Forest Change in the Mangroves of the Ganges-Brahmaputra Delta 1906-2014

Abstract

For the first time in the history of mangroves, the Ganges-Brahmaputra delta in particular, this doctoral research aims to quantify forest change with an unprecedented period spanning from 1906 to 2014. The world’s largest mangroves – the Sundarbans of the Ganges-Brahmaputra delta accounts for 4.5% of the world’s mangrove forests, has been used as the test site. The research quantified the forest change into two distinct strands: (a) an assessment of vegetation dynamics over 106-years (1906-2012) in the Bangladesh Sundarbans and (b) a 42-years long (1973-2014) satellite imagery-based assessment of spatial change patterns of the Sundarbans. To address those objectives, this study used three different datasets: (1) a three month long extensive ground survey in 2011-12; (2) synthesis of six epochs of vegetation estimates: 1906-10, 1923-26, 1957-59, 1982-84, 1996-97 and 2011-12; and (3) mapping forest and forest types from six epochs of Landsat time-series: 1973 (MSS1), 1979 (MSS3), 1989 (ETM+ 7), 2010 (TM5), and 2014 (OLI8) using ground-based training samples and high (5m-RapidEye) to very high (<5m-GeoEye, IKONOS, QuickBird, OrbView) spatial resolution optical imagery spanning from 1967 to 2014.
Ground survey of 164 sample plots, 400 m2 per plot, has recorded 21 tree species from 33 recorded plant species having median values of stem diameter and total height estimated to 3 cm and 5.4 m, respectively. Out of 47,293 stems, gewa was found dominating (42%) which together with sundri constitute about 80% of the stems. Sundri stems were found to be taller than gewa but opposite in the case of stem median diameter and are significant in both cases. Stocking has declined to 50 stems.ha-1 from 650 stems.ha-1 from ≥20 cm to ≥10 cm diameter. Tree richness, diameter and total height has found declining with increasing salinity. The southeast region of the Bangladesh Sundarbans has recorded the highest diameter and taller stems. Overall, total height of stems has declined while tree mortality from top-dying has increased 4.5 times.
Over the last 106-years (1906-2012), a 58% decline of ≥15 cm diameter sundri has been estimated. From 1923-2012, the sundri-dominated forest has declined by 34% while gewa and other-dominated forests have increased by 11% and 24% respectively. Decline in larger diameter stems (≥ 5cm) has provided space for younger stems to grow and dominate which showed an overall increase in stocking of sundri, gewa and other-dominated forest types. These long-term forest change estimates will benefit in understanding ecology and carbon dynamics of mangroves that eventually contribute in UN-led reduced emission from deforestation and forest degradation (REDD+) programmes for mangroves of the Ganges-Brahmaputra delta.