Résumé
The role of substrate elevation in the establishment of mangroves has been highlighted in the literature. However, the link between elevation and mangrove growth over large areas remains poorly known. To address this issue, we conducted an experiment in French Guiana, where extensive mud banks provide large areas of accreting mudflats over which occurs rapid mangrove expansion. About 15 hectares of a mudflat were topographically surveyed monthly to bimonthly over 19 months using Unmanned Aerial System (UAS)-based high-resolution photogrammetry, and environmental variables (vegetation, water depth, temperature, salinity) continuously measured. During the experiment, the mudflat elevation increased by 0.9 m, from 2.4 to 3.5 m above Lowest Astronomical Tide level, in the wake of mud-bank sedimentation. About 5.5 hectares were colonised by Laguncularia racemosa and Avicennia germinans mangroves in a couple of months. Once the elevation reached 2.8 m, close to Mean High Water Neaps, seedlings rapidly established and spread. High-frequency mapping of elevation using UAS-photogrammetry reveals, thus, the pivotal role of coastal accretion in mangrove establishment. We discuss the advantages of this cost-effective tool in confidently identifying critical elevation levels for natural mangrove establishment and for viable mangrove restorations, many of which fail due to poor knowledge of substrate elevation. We show the limitations of photogrammetric surveys once the mangrove cover develops. Monitoring the link between mangrove establishment and elevation is an essential aid in anticipating coastal change and determining biodiversity loss, and in promoting successful mangrove use in ecological restoration and as a nature-based solution.