Approximately 60% of the southern Thai coastline used to be occupied by mangroves according to the first mangrove forest assessment in
1961. During the past three decades, these mangrove areas have been reduced to about 50% with less than 10% left on the east coast. Coastal
erosion and accretion occur irregularly along the coast but an intensification of erosion has been noticed during the past decade. This study
assessed the relationship between mangrove presence and changes in coastal area. Mangrove colonization rates were assessed using in situ transects
and remote sensing time series. Both methods led to comparable estimates ranging between 5 and 40 m y1. Quantitative data on changes
of coastal segments along southern Thai coastlines as well as available possible factors responsible for these changes were compiled. Overall, net
erosion prevailed (1.3 0.4 m y1). The Gulf of Thailand coastline in the East of the country was found to be most dynamic: change occurred
along more coastal segments than in the West (43% vs. 16%). Rates of erosion and accretion were also higher, 3.6 versus 2.9 m y1 and 2.6
versus 1.5 m y1, respectively. Total area losses accounted for 0.91 km2 y1 for the Gulf coast and 0.25 km2 y1 for the West. Coasts with
and without mangroves behaved differently: in the presence of mangroves less erosion was observed whilst expansion occurred at particular
coastal types with mangrove existence, i.e. river mouths and sheltered bays. Possible underlying causes were examined using multivariate analysis.
Eroded areas were found to increase with increased area of shrimp farms, increased fetch to the prevailing monsoon, and when dams reduced
riverine inputs. Notably, however, in areas where erosion prevailed, the presence of mangroves reduced these erosion rates. Mangrove loss
was found to be higher in the presence of shrimp farms and in areas where mangrove forests used to be extensive in the past.
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