Khulna and Satkhira; East covers Bagerhat and a small part of Khulna. Most Sundari trees are found in the eastern sector -- precisely where parasitic pressure is now most visible.
From Tetultala Char in Maheshwaripur Union, Koyra Upazila, Khulna -- on the forest's edge -- resident Islam Sana said he has lived off the Sundarbans for 40 years, but has noticed marked changes.
"Areas that were dense Sundari jungle five or six years ago are now almost bare. Many Sundari trees have dried out."
For Shahjahan Akon of Sharonkhola, who has harvested wild honey for three decades, the change is sensory.
"Places that used to be thick jungle are now open. In early May, Sundari blooms and the forest turns white (blanketed with flowers)," he says.
"But many Sundari trees have dried out. The bees aren't nesting in those areas." Fewer blossoms mean fewer hives; fewer hives mean weaker livelihoods for the people who have learned to live lightly with the mangrove.
Forest users say Sundari is a load-bearing pillar crucial for soil retention, coastal protection from cyclones and storm surges, and wildlife breeding. Its roots shelter juvenile fish and shrimp, birds nest in its canopy, and deer and the Royal Bengal Tiger rely on its cover.
What's driving the unravelling is both local and planetary. Salinity has crept upward in the forest's soils and channels -- an outcome of sea-level rise, shifting freshwater flows, and repeated storm surges pushing salt inland.
Sundari, while salt-tolerant, has limits. Sapling mortality rises as thresholds are crossed, cutting off natural regeneration just as parasitic vines sap the strength of what remains.
If Sundari declines further, coastal protection weakens, fisheries nurseries thin, and wildlife cover shrinks. The forest will still wear the name. It may no longer fit.
For now, Sana and Shahjahan wait under a canopy that blooms less each May, counting beehives like missing friends.
Prof Wasiul Islam of Khulna University's Forestry and Wood Technology Discipline said the Sundari tree has long suffered from the top-dying disease. However, rising salinity has intensified both this disease and the parasitic vine infestation.
"The parasitic vine spreads more easily to trees already weakened by the [top-dying] disease. Trees in high-salinity areas or in parts with limited tidal flow are particularly vulnerable."
Prof Wasiul said, "These vines usually flower and fruit during the winter. If they are removed beforehand, reproduction can be prevented. Birds spread their seeds from tree to tree."
He warned that if the population of Sundari and other mangrove species declines, the entire Sundarbans ecosystem could collapse. Effective solutions are impossible without extensive research.
Nahid of the Forest Research Institute said, "The decline of Sundari trees results from a combination of factors. Research is ongoing to analyse the causes of disease. Our studies show that older Sundari trees in highly saline areas are more susceptible.
"Saplings of less salt-tolerant Sundari varieties and other plants have lower survival rates. Conversely, trees locally known as Geowa and Bola are increasing in certain saline zones."
Researchers note that coastal rivers typically have salinity levels of 2-5 parts per thousand. Due to climate change, rising sea levels and reduced upstream freshwater flow have long pushed the forest's water and soil salinity beyond safe thresholds. Simultaneously, sedimentation has increased.
Higher salinity prevents saplings from establishing and reduces new tree growth. The tall, sparsely branched Sundari trees are more vulnerable to storm damage, decreasing both forest density and canopy cover.
Elevated river and creek water levels also restrict the trees' water absorption, leaving them stunted and disease-prone. Parasites then colonise weakened trees, accelerating mortality.
SALTWATER STRIKES DEEPER
Experts say climate change is wreaking havoc on the Sundarbans, reducing the freshwater character of the mangroves and accelerating saltwater intrusion.
Prof Abdullah Harun Chowdhury of Khulna University's Environmental Science Discipline said: "Rising sea levels due to climate change have increased the pressure of saline tidal waters in the forest, while upstream freshwater inflows have decreased.
"Consequently, soil and water salinity have risen, increasing tree diseases and mortality, and reducing forest density."
Prof Harun added that more research is needed on the causes, current status, and remedies for Sundari tree diseases.
AZM Hasanur Rahman, Divisional Forest Officer of the Western Sundarbans, said: "Sundari trees thrive in freshwater or slightly saline water. But with reduced upstream freshwater, overall forest salinity has risen, particularly in Satkhira and Khulna sectors."
Prof Anwarul Kadir, executive director of the Sundarbans Academy, said the forest is a product of natural mangroves, wetlands, marine and coastal ecosystems. Climate change has diminished freshwater sources.
"Many creeks are silted with sediment, and new shoals have emerged, increasing saline water coverage. This excess salinity is killing Sundari and other trees through disease."
He said, despite proposed conservation initiatives, their implementation has been inadequate.
Obaidul Kabir Samrat, media coordinator for the Climate Awareness and Sundarbans Protection Movement, said: "Restoring freshwater flow through upstream rivers could rejuvenate the forest.
"Diverting water from the Pashur and Baleshwar rivers would be effective. Conservation efforts should follow scientific evidence."
Imran Ahmed, conservator of forests in the Khulna region, said: "The Sundarbans, a UNESCO World Heritage site, is vital for biodiversity and environmental balance. The Sundari tree is crucial not only ecologically but also for the local economy."
On the vine infestation, he said: "We are monitoring the situation closely. Research on this parasitic plant to save Sundari trees will begin soon.
"Prof Ashfaque Ahmed of Dhaka University's botany department has already surveyed the forest and identified the vine species. Research work will commence shortly."