Detection of Kankra (Bruguiera gymnorhiza) tree expansion in the northern Sundarbans

Abstract

Mangroves are vulnerable to environmental changes, which can shift in the dominant species composition across ecological zones. This research aims to develop an object-based image analysis (OBIA) methodology combining Landsat imagery with very high spatial resolution reference imagery for mapping changes in the dominant species composition in a mangrove ecosystem in the Sundarbans, Bangladesh. The study analysed Landsat images (1988, 1989, 2000, 2010, and 2022) alongside QuickBird (2004–2005) and WorldView-2 (2021) as reference data. Satellite images were interpreted using field data, published maps, and relevant documents. The study found that 1,785 ha of Sundri–Gewa–dominated areas were converted to high-density Kankra ( Bruguiera gymnorhiza )-dominated regions, while 3,000 ha transitioned to moderate-density Kankra-dominated areas. Kankra is gradually replacing Sundri ( Heritiera fomes ) and mixed Sundri-Gewa ( Excoecaria agallocha ) stands in several parts of the study region. The study addresses its objectives by delivering an OBIA-based methodology for detecting expansion of one species by replacing a group of species, achieving overall classification accuracies of 90.67% (1988) and 92.67% (2022); and applying this methodology to quantify, for the first time, multi-decadal Kankra expansion in the Chandpai Range. The findings provide a reproducible framework for monitoring dominant species dynamics in mangrove ecosystems and offer quantitative evidence of ongoing compositional change in one of the world’s most ecologically significant wetland systems.