Assessing cyclone disturbances (1988-2016) in the Sundarbans mangrove forests using Landsat and Google Earth Engine

Cyclone disturbances can cause significant damage to forest vegetation. The Sundarbans spreading across Bangladesh and India, the world's largest mangrove forest, is frequently exposed to cyclones of various magnitudes. However, the extent and pattern of forest disturbances caused by cyclones in the Sundarbans (both parts) remain poorly understood, and a long-term dataset focused on cyclones and forest disturbances is required. In this study, Google Earth Engine and Landsat images were used to evaluate changes in the normalized difference vegetation index (NDVI) before versus after 21 cyclones that occurred between 1988 and 2016. Supervised classification successfully classified the forest area with an overall accuracy of 86% and Kappa coefficient of 0.80. The percentage of affected forest area (i.e., the area that exhibited negative changes in NDVI values following a cyclone) ranged from 0.5 to 24.1% of the total forest area. Of the 21 focal cyclones, 18 affected less than 10% of the forest area, while two cyclones, Sidr in 2007 (category H5) and a cyclone in 1988 (category H3), affected 24.1% and 20.4%, respectively. Among the cyclone parameters (i.e., maximum wind speed, distance from the Sundarbans, and river water level), wind speed was significantly and positively correlated with affected forest area. Wind speed and affected forest area were nonlinearly related indicated by the piecewise linear regression and cubic regression. The piecewise model estimated a threshold point, suggesting that wind speed had little effects below a breakpoint of 101.9 km h(-1). Our analyses, based on a 29-year dataset, suggest that, although the region experienced cyclones almost every year, only the largest cyclones (i.e., in the H3 category or higher) affected 20% or more of the mangrove forest area, and these occurred around once per 7- to 12-year period. Trees with broken stems or uprooted canopies as a result of strong winds are likely to contribute to the reduction in NDVI in the aftermath of a cyclone. From a long-term perspective, such rare yet intense cyclones may have a significant effect on regeneration and species composition in the Sundarbans mangrove forest. Since previous studies only focused on a few cyclones, our results based on 21 cyclones will certainly help better understanding of the effects of cyclones on mangrove forest disturbance.