COMPARISON OF 3 METHODS FOR MEASURING THE TRANSPIRATION RATE OF YOUNG TREES

Three different methods of measuring the transpiration in young potted walnut frees (Juglans regia L, cv Lara) were compared over a 115-d period in 1990. Weighing of the pots was used as the reference for comparison with the sap flow rate given by a thermal method and the rate of transpiration by gas exchange determination. Several sap flow meters were installed at different heights on the same tree in order to estimate the storage of water between them. A good calibration of the sap flow measurement method was obtained in those trees which had been subjected to a short period of severe water stress, leading to an excellent agreement with hourly or daily values of transpiration by weighing. Gas exchange determination systematically underestimated transpiration. Nevertheless if seems possible to improve gas exchange determination of transpiration by: i) accurate calibration of all the sensors; and ii) introducing a pressure correction in the calculation of water vapour density in the air. Although the accuracy of the sap flow method was high (3%), it was not possible to assess within tree water movement correctly.