Quantifying the impact of Great Green Wall and Corporate plantations on tree density and biomass in Sahelian Senegal

International audience

إنجليزي. The Great Green Wall (GGW) is an international initiative to combat land degradation and restore native plant life in the Sahel, but due to a lack of monitoring tools, it remains unknown to be considered as success or failure. Here, we quantify the impact of GGW plantations and Corporate plantations (privately owned) in Sahelian Senegal based on remote sensing data and deep learning by mapping individual trees and their biomass across the Sahel region. Tree features (cover, density and above-ground biomass) have been computed in every hectare of 42 plantations (of both, corporate and GGW) and their surrounding non-planted areas, subsequently used for a comparative analysis of tree features. Results showed that gains in tree features varied substantially between plantations. At plot scale, among Corporate plantations, the average gain in tree density was 61.16 +/- 42.12 trees/ha while it was 5.7 +/- 5.8 trees/ha for GGW plantations. In regards to tree cover, the average gain was 618.5 +/- 588.5 m2/ha for Corporate plantations and 71.72 +/- 108.89 m2/ha for GGW plantations. For the above-ground biomass, the average gain was 3.36 +/- 3.29 tons/ha in the Corporate plantations and 0.46 +/- 0.67 tons/ha in the GGW plantations. The average gain in foliar biomass in the Corporate plantations was 0.15 +/- 0.13 tons/ha and in the GGW plantations, it was 0.02 +/- 0.03 tons/ha. The average gain in wood biomass was 3.21 +/- 3.12 tons/ha among the Corporate plantations and was 0.43 +/- 0.64 tons/ha among the GGW plantations. Notably, regarding the relative benefit in terms of ecosystem services per unit of density, each tree in GGW plantations contribute more to ecosystem services per unit of density compared to Corporate plantations. In GGW, each gained tree contributes 18 m2 of cover, 116.1 kg of above-ground biomass, 5.6 kg of foliar biomass, and 114.2 kg of woody biomass, while in corporate plantations, on the other hand, each gained tree adds 9 m2 of cover, 48.4 kg of above-ground biomass, 2.3 kg of foliar biomass, and 46.3 kg of woody biomass. These findings are opposed to conventional paradigms, suggesting that the Great Green Wall, while perhaps missing its tree density targets, has succeeded in its mission to produce ecosystem services per tree. This raises important questions about the redefinition of objectives in reforestation projects, focusing on quality rather than quantity, a perspective that could transform our understanding of the successes and failures of these essential ecological restoration initiatives. However, the assessment primarily relies on indicators such as cover and biomass, potentially overlooking other crucial ecosystem services. Therefore, while our conclusions underscore the effectiveness of reforestation initiatives, future research should aim to establish a more comprehensive understanding of ecosystem services by incorporating additional indicators beyond cover and biomass, such as species diversity, soil health, water retention, and habitat quality.