MEASURING AND MODELLING FOREST TRANSPIRATION

The experimental part of the study was performed in the floodplain forest growing along the bank of Dyje River close to Pohansko (about 10 km east of the town of Břeclav, southernmost Moravia). Sap flow was measured by the trunk heat balance (THB) method with direct electric heating of xylem tissues. Two transpiration models were tested, one with plant control, and another without plant control. The principle of the model with plant control is that the physical mechanism of transpiration is evapo-ration, which is actively controlled by plants. The supposed mechanism is: part of the heat (heat equivalent to the energy absorbed from solar radiation), which would cause heating of the plant above 25°C, is dissipated by evaporation. The model has five physical parameters, which are in principle measurable independently. The model without plant control is based on the assumption that transpiration dissipates a constant fraction f of the incoming solar radiation. The later model needs only one parameter which can by found by means of parameter search (f is about 45 % in this case). Both models were tested by comparing their results with actual measured transpiration (via sap flow) in the floodplain forest growing along the bank of Dyje River close to Pohansko (Moravia, Czech Republic). The results of the two models were almost identical