SUMMARY

The sap flow rate in trees is estimated from the heat balance computed in a segment of the trunk surrounded by a heating tape delivering a known rate of heating. Two pairs of thermocouples, radially inserted at the ends of the heated segment of the trunk, and a thermopile surrounding the heating tape are used to determine the different conductive heat flows and the heating of the sap. The rate of heat storage must be considered; it is computed from the measurement of the temperature in the heated volume.

This method was used to estimate the water use of apple trees in an orchard. The sap flow in the different trees increases with the leaf area. The mean sap flow rate of the trees is compared to the net radiation and to the radiation absorbed by the trees. The daily transpiration of the trees is lower than Penman potential evapotranspiration. The correlation with orchard net radiation or absorbed radiation is quite good.

The same method was used to compare sap flow of two walnut trees grown in containers and subject to normal and drought conditions. Sap fluxes were measured on the main trunk and on the three major branches supporting the foliage in order to test: a) the accuracy of the method; b) the relative importance of water flow within each branch.

Results show that: a) the rate of sap flow is very rapidly affected by the water stress conditions, more than the predawn water potential; b) in our case, due to the form and orientation of foliage, the changes in sap flow between the different branches, during a day, were synchronous. […]