Enhanced local erosion in newly established Scirpus mariqueter salt marshes: evidence from UAV photogrammetry

Newly established salt marsh vegetation may exert unexpected effects on sediment dynamics, yet direct high-resolution evidence remains scarce. To address this gap, this study monitored early-stage colonization of Scirpus mariqueter at Chongming Dongtan using Unmanned Aerial Vehicle (UAV) imagery acquired from July to September 2021. Across all monitoring periods, a dynamic thresholding approach using the Green Leaf Index (GLI) enabled high-accuracy classification of vegetated versus unvegetated mudflats, with overall accuracies exceeding 99% and Kappa coefficients above 0.97. This robust classification enabled the identification of newly established vegetation as target areas for erosion analysis, while also ensuring reliable delineation of bare-surface areas for constructing digital elevation models (DEMs). The resulting DEMs achieved a spatial resolution of 5 cm and a root-mean-square error (RMSE) of 0.034 m, enabling precise detection of microtopographic changes at the decimeter scale. Orthophoto analysis revealed that scour pits observed in September closely mirrored the contour shapes and spatial boundaries of these newly established vegetation areas, with strong spatial congruence in both centroid location and perimeter extent. Statistical analysis showed that the average erosion depth within the newly established vegetation areas reached 12.5 ± 5.2 cm, significantly deeper than the 6.4 ± 5.5 cm observed in the adjacent bare mudflats. These findings suggest that the newly established S. mariqueter areas may have intensified local hydrodynamic processes, thereby enhancing sediment erosion. This indicates that, under natural colonization conditions, newly established vegetation establishment may induce negative geomorphic feedbacks, challenging the conventional assumption that vegetation consistently promotes surface stabilization in tidal wetlands.