||& Keymessage CO2 fluxes were measured during 18months
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.
& Context 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 9.79°N, long
1.72°E, alt 414 m) and a cultivated savannah (lat 9.74°N, long
1.60°E, alt 449 m) in northern Benin. The two sites were among those equipped by the AMMA-CATCH observatory
and Ouémé 2025 project.
& Aims 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.
& Methods 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.
& Results 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.
& Conclusion 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.