A Gauge to Measure Mass Flow Rate of Sap in Stems and Trunks of Woody Plants

Abstract

A stem flow gauge designed for herbaceous plants was adapted for measuring the absolute mass flow rate of sap in large stems and trunks of woody plants. The method uses a steady-state heat balance method in which a constant, known amount of heat is supplied to a stem segment. The axial and radial conductive heat fluxes away from the heated segment are measured, as well as the rise in sap temperature. The device can be operated by commonly available dataloggers and does not require calibration. In a greenhouse experiment with a small tree, the sap mass flow rate, as measured by the the gauge, agreed with the measured transpiration rate within 4% when both were integrated over 24-hr periods or longer. Short term comparisons (≤4hr) were less accurate, due to the changes in water content of the tree above the gauge, which cause a lag between transpiration rate and sap flow rate. The dynamic response of the tree and gauge system to sudden changes in sap flow was ≈20 min under midday conditions. Other than the insertion of temperature-sensing thermocouples 2 mm into the trunk, the gauge components are non-invasive and do not disturb the tree physically or physiologically to a significant extent.