Shrub Mangroves Facilitate Self-Sustaining Conditions for Colonization: Insights From the Nanliu Delta, China

Shrub mangrove (SM) tidal flats are vital ecosystems in tropical and subtropical regions, yet they are threatened by rising sea levels and anthropogenic activities. With their lower canopy height, SMs influence hydro-sedimentary processes differently from the well-studied tree mangroves, highlighting the need for a deeper understanding of the stability of the tidal flats they colonize. Here, we analyze hydrodynamic and sediment transport processes over a full spring-neap tidal cycle on an Aegiceras corniculatum tidal flat in the Nanliu delta China, to explore the bio-morphodynamic feedback shaping SM tidal flats. Our findings reveal distinct differences in hydrodynamics and sediment transport between the flood and ebb phases, with the flood phase playing a significantly stronger influence. During the flood phase, the interaction between tidal flow and increasing vegetation density landward results in a significant reduction in flow velocity (up to 36%), particularly concentrated at the vegetation fringe. This reduction diminishes sediment transport capacity (up to 80%), leading to a decline in suspended sediment concentration as it moves landward, resulting in localized deposition in front of the densely vegetated area. This process is further supported by the substantial vertical accretion observed over an annual timescale. Our observations reveal that sediment deposition at the front edge of the dense SM zone is associated with a coarsening of surface sediments, which creates favorable conditions for seedling establishment and drives seaward vegetation expansion. These findings highlight the critical role of SMs in coastal ecosystem resilience and the evolution of tidal flats.