Simulation of sap flow in a beech tree by means of a dynamic data-based model as influenced by a solar eclipse

Responses of crucial plant processes such as transpiration and photosynthesis have often been modelled by mechanistic models or static regression models. The objective of this paper was to quantify the dynamics of sap flow rate in a beech tree to the rapid variations in short-wave solar radiation, air temperature and vapour pressure deficit using an alternative dynamic data-based modelling approach. In order to have enough dynamic information, data from the solar eclipse of 11 August 1999 were used. This permitted determination of the order of the sap flow dynamics, together with the relative contribution of short-wave radiation, air temperature and vapour pressure deficit. A multiple-input and single-output transfer function was used to simulate the sap flow rate responses in three branches at 22, 16 and 9 m accurately (R2 of 0.94, 0.86 and 0.90, respectively). The appropriate model structure was the same for the three branches and was characterised by second-order dynamics. An important advantage of the dynamic modelling approach presented was that it enabled the decomposition of the total sap flow rate response into partial responses to short-wave radiation, air temperature and vapour pressure deficit, respectively.