Mangrove expansion in a large river delta is dependent on foreshore geomorphology

Abstract Understanding the environmental conditions facilitating mangrove expansion across the large scale of entire deltas or estuaries (~100 to 1000 km 2 ) remains limited, despite its relevance for upscaling mangrove restoration strategies, which necessitates knowledge on where active planting of mangrove seedlings has a high versus low chance of success. In this study, we investigate the environmental conditions facilitating mangrove development across the Guayas Delta in Ecuador, a large tropical river delta (~ 3200 km 2 ) where young mangrove forests are expanding across extensive intertidal mudflats. By combining satellite‐based remote‐sensing, drone‐based field verification and statistical modelling, we quantified differences in environmental conditions between mudflats showing active mangrove expansion versus mudflats with stable mangrove edges. We analysed key variables, including mudflat elevation, slope, curvature, wind fetch, mudflat length and tidal inundation duration, frequency and depth. Our results reveal that the morphology of the tidal flat foreshore is a key determinant of mangrove edge dynamics, and a stronger predictor than more commonly used inundation metrics such as the elevation, inundation duration, frequency and depth at the mangrove edge. In general, mangrove expansion is facilitated by wide foreshores that have low slopes and straight profile shapes, which are attributed to more efficient attenuation of waves and tidal currents. Conversely, mangroves are much less likely to expand when they border narrow, steeper and concave foreshores that provide minimal protection and that occur at lower intertidal elevations, where they experience increased inundation stress. Synthesis and applications . Our findings provide guidance for large‐scale mangrove restoration, particularly in tropical river deltas, by identifying sites where (1) mangroves can potentially expand in the future, and (2) conditions are favourable for active mangrove seedling planting. The presence of a mudflat in front of existing mangroves is not sufficient. Instead, mangrove planting should be prioritized on mudflats with low tidal inundation stress, low slope, large width and convex topographic profiles, while low intertidal areas with narrow, steep and concave mudflat foreshores should be avoided. These specific geomorphic mudflat characteristics may serve as indicators to select suitable mangrove planting sites at a large deltaic scale.