FLUXES OF CARBON-DIOXIDE AND WATER-VAPOR FROM A SAHELIAN SAVANNA

Simultaneous measurements of atmospheric CO2 flux, F-c, and latent hea t flux, E, from a shrub savanna in Niger, West Africa, were made by ed dy correlation. The vegetation at the study site consisted of scattere d shrubs with an understorey of grasses and herbs. The measurements ma de available some of the first data on CO2 and H2O exchange for an sem i-natural, mixed plant community, growing in the semi-arid tropics. Su ch data are necessary for the development of improved soil-vegetation- atmosphere models, able to describe the complex interplay between atmo spheric CO2, vegetation conductance and the surface energy balance of the Sahel in global climate models. In this framework, the effect of t he extreme and highly variable environment, represented by the saturat ion deficit, D, and integrated water content of the upper soil layers, Theta, was discussed. F-c and E were measured throughout the transiti on from the wet to the dry season (September-October) during the HAPEX -Sahel Intensive Observation period, in 1992. At the same time, leaf s tomatal conductances, g(1), of the shrubs and two understorey herb spe cies was measured. Daily totals of F-c decreased by 50% (peak values d eclined from -10 to -5 mu mol m(-2) s(-1)) over 3 weeks following the last rainfall of the wet season. During the same period, g(1) decrease d roughly four-fold for all sampled species. D appeared to be the main controlling parameter in the exchange of CO2 and H2O. g(1) and 'water use efficiency', \F-c/E\, were well correlated with D, which also con siderably influenced the response of F-c to photosynthetically active radiation, Q(p). Simultaneously, a decreasing Theta caused lower value s of g(1) and F-c. However, soil moisture had little effect on the emp irical relationships found between atmospheric variables (D or Q(p)) a nd g(1) or F-c.