Remote sensing assessment of the world’s largest mangrove reforestation program (Senegal, 2009–2012): Implications for carbon crediting and future restoration efforts dedicated to climate change offsetting

• Remote sensing indicates that less than 30 % of the VCU has been stored by the project. • Only 31.7% of the project appears as mangrove on remote sensing • Only 19 % of the project is covered by a mangrove attributable to replantation. • Carbon validation should implement a method dedicated to assessing success rate. • Failure rate is explained by too large spatial cover and poor species choice. In the race to mitigate climate change, the urgent need to sequester carbon through nature-based solutions demands not only ambition but also the standards of scientific rigor—ensuring that every ton of CO₂ claimed is secured. Between 2009 and 2012, Oceanium organized a mangrove replantation of 10,415 ha, and 228,542 tCO₂ were credited in 2020. A stacked K-means classification with radiometric interpretation was applied to LANDSAT imagery to produce a change map between 2009 and 2024. The resulting map was validated using field data and 4022 visual interpretations of high-resolution imagery, with an error margin ranging between 1.04% and 1.3%. Only 1,910 to 2,130 hectares showed a gain in mangrove specifically attributed to the planting activities, corresponding to a replantation success rate between 18.3 and 20.5 %. Agresta took some failure rates into consideration, however, their expertise did not extend to a spatially exhaustive assessment of success rates. This led to a 3.5 times overestimation of carbon stocks. The tree carbon stock, previously estimated at 36,480 tCO₂, should be revised to a value between 9,821 and 10,952 tCO₂. Similarly, the soil carbon stock, previously estimated at 217,090 tCO₂, should be corrected to a value between 59,669 and 65,288 tCO₂. Based on these results, recommendations are proposed. First, a similar spatially explicit method should be implemented prior to the next credit validation. Second, future replantation efforts should target mudflats adjacent to existing mangroves, relying on more accurate hydroperiod and soil analyses and replanting the species more fit to the tidal range.