Measuring and modelling forest transpiration

Abstract

Two transpiration models were tested, one with evaporation control, and another without evaporation control. The principle of the model with evaporation control is that the physical mechanism of transpiration is evaporation, which is actively controlled by plants. The supposed mechanism is: part of the heat (heat equivalent to the energy absorbed from solar ra-diation) that would cause heating of the plant above 25 °C is dissipated by evaporation. The model has four physical parameters, which are in principle measurable independently. The model without evaporation control is based on the assumption that evaporation dissipates a constant fraction of the incoming solar radiation. In this model, no physical mechanism of tran-spiration is given. The latter model needs only one parameter, which can be found by means of parameter search. Both models were tested by comparing their results with transpiration meas-ured in the floodplain forest growing along the bank of the Dyje River close to Pohansko (Mo-ravia, Czech Republic). Both models produced a typical difference of about 1.5 mm d -1 be-tween measured and calculated daily transpiration totals. We can therefore say that both mod-els are equally valid. 1. Introduction The aim of this study was to evaluate the validity of two transpiration models based on two different approaches. The first supposes that plants have the ability to control evaporation in order to optimise their temperature [1]. The second is based on the assumption that plants use some constant fraction of the global radiation above the forest canopy for evaporation. The direct measurement of field transpi-ration in floodplain forests was used as a reference.