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

The sensible and latent heat losses of a maritime pine stand (Pinus pinaster Aiton) and of its understorey were measured in Portugal, in conditions of mild water stress, using an eddy covariance technique (monodimensional sonic anemometer coupled with a thin thermocouple and a fast-response hygrometer) at two levels above and under the canopy: canopy exchanges were estimated by difference. This paper first discusses the corrections 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 estimations 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.