Analysing the applicability of the heat balance method for estimating sap flow in boreal forest conditions

A simple quasistationary dynamic model was constructed to analyse the performance of the stem heat balance method for estimating sap flow in tree stems. Model predictions were compared with field measurements of sap flow in a 35-year-old dominant Scots pine tree and in a smaller understorey mountain ash. The sap flow was measured with the Dynamax Flow32&tm; Stem-Flow Gauge system. Results indicate that the heat balance method underestimates the sap flow in steady-state conditions, especially with stems of larger diameters. The reason for the underestimate of the flow in the case of bigger stems is the inaccurate estimation of sap temperature increase DT when measured on the surface of the stem. Additionally, the difference between air and sap temperatures, which is typical of boreal conditions in the early summer mornings, may cause substantial peaks in the sap flow estimates. Increasing the power input will decrease the latter problem but this may result in problems with overheating of the stem.