Individual tree branch-level simulation of light attenuation and water flow of three <em>F. sylvatica</em> L. trees.

A leaf stomatal conductance model was combined with a hydrological tree and soil water flow model and a spatially explicit three-dimensional canopy light model. The model was applied to single, old-growth Fagus sylvatica L. trees, and the measured daily values of stem sap flux could be reproduced with a normalized root mean square error of 0.10 for an observation period of 32&nbsp;days in the summer of 2009. The high temporal resolution of the model also makes it possible to simulate the diurnal dynamics of transpiration, stem sap flux, and root water uptake. We applied new data-processing algorithms to information from terrestrial laser scans to represent the canopies of the functional-structural model. The high spatial resolution of the root and branch geometry and connectivity makes the detailed modeling of the water usage of single trees possible and allows for the analysis of the interaction between single trees and the influence of the canopy light regime on the water flow inside the xylem. In addition to the laser scans of the observed trees, the model needs tree-species-specific physiological input parameters, which are easy to obtain. The model can be applied at various sites and to different tree species, allowing the up-scaling of the water usage of single trees to the total transpiration of mixed stands