Transpiration of a 64-year old maritime pine stand in Portugal : 2. Evapotranspiration and canopy stomatal conductance measured by an eddy covariance technique

Abstract The sensible and latent heat losses of a mari- time pine stand (Pinus pinaster Aiton) and of its under- storey were measured in Portugal, in conditions of mild water stress, using an eddy covariance technique (mono- dimensional sonic anemometer coupled with a thin ther- mocouple and a fast-response hygrometer) at two levels above and under the canopy: canopy exchanges were es- timated by difference. This paper first discusses the cor- rections to be made to the raw fluxes, then shows that, over a 24-h interval, the energy balance closure was very satisfactory, as well as the comparison between estima- tions of canopy evaporation by eddy covariance and sap flow. Moreover, the phase shift between the two methods was small when sap flow was measured just at the base of the canopy. Canopy stomatal conductance, estimated with a flux-gradient model, increased very sharply at dawn, then decreased progressively in compensation for the increase in air saturation deficit.