On 12-14 Swiss ICP-Forests Level II (LWF) plots, we estimated the atmospheric deposition of acidity and nitrogen using throughfall data and we measured ambient concentrations of O₃ with passive samplers. We determined the critical loads of acidity and nitrogen using the steady state mass balance approach and calculated the critical O₃ levels using the AOT40 concept. Exceedances of critical loads (nitrogen, acidity) and levels (O₃) were related to the following indicators: 1) the nitrogen (N) status in terms of mineral nutrition of the respective forest stand, based on foliar analyses; 2) the acidity status of the forest soils, based on the base cations I aluminium (Be/Al) ratio in the soil solution; 3) the development of O₃-induced visible injury on trees and shrubs.
For nitrogen, critical loads were exceeded on 8 plots. Foliar N concentrations were correlated with both N deposition and the exceedances of the critical loads for N, particularly for broadleaved species. At one plot where N deposition was very high (Schlinis), foliar N concentrations were above the optimum range.
Acid deposition exceeded the critical loads on 2 plots and almost reached them on 4 plots, but the median of the Be/Al in the soil solution was above the critical value of 1 for all plots. There was a slight correlation between Be/Al and the ratio between actual deposition and critical loads calculated with site specific rooting depths.
The critical levels for O₃ were exceeded on 12 plots, but this did not lead to the development of typical Orinduced visible injury on all 12 plots due to (i) the site specific composition of O₃ sensitive and tolerant plant species and (ii) the influence of microclimatic site conditions on the stomatal behaviour, i.e. O₃ uptake.