FLUXES OF CARBON, WATER AND ENERGY OVER BRAZILIAN CERRADO - AN ANALYSIS USING EDDY COVARIANCE AND STABLE ISOTOPES

We present the energy and mass balance of cerrado sensu stricto (a Bra zilian form of savanna), in which a mixture of shrubs, trees and grass es forms a vegetation with a leaf area index of 1.0 in the wet season and 0.4 in the dry season. In the wet season the available energy was equally dissipated between sensible heat and evaporation, but in the d ry season at high irradiance the sensible heat greatly exceeded evapor ation. Ecosystem surface conductance g(s) in the wet season rose abrup tly to 0.3 mol m(-2) s(-1) and fell gradually as the day progressed. M uch of the total variation in g(s) was associated with variation in th e leaf-to-air vapour pressure deficit of water and the solar irradianc e. In the dry season the maximal g(s) values were only 0.1 mol m(-2) s (-1). Maximal net ecosystem fluxes of CO2 in the wet and dry season we re -10 and -15 mu mol CO2 m(-2) s(-1) respectively (sign convention: n egative denotes fluxes from atmosphere to vegetation). The canopy was well coupled to the atmosphere, and there was rarely a significant bui ld-up of respiratory CO2 during the night. For observations in the wet season, the vegetation was a carbon dioxide sink, of maximal strength 0.15 mol m(-2) d(-1). However, it was a source of carbon dioxide for a brief period at the height of the dry season. Leaf carbon isotopic c omposition showed all the grasses except for one species to be C-4, an d all the palms and woody plants to be C-3. The CO2 coming from the so il had an isotopic composition that suggested 40% of it was of C-4 ori gin.