Mapping vegetation change and cyclonic effects along the coastal regions of Bangladesh: A spatio-temporal perspective

Abstract

Being a climate-vulnerable deltaic country, Bangladesh faces substantial threats to its coastal vegetation due to frequent cyclones. Hence, monitoring the changes in vegetation greenness is essential to understand the vulnerabilities and resilience of this region’s ecosystem. Therefore, this study assesses the long-term trends and changes in vegetation greenness across the western, central and eastern coastal zones of Bangladesh from 1990 to 2023, using Normalized Difference Vegetation Index (NDVI). Then it inspects the immediate impacts of cyclones- Bulbul and Amphan on vegetation areas and density by comparing pre- and post-cyclone NDVI values to evaluate vegetation resilience. Landsat derived NDVI from Google Earth Engine were used for temporal trend analysis utilizing Mann-Kendall trend and Sen’s Slope tests, focusing on the dry period. Then, for detecting the spatial variation, the study reclassified the NDVI values in no, slight, medium and high-density vegetation classes. Results indicate a significant upward trend in NDVI for the study period, particularly in areas with moderate (0.3–0.6) and high (0.6–1.0) vegetation density. Post-cyclonic evaluation following both cyclones, reveals significant reduction in high-density vegetation along with a compensatory increase in low density vegetation classes. The average NDVI values in the region fell by 13 % and 31 % immediately after cyclone Bulbul and Amphan respectively. The results also imply similar changes immediately after both cyclones for the Sundarbans which often is regarded as the first line of defense against tropical cyclones in Bangladesh. This study combines long-term analysis and cyclonic event-based assessments to provide critical insights into the coastal vegetation dynamics in Bangladesh where the long-term greening trend suggests possible land use changes and expanded agricultural activities. And the immediate loss of greenery after cyclones coupled with the gradual recovery within a few fortnights, showcases both the devastating impacts of cyclones on coastal vegetation and the resilience of these ecosystems to recover.