ESTIMATION OF TRANSPIRATION BY SINGLE TREES - COMPARISON OF SAP FLOW MEASUREMENTS WITH A COMBINATION EQUATION

Sap flow estimates for whole trees (scaled from measurements on select ed branches using the heat balance method) were compared with estimate s 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 agre ement with the transpiration rate estimated using the Penman-Monteith equation with measured stomatal conductance (R-2 = 0.886) on six selec ted days during the season. The dominant environmental variable influe ncing transpiration was the vapour pressure deficit, as the ''aerodyna mic term'' in the Penman-Monteith equation accounted for more than 70% of daily total transpiration, with the rest due to the ''radiation co mponent''. Stomatal conductance, estimated by inverting the Penman-Mon teith equation from continuous measurements of sap flow over 55 days, was used to determine the parameters for a multiplicative stomatal con ductance model. For an independent data set there was better agreement between measured sap flow and transpiration predicted from the stomat al conductance (R-2 = 0.90) than for calculated and predicted stomatal conductance (R-2 = 0.51). (C) 1997 Elsevier Science B.V.