Mapping mangrove forests in the Red River Delta, Vietnam

Mangrove forests (MFs), which occur along tropical and subtropical coastal zone, are among the most productive and richest carbon-rich ecosystems in the world. However, MFs have experienced great losses worldwide due to the impacts of intensive human activities and relative sea level rise. Here, the recent dynamic changes in the MFs of the Red River Delta (RRD), the second largest delta in Vietnam, were quantified using the random forest algorithm based on 328 remote images obtained by Landsat TM/ETM+/OLI during 1986-2019. The results show obvious increasing trends in the MF area with a change rate of 39 ha/y from 1,655 ha in 1986 to 2,944 ha in 2019. The change rates in the northern RRD (NP) and southern RRD (SP) were 8.44 ha/y and 30.62 ha/y, respectively, even though MFs in the tropics worldwide encountered significant losses. Further, approximately 1,091 ha of MF was converted into aquaculture ponds during 2000-2006 in the interior of the RRD, while the shoreline of MFs in the NP and SP continually expanded seaward to the northeast and southwest at average rates of 38.9 m/y and 17.9 m/y, respectively. Moreover, although the fluvial sediment supply declined dramatically from 199.57 to 16.41 mt/y during 1986-2017, the sediment supply was not responsible for the present seaward expansion of MFs. Wave action drove sediments to overwash barriers, and tidal dynamics carried this sediment into tidal channels, depositing sediments behind the barriers and on both sides of tidal channels with continuing MF growth. In addition, the conversion of MFs into aquaculture ponds by human activities was responsible for the massive losses of MFs in 2000-2006, but these impacts were alleviated as conservation policies were implemented by local governments in subsequent years. This study indicates that the growth, loss and recovery of MFs are affected by various factors, and it is vital to map the variations in MFs to better implement regional ecological restoration programs for coastal MFs.

Automated summary

The study demonstrated a significant increasing trend in the Mangrove forests (MFs) of the Red River Delta over the past 30 years, with some areas being converted into aquaculture ponds. However, the mangroves continued to expand seaward to the northeast and southwest despite a drastic decline in fluvial sediment supply, as wave action and tidal dynamics played a role in sediment distribution behind barriers and along tidal channels. The conversion of MFs into aquaculture ponds by human activities led to substantial losses of MFs, but conservation policies implemented in later years helped alleviate these impacts.