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 pi naster Aiton) and of its understorey were measured in Portugal, in con ditions of mild water stress, using an eddy covariance technique (mono dimensional sonic anemometer coupled with a thin thermocouple and a fa st-response hygrometer) at two levels above and under the canopy: cano py exchanges were estimated 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 w ell as the comparison between estimations of canopy evaporation by edd y covariance and sap flow. Moreover, the phase shift between the two m ethods was small when sap flow was measured just at the base of the ca nopy. Canopy stomatal conductance, estimated with a flux-gradient mode l, increased very sharply at dawn, then decreased progressively in com pensation for the increase in air saturation deficit.