Because of their surface roughness, higher leaf area index and physiological leaf characteristics, forests receive a higher atmospheric input of gases and aerosols compared to open field circumstances. These increased deposition levels may have adverse effects on the structure and functioning of forest ecosystems. To quantify the total atmospheric deposition onto forests, a so-called canopy budget model that estimates dry deposition and ion exchange reactions within forest canopies, based on precipitation and throughfall measurements, is applied worldwide. Up to now, this model has only been validated twice with the results of an inferential method. In both studies the results of the two methods didn’t correspond sufficiently for one or more elements.  The aim of this project is to evaluate and adapt the several assumptions made in the canopy budget model for different tree species, phenological periods, atmospheric pollution levels and site conditions by working at different spatial levels. For one year dry deposition and canopy exchange processes will be studied at the leaf and branch level of an adult beech and young beech, birch and pine trees under different N-supply, rain intensity and atmospheric pollution level. At the same time, physiological leaf characteristics will be measured and linked to the observed processes. This results can be used to adapt the canopy budget model and extend the existing FORUG model, that simulates nutrient transfer using meteorological en atmospheric conditions. Using canopy architecture data, this FORUG-model can be upscaled to the canopy level. Both models will be compared en validated at the stand level (a Level II observation plot in wich water and ion fluxed in throughfall and stemflow have been measured since 1994) with an inferential method for measuring dry deposition.