Radial Variation in the Axial Conductivity ofPopulusand its Significance in Heat Pulse Velocity Measurement

Conductivity (defined as permeability divided by viscosity) was estimated as a function of radial position in the sapwood at three height levels, for two species of Populus. Stem sections 210–260 mm long and comprising the entire cross section were de-aerated and then infused axially with toluidine blue dye solution until emergence just occurred from the opposite end. Measurements were made of: (i) maximum penetration within each ring, and (ii) axial penetration at 10 mm radial increments starting at the cambium. The corresponding conductivity values were calculated for each measurement, and the values were fitted to a quadratic equation for each stem section. The equations were found to be suitable for calculating stem flux from sap velocities derived from heat pulse velocities measured at different depths in the xylem.