Minisymposium 15: In Response to Water
Abs #
27004: Modelling the diurnal rhythm of water flow in young oak and beech trees
The flow of water through plants in the so-called “Soil Plant Atmosphere Continuum” is controlled by both the vapour pressure deficit of the atmosphere and by the availability of soil water. The electrical analogy model based on Ohm’s law is often used to describe mathematically the flow of water through plants in SPAC-models. This model, also known as the “van den Honert” concept, can be easily used, but is limited by the assumption of stationary water flow. In reality, water flow is non-stationary and phenomena such as the daily stem diameter fluctuation, the water storage in stem and branches and the time lag between transpiration rate at leaf level and sap flow rate at stem level have been frequently observed. Such phenomena clearly indicate the dynamic nature of water transport through plants. The purpose of this presentation is to show that dynamic flow patterns have to be incorporated in a realistic model, characterised by parameters expressing plant features such as hydraulic capacitance and resistance.
For these reasons, an experimental set-up was built to gather the experimental data needed to design and validate a dynamic model for water transport. This set-up allows continuous measurement of a complex set of physiological processes of young model trees. Beech (Fagus sylvatica L.) and oak tree (Quercus robur L.) were selected as test species because of their differences in wood anatomy. The transpiration rate, the sap flow rate at root, stem and branch level and the diameter fluctuations of branch and stem were measured at 10 min intervals during several days. The microclimatological variables driving these processes were recorded as well.
The experimental data permitted validation of two different modelling approaches, i.e. (1) an electrical analogon based on an RC circuit and (2) a hydraulic flow system of dynamic nature. The models contain different system components based on both physical and physiological characteristics which differ between oak (ring-porous tree) and beech (diffuse-porous tree). Both models use transpiration rate as input variable and simulate the diurnal courses of stem sap flow, water storage and stem diameter fluctuation. Some differences of model parameters observed between the oak and the beech trees could be explained by analysis of 3D images of xylem anatomy obtained from micro-tomography.
