THERMAL MEASUREMENTS OF SAP FLUX IN TRUNKS AND ROOTS AND HYDROUS BEHAVIOR OF TREES .1. THEORETICAL-ANALYSIS OF THE ERRORS IN SAP-FLUX ESTIMATIONS AND MEASUREMENT VALIDATION IN THE PRESENCE OF EXTERNAL THERMAL-GRADIENTS

A thermal method for estimating sap flow in apple-tree trunks and roots, based on measuring the temperature of a constant heat source inserted into the xylem had been applied for investigating water use by trees and Soil-Water uptake by roots under field conditions (figs 1, 2). A theoretical analysis of the errors in flux measurement determined the percentage of confidence for the measurements of sap flux in 2 different types of organs studied (fig 3 and table I) and the contribution of each variable in the calculation model to the formation of the total error (table II). The study of effect of the external thermal gradient on flux values (fig 4), especially important in the root zone, led us to propose a corrective term for this measurement (figs 5, 6; table III). Moreover, it has been shown that, for a given treatment, the flux measurements taken on trees with representative morphological and vegetative characteristics are representative at the level of the treatment or of the plot in the case of uniform treatment (fig 7, table IV).