Measurement of Sap Flow and Transpiration in Ring-porous Oaks Using a Heat Puise Velocity Technique

Abstract

A “modified” heat puise velocity (HPV) technique proved to be a satisfactory method to monitor sap velocities in ring-porous oaks. Velocities were calculated from the time of the first onset of a heat puise at a thermistor located 2 cm downstream from the heat source. The thermistors were embedded through the bark and located at the cambium-xylem interface. HPVs in black oaks ranged from 0.6 to 22 m hr^–1. HPVs on sunlit sides of a tree were up to 4 times higher than those measured on a shaded side and responded very rapidly to changes in direct sunlight exposure. Average around-the-bole HPVs were linearly related to potential evapotranspiration and to rates of water uptake through stem girdles. increased with increasing solar radiation flux densities from 0.3 to 0.6 cal cm^–2 min^–1. At higher radiation levels were primarily functions of humidity. Forest Sci. 26:485–494.