Profiling of mangrove forest dynamics in the Fly River delta, Papua New Guinea.

Mangrove forests (MFs), as vital ecosystems in tropical and subtropical coastal regions, play a significant role in the global carbon cycle. However, MFs are currently facing unprecedented risks of degradation due to both natural and anthropogenic factors. Here, using 477 remote sensing data from Landsat TM, ETM, and OLI series of satellites, the dynamic alterations of MFs in the Fly River Delta (FRD) of Papua New Guinea were analyzed by the Random Forest method. The Random Forest model in this study achieved an overall accuracy of 97 % and a kappa coefficient of 0.96, ensuring high-precision classification. The results indicate a total decrease of 33.83 %, with the mangrove area in the FRD declining from 81,979.2 ha in 1988 to 54,247.73 ha in 2023. Meanwhile, the delta coastal region experienced significant degradation, with a 54.71 % reduction over the 36-year period, while the delta island region exhibited a slower decline of only 2.05 % during the same timeframe. Moreover, seaward expansion of 1309.32 ha was observed in the easternmost islands of the delta island region, localized areas of the eastern shoreline, and river channels of the delta coastal region, partially offsetting the landward losses. Deforestation and the expansion of aquaculture ponds are significant contributors to the degradation of mangroves in the FRD. Although fluvial suspended sediment discharge delivered into the estuary promote mangrove growth, southeast waves are the primary cause of shoreline retreat, surpassing the effects of local sea level rise. Nevertheless, strong tidal currents transport sediments into the tidal channels of the delta, filling them and creating suitable habitats for local mangrove growth in the delta coastal region. These findings offer important insights into the factors influencing mangrove dynamics and could inform necessary management strategies for global mangrove restoration efforts.