Fluxes of carbon dioxide and water vapour from a Sahelian savanna

Simultaneous measurements of atmospheric CO 2 flux, F c, and latent heat flux, E, from a shrub savanna in Niger, West Africa, were made by eddy correlation. The vegetation at the study site consisted of scattered shrubs with an understorey of grasses and herbs. The measurements made available some of the first data on CO 2 and H 2O exchange for an semi-natural, mixed plant community, growing in the semi-arid tropics. Such data are necessary for the development of improved soil-vegetation-atmosphere models, able to describe the complex interplay between atmospheric CO 2, vegetation conductance and the surface energy balance of the Sahel in global climate models. In this framework, the effect of the extreme and highly variable environment, represented by the saturation deficit, D, and integrated water content of the upper soil layers, Φ, was discussed. F c and E were measured throughout the transition from the wet to the dry season (September–October) during the HAPEX-Sahel Intensive Observation period, in 1992. At the same time, leaf stomatal conductances, g 1, of the shrubs and two understorey herb species was measured. Daily totals of F c decreased by 50% (peak values declined from −10 to −5 μmol m −2s −1) over 3 weeks following the last rainfall of the wet season. During the same period, g 1 decreased roughly four-fold for all sampled species. D appeared to be the main controlling parameter in the exchange of CO 2 and H 2O. g 1 and ‘water use efficiency’, | F c E |, were well correlated with D, which also considerably influenced the response of F c to photosynthetically active radiation, Q p. Simultaneously, a decreasing Φ caused lower values of g 1 and F c. However, soil moisture had little effect on the empirical relationships found between atmospheric variables ( D or Q p) and g 1 or F c.