Fluxes of carbon dioxide and water vapour from a Sahelian savanna

A. Verhoef, S.J. Allen, H.A.R. De Bruin, C.M.J. Jacobs, B.G. Heusinkveld

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Simultaneous measurements of atmospheric CO2 flux, Fc, 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 CO2 and H2O 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 CO2, 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, Θ, was discussed. Fc 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, gl, of the shrubs and two understorey herb species was measured. Daily totals of Fc decreased by 50% (peak values declined from -10 to -5 μmolm-2s-1) over 3 weeks following the last rainfall of the wet season. During the same period, gl decreased roughly four-fold for all sampled species. D appeared to be the main controlling parameter in the exchange of CO2 and H2O. gl and 'water use efficiency', |Fc/E|, were well correlated with D, which also considerably influenced the response of Fc to photosynthetically active radiation, Qp. Simultaneously, a decreasing Θ caused lower values of gl and Fc. However, soil moisture had little effect on the empirical relationships found between atmospheric variables (D or Qp) and gl or Fc.
Original languageEnglish
Pages (from-to)231-248
JournalAgricultural and Forest Meteorology
Issue number2-4
Publication statusPublished - 1996
Externally publishedYes


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