Measurement of Sap Flow in Conifers by Heat Transport.

Abstract

Mathematical theory of heat transfer applied to the heat pulse method of measuring sap flow shows that the assumption that velocities of heat pulse and sap are identical is not correct. Apparatus incorporating a thermistor to measure temperature rises of 0.5[degree]F confirmed theory experimentally in the case of Pinus radiata. Heater wire and thermistor probe are inserted radially into the sapwood to avoid unknown heat losses. Heat-pulse velocity and thermal diffusivity of sapwood are calculated from a single temperature-time curve. For softwoods sap flux (in ml/(cm2/hour) = Pb (mc + 0.33) x heat-pulse velocity (in cm/hour), where [rho]b is the basic density of sapwood and mc its moisture content (as a decimal fraction). Measurements showed considerable dispersion of sap speed over a few cm2 of cross section of the xylem of P. radiata. Previous work is critically reassessed in terms of the theory.