CO2 fluxes at leaf and canopy scale in millet, fallow and tiger bush vegetation at the HAPEX-Sahel southern super-site

Measurements of canopy and leaf scale CO2 flux from the three sub-sites at the HAPEX-Sahel Southern supersite are presented. These are analysed in relation to biological and environmental variables. At leaf scale, the flux is most strongly influenced by photosynthetic photon flux density (PPFD) and stomatal conductance. Together with measurements of canopy structure at each site, the measurements of leaf photosynthesis, stomatal conductance and stem respiration were used to parameterise sub-models within the canopy model MAESTRO, which predicts canopy net CO2 flux. Comparison of the independent canopy flux measurements with predictions is informative, as the model represents an integration of our knowledge of the system, and so differences highlight weak points in our understanding as well as measurement artefacts. These differences are largest in tiger bush and smallest in millet, and are attributed to the effect of canopy heterogeneity on measurements rather than biological processes. Generally, good agreement was found at all three sites and the model can be regarded as validated. The model was used to extrapolate measurements in time, and, using a year's weather data, predicted a value for carbon sequestration at the millet site over the growing season very close to harvest measurements