A MICROCLIMATIC STUDY OF THE EFFECT OF DROUGHT ON EVAPOTRANSPIRATION IN A MARITIME PINE STAND AND ITS UNDERSTOREY

The evapotranspiration of a Maritime pine stand was studied in the Landes forest (south-west France) during the severe drought of 1989 (May-September) by means of a microclimatic approach as well as sap flow measurements: heat flux was measured at 2 levels above the canopy and understorey by an eddy correlation method (2 monodimensional sonic anemometers coupled with fast-response temperature sensors), then latent heat flux was deduced from the energy balance of the 2 layers. Vertical profiles of air temperature and vapour pressure, leaf temperature measurements in the understorey, as well as assumptions regarding surface temperature of pine needles allow the computation of mean surface conductances of the tree canopy and understorey. In our experimental conditions, these conductances can be described fairly well by a linear function of the ratio of the net radiation absorbed by the layer on the air saturation deficit at its level (R(n)/D). This leads to a simple relationship between surface conductance and the ratio LE/R(n) (LE is the latent heat flux of each layer). The variations of daily LE/Rn values for pines seem to be related to the amount of water stored in the ground (0-75 cm). However, when drought is established, evapotranspiration increases after rainfalls, even if they do not obviously affect overall water storage. For the understorey, LE/R(n) remains at high levels throughout the experiment, possibly due to the low climatic demand, as understorey net radiation is rather low.