AN IMPROVED STEM HEAT-BALANCE METHOD USING ANALOG HEAT CONTROL

This paper describes a modified sap flow meter appropriate for sap flow measurements even on plants having an irregular stem surface and/or heat-sensitive bark, such as apple trees. The modifications build on the plant-friendly but not very user-friendly design of Weibel and De Vos (Plant and Soil, 1994). Three important improvements are achieved (1) the number of heat-controlling gauges that can be connected to one 'Campbell 21X' datalogger is increased from three to 32 by using an analog algorithm together with additional circuitry consisting of sample-and-hold and power amplifiers; (2) the sap flow gauge is easier to install; (3) the gauge has a better adaptability to stem growth in the radial direction and to diurnal stem shrinking. Accuracy tests with the modified system were carried out under glasshouse conditions with potted apple trees having stem diameters between 15 and 45 mm. Using gravimetric weight loss as a reference method we found good reliability of all technical components and excellent measuring accuracy, with errors of less than 4% over a 24h measuring interval and less than 15% over a period of 15 min. Also, in a 4 month field experiment with sap flow measurements on six 3-year-old apple trees rooted in the ground, the modified system proved highly reliable. After removing the gauges the stems were not 'strangled', nor was the tissue of the bark affected. Error analyses on the need to include a term for stored energy in the heat balance equation showed that the importance of stored energy increases substantially when the tree stem diameter is more than 30 mm.