Dynamic changes in mangroves of the largest delta in northern Beibu Gulf, China: Reasons and causes

Many mangrove forests occur along estuaries and deltas are experiencing irreparable losses throughout the world due to changes in natural factors and intensive anthropogenic interferences. However, little information is available regarding variations in mangrove forests and associated reasons and causes. Here, the long-term dynamic patterns of mangrove forests in the Nanliu River Delta (NRD), the largest delta in the Beibu Gulf, China, were detected based on a series of hydrosediment data and remote sensing images between 1986 and 2020. The results indicated that the total mangrove forest area of the NRD increased in an incremental manner, even though a rapid decline occurred in the western terrestrial margin of the delta before 1998. Additionally, mangrove forests have expanded southeastward to the sea in the eastern NRD, and they have colonized to the southwest in the western NRD. Moreover, long-lasting horizontal seaward expansion with vertical accretion in the bare tidal flat of the NRD created new sites for potential mangrove regeneration. Furthermore, an average annual sea level rise of 0.2 mm per year and an 86% decline in fluvial sediment supply could not cause expansions and local losses of mangrove forests. The combination of local tidal currents and waves transports sufficient estuarine sediment to the northeast into the delta to provide important sediment material for the deposition of mangrove tidal flats. The mangrove forest destruction induced by local residents and the ecological restoration implemented by the government are the causes of the continuous serious losses and extensive gains of mangrove forests, respectively. The results highlighted that the dynamic changes in mangrove forests in the NRD caused by driving forces from natural and human interferences can serve as significant references for mangrove forest restoration and decision-making policy management in similar deltas around the world.