SAP FLOW MEASUREMENTS FROM STEM HEAT BALANCES - A COMPARISON OF CONSTANT WITH VARIABLE POWER METHODS

Two methods of sap flow measurement using the stem surface heat balanc e approach were compared: the variable power (VP) and constant power ( CP) methods. Comparisons were carried out on woody stems in a glasshou se in the UK (Ficus benjamina) and in the field during the rainy seaso n in Niger (Guiera senegalensis). Substantial errors occurred if chang es in stem temperature were neglected in the CP method. The simple VP method used neglected conductive losses, but was less affected by chan ges in stem temperature than the CP method. Therefore, at low flows th e VP method overestimated sap fluxes, but at high flows it had a bette r dynamic response than the CP method. Daily totals of sap flow over 9 days in G. senegalensis stems with leaf areas of 1.3-2.3 m2 ranged fr om 2 to 9 kg day-1, and the two methods agreed well, although there wa s a consistent 8-12% higher estimate by the CP method.