Estimation of transpiration by single trees: comparison of sap flow measurements with a combination equation

Abstract

Sap flow estimates for whole trees (scaled from measurements on selected branches using the heat balance method) were compared with estimates of transpiration based on porometry in a study of poplar trees in an agroforestry system in the south of the UK. Sap flow showed good agreement with the transpiration rate estimated using the Penman-Monteith equation with measured stomatal conductance ( R 2 = 0.886) on six selected days during the season. The dominant environmental variable influencing transpiration was the vapour pressure deficit, as the “aerodynamic term” in the Penman-Monteith equation accounted for more than 70% of daily total transpiration, with the rest due to the “radiation component”. Stomatal conductance, estimated by inverting the Penman-Monteith equation from continuous measurements of sap flow over 55 days, was used to determine the parameters for a multiplicative stomatal conductance model. For an independent data set there was better agreement between measured sap flow and transpiration predicted from the stomatal conductance ( R 2 = 0.90) than for calculated and predicted stomatal conductance ( R 2 = 0.51).