The impact of rain events on CO₂ emissions from contrasting land use systems in semi-arid West African savannas

In the future the Sudanian savanna – one of West Africa's high-potential “bread baskets” – will likely face shorter rainy seasons with more extreme rains and droughts. That could have serious impacts on the vegetation and its carbon dioxide (CO₂) exchange with potentially increasing CO₂ emissions accelerating climate warming. Understanding how the CO₂ fluxes in this area respond to environmental variables, in particular rain events, is essential, but available data are scarce. In this study, we monitored net ecosystem exchange (NEE) of CO₂, rainfall and other environmental parameters during four years at three savannas. Savannas were characterized by different vegetation due to different land use: i) woody and nearly pristine, ii) mixture of cropland and grassland and iii) intensive grazing. The impact of rain events on CO₂ exchange for these contrasting ecosystems were analyzed for single rain events (short-term) and on a yearly time scale (long-term) using three eddy covariance towers. We found that the woody pristine savanna site was a prominent sink of CO₂ (−864 to −1299 g CO₂ m⁻² y⁻¹) while the degraded sites were net CO₂ sources (118 to 605 g CO₂ m⁻² y⁻¹) with a complicated relation with annual rainfall amounts. The NEE responses to single rain events revealed that daytime rain systematically decreased the sink strengths at all sites, which might be associated with decreased light availability. At the degraded sites, additional factors increasing CO₂ losses were rain duration and dry spell length. The observed patterns of immediate CO₂ flux responses to rainfall at differently used savannas indicate strong internal feedbacks between vegetation and land use changes and raise the question whether the CO₂ sink strengths might be overestimated with possible implications for global CO₂ budgets. Sustainable adaptation strategies need to be developed for West Africa.