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CO (2) fluxes were measured during 18 months in a forest and a savannah in northern Benin. Higher values of carbon fluxes were found during the wet season at each site. A strong dependency of carbon fluxes on water relations was found in two contrasting sites. The forest sequestered 640 +/- 50 and the savannah 190 +/- 40 g C m (-2) year (-1) . In West Africa, the main mechanisms or factors governing the dynamics of ecosystems, especially the dynamics of the carbon fluxes and productivity, still remain less known. This study reports the carbon fluxes over two contrasting ecosystems, notably a protected forest (lat 9.79A degrees N, long 1.72A degrees E, alt 414 m) and a cultivated savannah (lat 9.74A degrees N, long 1.60A degrees E, alt 449 m) in northern Benin. The two sites were among those equipped by the AMMA-CATCH observatory and Ou,m, 2025 project. Flux data were analyzed at the daily and seasonal scales in order to understand their controlling variables. We discussed the patterns of CO2 fluxes and the characteristics of the two ecosystems. The study also focused on the different water usage strategies developed by the two ecosystems since the alternation between dry and wet seasons highly influenced the seasonal dynamics. Finally, the annual carbon sequestration was estimated together with its uncertainty. The carbon fluxes were measured during 18 months (July 2008-December 2009) by an eddy-covariance system over two contrasting sites in northern Benin. Fluxes data were computed following the standard procedure. The responses of CO2 fluxes to the principal climatic and edaphic factors, and the canopy conductance were studied. A clear CO2 fluxes response to main environmental factors was observed, however with difference according to the seasons and vegetation types. The ecosystem respiration showed the highest values during the wet season and a progressive decrease from wet to dry periods. Also, the carbon uptake values were high during the wet period, but low during the dry period. However, the CO2 fluxes for the protected forest were always higher than that for the cultivated savannah within each defined period. This was due to the seasonal changes not only in phenology and physiology but also to the acclimation to environmental conditions, especially to the soil water availability. The water use efficiency was influenced by VPD during the day conditions for two ecosystems. However, the VPD response curve of water usage was relatively constant for the protected forest during the transitional and wet seasons. In contrary, for the cultivated savannah the VPD response decreased about 46 % from transitional dry-wet to wet seasons and remained relatively constant until transitional wet-dry season. The close relationships between the net CO2 assimilation and the canopy conductance were found for the two ecosystems. This suggests a regulation of the stomata by a partial stomatal closure besides the radiation control. Finally, the forest sequestered at the annual scale 640 +/- 50 and the savannah 190 +/- 40 g C m(-2) year(-1). We conclude with a strong relation between the carbon fluxes and water in the two investigated ecosystems. Apart from the radiation control, the stomata also play an important role in the regulation of the CO2 assimilation in the two ecosystems.

Turbulent CO2 exchanges between a cultivated Sudanian savanna and the atmosphere were measured during 29 months (August 2007-December 2009) by an eddy-covariance system in North-Western Benin, West Africa. The site (Lat 9.74 degrees N, Long 1.60 degrees E, Alt: 449 m) is the one of three sites fitted out by the international AMMA-CATCH program. The flux station footprint area is mainly composed of herbs and crops with some sparse trees and shrubs. Fluxes data were completed by an inventory of dominating species around the tower and the meteorological measurements. Flux response to climatic and edaphic factors was studied. Water was found the main controlling factor of ecosystem dynamics: much larger uptake was found in wet than dry season. During wet season, a very clear answer of net CO2 fluxes to photosynthetic photon fluxes density (PPFD) was observed. A low limitation in response to saturation deficit and soil water variability was however observed. The total ecosystem respiration (TER) was found highly dependent on soil moisture below 0.1 m(3) m(-3), but saturates above this threshold. The average annual carbon sequestration was 232 +/- 27 gC m(-2) with its inter-annual variability mainly controlled by TER. Finally, the ecosystem appeared more efficient during morning and wet season than during afternoon and dry period.