Inside the Sundarbans
Shihabur Rahman, Wikipedia
For decades, the Sundarbans has stood as one of the world's greatest natural fortresses. Straddling India and Bangladesh across nearly 10,000 square kilometres, this UNESCO World Heritage Site is far more than a dense expanse of mangroves. It is a living shield against cyclones, a globally significant carbon sink, a nursery for fisheries, and home to iconic species including the Royal Bengal tiger, estuarine crocodile, Irrawaddy dolphin and hundreds of birds, reptiles and plants. More importantly, it supports the livelihoods and security of more than 3.5 million people living in and around its fragile deltaic landscape.
Yet beneath this seemingly resilient landscape, a quieter ecological crisis is unfolding.
A new study published in Ecological Indicators by Siddhartho S. Paul and colleagues reveals that while the overall extent of the Sundarbans has remained relatively stable over the past three decades, the health of its mangrove vegetation has steadily deteriorated. The findings challenge a long-held assumption in conservation—that if forest cover remains intact, the ecosystem itself must also be healthy. The research demonstrates that this is no longer true for the Sundarbans.
Using more than three decades of satellite observations between 1988 and 2020, the study shows that nearly four-fifths of the mangrove forest has experienced high to very high fluctuations in vegetation health. Even more concerning, 77 percent of the forest exhibits signs of long-term "browning" when measured using advanced vegetation indices that capture changes in canopy condition and biomass. The forest, in other words, is still standing, but it is gradually losing its ecological vitality.
The implications extend well beyond the Sundarbans. Around the world, forests are increasingly threatened not only by outright deforestation but also by subtle degradation that weakens ecosystem functions long before trees disappear from satellite maps. The Sundarbans offers one of the clearest examples of why conservation can no longer focus solely on hectares protected; it must also account for the health, resilience and functioning of ecosystems.
Looking beyond forest cover
Conventional assessments of forests often rely on a straightforward question: how much forest remains? While this approach is useful for detecting large-scale deforestation, it frequently misses more gradual forms of ecological decline.
Mangroves are particularly vulnerable to this oversight. Rising salinity, prolonged inundation, repeated cyclone damage, declining freshwater flows and human disturbances may not immediately reduce forest area. Instead, they weaken tree canopies, reduce biomass, alter species composition and diminish the forest's ability to provide ecosystem services. A mangrove forest may therefore appear intact in land-cover maps while simultaneously becoming less productive, less diverse and less capable of protecting coastlines.
The new study addresses this limitation through an innovative monitoring framework that focuses on vegetation health rather than forest extent alone.
The researchers analysed annual Landsat satellite imagery covering the entire Sundarbans across India and Bangladesh from 1988 to 2020, creating a continuous 33-year record using the Best Available Pixel (BAP) algorithm on Google Earth Engine. By selecting the highest-quality cloud-free observations each year, the BAP approach overcomes the persistent cloud cover that often limits tropical remote sensing and enables reliable long-term monitoring of vegetation.
Unlike earlier studies that examined either the Indian or Bangladeshi Sundarbans separately, this research assessed the mangrove ecosystem as a single transboundary landscape, providing a more realistic understanding of ecological change across the region.
The study also moved beyond the conventional Normalised Difference Vegetation Index (NDVI) by incorporating Green NDVI (GNDVI) and the Enhanced Vegetation Index (EVI). While NDVI effectively measures vegetation greenness, EVI is better suited to dense mangrove forests because it detects subtle changes in canopy structure and biomass that traditional indices often miss.
These satellite-derived indicators were combined with statistical trend analyses, including the Mann-Kendall test and Sen's slope estimator, as well as machine-learning models to identify the environmental and human drivers of long-term vegetation change. Together, these methods provide one of the most comprehensive assessments to date of the ecological health of the Sundarbans.
A forest in constant flux
One of the study's most significant findings is the remarkable instability of mangrove vegetation over the past three decades. Around 80 percent of the Sundarbans experienced high to very high fluctuations in vegetation between 1988 and 2020, reflecting repeated cycles of damage and recovery driven primarily by cyclones. Satellite records show sharp declines in vegetation following major storms, including those between 1999 and 2002 and after Cyclone Sidr in 2007. Although forests recovered over time, successive cyclones repeatedly interrupted regeneration, leaving the ecosystem in a near-constant state of recovery.
The central Sundarbans emerged as the most vulnerable region, where rising salinity and the decline of the dominant Sundri (Heritiera fomes) tree, affected by the "top dying" disease, have intensified vegetation stress. Climate change has further compounded these pressures through sea-level rise, increasing salinity intrusion and more frequent extreme weather events.
The study also found that the choice of vegetation index significantly influences how forest health is interpreted. While NDVI and GNDVI suggested browning across about 40 percent of the forest, the more sensitive Enhanced Vegetation Index (EVI) detected vegetation decline across 77 percent of the Sundarbans. This suggests that conventional monitoring may substantially underestimate ecological degradation in dense mangrove forests. The findings highlight the need to move beyond measuring forest extent and instead adopt monitoring systems that track canopy health and biomass, enabling early interventions before degradation becomes irreversible.
What is driving the decline?
The study highlights that the degradation of the Sundarbans is driven by an interconnected system of climate variability, hydrological shifts, land-use changes, and geographical factors rather than a single isolated threat. Key drivers reshaping this ecosystem include:
Precipitation Anomalies: While mangroves rely on water, excessive rainfall accounts for nearly one-third of observed vegetation changes. Beyond certain thresholds, prolonged inundation deprives roots of oxygen, alters sediment deposition, impairs nutrient absorption, and worsens vegetation health—especially when combined with poor drainage and sea-level rise.
Upstream Hydrological Alterations: Decades of dam building and river regulation have drastically curtailed freshwater inflows. The resulting saltwater intrusion has raised salinity levels across the central and southern delta, stressing species adapted to stable salinity gradients.
Geographical & Spatial Variations: Forest health degrades noticeably from west to east, with a distinct drop in vitality across the Bangladeshi border. Spatial variables—longitude (explaining ~30% of variation) and latitude (13%)—reflect regional differences in freshwater access, salinity, cyclone vulnerability, and land pressure, proving the delta cannot be treated as a uniform landscape.
Anthropogenic Pressures: Shrimp farming, agricultural expansion, encroachment, and pollution steadily undermine resilience, while rising sea levels and erosion are converting forest land into open water.
Ultimately, the study emphasizes the compound effect of these stressors. Multi-layered pressures—such as a severe cyclone striking an already flooded and saline-stressed forest—accelerate long-term ecological decline. Effective conservation requires holistic management of the entire interconnected network of threats.
Why mangrove health matters far beyond the delta
The degradation of the Sundarbans extends far beyond biodiversity loss, threatening vital ecosystem services long before forests completely vanish. As canopy density and vegetation health deteriorate, the forest's ability to act as a crucial carbon sink, buffer coastal storm surges, filter water, and nourish commercial fisheries steadily declines.
These ecological shifts directly translate into severe socio-economic insecurity for millions of local residents:
Fishing & Resource Communities: Catch yields decline as nursery habitats degrade, while traditional foragers face shifting species distributions.
Agriculture & Water Security: Saltwater intrusion disrupts crop yields and reduces access to fresh drinking water.
Coastal Protection: Weakened mangrove barriers leave settlements increasingly vulnerable to devastating cyclone impacts.
Consequently, two key policy priorities emerge:
Strategic Conservation: Mangrove protection is not merely an environmental goal, but a core strategy for climate adaptation, disaster risk reduction, and economic stability.
Updated Monitoring: Tracking total forest coverage alone provides a false sense of security. Conservation systems must evolve to monitor canopy health, biomass, and functional stress to enable early intervention before irreversible decline occurs.
From monitoring to management
A practical path forward for the Sundarbans requires moving from passive observation to an operational management framework. Leveraging freely available Landsat imagery alongside vegetation indices like EVI offers a low-cost, scalable method for continuous ecosystem surveillance. By identifying browning hotspots early, conservation agencies in India and Bangladesh can intervene before structural degradation becomes irreversible.
Monitoring alone, however, cannot rebuild resilience. Freshwater management must form the core of all conservation strategies. Restoring upstream environmental flows, reducing hydrological fragmentation, and managing sediment transport are essential to counter rapid salinity intrusion. Because mangroves cannot thrive in degraded water regimes, hydrological restoration must precede tree planting. Furthermore, interventions must be regionally tailored to address distinct local pressures, such as extreme salinity in the central delta, cyclone vulnerability in the south, and aquaculture expansion in the north. Success must ultimately be evaluated through canopy density, biomass accumulation, and overall ecosystem health rather than sapling counts.
Because the Sundarbans is a unified ecological system divided by political borders, effective management requires deeper transboundary collaboration. Joint monitoring protocols, harmonized data sharing, and coordinated planning between India and Bangladesh are essential for managing cross-border water flows and storm impacts. Future efforts should also integrate high-resolution salinity mapping, field measurements, and emerging technologies like radar and drone surveillance.
While the Sundarbans appears remarkably resilient, surviving repeated cyclones and human pressures, its functional health is quietly eroding. The absence of widespread deforestation masks a deeper decline. Securing the ecosystem's future requires shifting policy focus from merely measuring surviving forest area to restoring functional vitality, ensuring it can continue shielding coastlines, supporting livelihoods, and storing carbon.