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 Brazilian form of savanna), in which a mixture of shrubs, trees and grasses 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 dry season at high irradiance the sensible heat greatly exceeded evaporation. Ecosystem surface conductance g(s) in the wet season rose abruptly to 0.3 mol m(-2) s(-1) and fell gradually as the day progressed. Much of the total variation in g(s) was associated with variation in the leaf-to-air vapour pressure deficit of water and the solar irradiance. 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 were -10 and -15 mu mol CO2 m(-2) s(-1) respectively (sign convention: negative denotes fluxes from atmosphere to vegetation). The canopy was well coupled to the atmosphere, and there was rarely a significant build-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 composition showed all the grasses except for one species to be C-4, and all the palms and woody plants to be C-3. The CO2 coming from the soil had an isotopic composition that suggested 40% of it was of C-4 origin.