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

TLDR

Sap flow measurement in woody plants is essential for understanding water transport, but existing gauges were designed for herbaceous species. This study adapts a stem flow gauge originally for herbaceous plants to measure absolute sap mass flow rates in large woody stems and trunks. The gauge employs a steady‑state heat balance, supplying constant heat to a stem segment and measuring axial and radial conductive heat fluxes and sap temperature rise, and can be operated with standard dataloggers without calibration. In a greenhouse experiment with a small tree, the gauge’s sap mass flow rate matched transpiration within 4 % over 24‑hr periods, but short‑term accuracy was lower due to lag, with a dynamic response of about 20 min midday, and the device is non‑invasive except for 2 mm thermocouple insertion.

Abstract

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.