Drone-Based Mapping of Vegetation Edge Limits in Dutch Wadden Sea Salt Marshes::Spatial Variability, Human Interventions, and Environmental Drivers

Salt marshes in the Wadden Sea, a mesotidal system along the North Sea coasts of the Netherlands, Germany, and Denmark, provide essential services for biodiversity and coastal protection. However, the processes that shape marsh development, particularly at the seaward vegetation edge, remain poorly understood. The seaward vegetation edge is critical for evaluating the long-term potential of marshes as natural coastal defences.<br/>This study investigates the elevation of the seaward vegetation edge across four Dutch Wadden Sea marshes, identifying environmental factors that influence its position and assessing the impact of brushwood dams, a common restoration strategy in the region. Over two years, high-resolution aerial imagery was acquired using a hybrid vertical take-off and landing fixed-wing UAV (Sony 203 DSC-RX1R II 42 MP full-frame RGB camera). Orthomosaics and digital elevation models (DEMs) were generated via Structure-from-Motion photogrammetry from the overlapping RGB images. Vegetation boundaries were delineated using Iso Cluster unsupervised classification in ArcMap and reclassified into binary vegetated/unvegetated rasters, following the method described by Farris et al. (2019). These were vectorised to extract vegetation edge lines spatially and temporally. Elevation data were extracted from the DEMs.<br/><br/>The results indicate that vegetation establishment thresholds vary between sites, with differences in median thresholds reaching up to 30 cm. Marshes sheltered by human interventions, as brushwood dams, support vegetation at lower elevations than neighbouring exposed marshes, suggesting that hydrodynamic sheltering can enhance establishment at lower tidal heights. By integrating fine-scale remote sensing with local hydrodynamic context, this approach highlights how both natural drivers and human interventions interact to shape salt marsh dynamics. Overall, the workflow provides a transferable framework for monitoring vegetation change in intertidal zones and assessing the effectiveness of marsh restoration strategies under varying environmental conditions.