Afforestation as climate change mitigation strategy in West Africa: potential impacts on the terrestrial carbon cycle

The forest landscape in West Africa faces significant challenges from rapid population growth, agricultural expansion, and urbanization. These anthropogenic land-use and land-cover changes (LULCC), including deforestation and afforestation, impact ecosystem-climate-carbon cycle interactions through biogeochemical emissions and greenhouse gas uptake. However, the capacity of the land-based carbon sink, encompassing LULCC emissions and CO2 uptake, remains uncertain. This study employs the fully coupled WRF-Hydro system, incorporating surface and subsurface hydrology and a dynamic carbon cycle, to perform high-resolution (3 km) convection-permitting simulations for the period 2011-2022. It assesses regional impacts of idealized LULCC scenarios by comparing several land use and afforestation simulations representing specific land cover transitions in the Sudan savannah belt of Burkina Faso and Ghana.

Model performance was validated using gross primary production (GPP) data from four eddy covariance sites along a land-use gradient (pristine savanna forest, cropland, and degraded grassland) in the Sudan savannah belt of Burkina Faso and Ghana and further evaluated by comparing simulated GPP and leaf area index (LAI) with Copernicus Land Monitoring satellite products. Overall, the model showed the best performance at the pristine savanna forest site with homogeneous vegetation.

Analysing of carbon cycle variables, including GPP, NPP, NEE, carbon residence time, and soil and vegetation carbon stocks, our results reveal that deforestation reduces GPP by 60% (-1.08 ± 0.1 gC/m²), carbon stocks by 45% (-1.79 ± 0.19 kgC/m²), and carbon residence time by 25% (-3.6 ± 0.9 years). Conversely, afforestation strategies, such as converting grassland to evergreen or mixed forest, can mitigate carbon losses by significantly increasing total carbon stocks (1.6 ± 0.19 kgC/m²) through increased canopy cover. Furthermore, our results indicate that converting grassland to evergreen forest can approximately double the carbon residence time in soils and ecosystems compared to afforestation options involving woody savanna or savanna. The study also investigates the underlying physical mechanisms behind LULCC-induced terrestrial carbon cycle responses.