Critical loads for N, S and total acidity, and amounts by which they are exceeded by present atmospheric loads, were derived for coniferous and deciduous forests in Europe using the one-layer steady-state model START. Results indicated that present acid loads exceed critical values in approximately 45% of the forested area i.e. 52% of all coniferous forests and 33% of all deciduous forests. The area exceeding critical loads was nearly equal for N (50%) and S (52%). However, the maximum exceedances were much higher for S (up to 12000 molc ha i yr-1 in Czechoslovakia, Poland and Germany) than for N (up to 3500 molc ha -1 yr -1 in the Netherlands, Belgium and Germany). Furthermore, the critical N loads derived refer to the risk of increased vegetation changes. Higher values, i.e. lower exceedances, were found for N when it was related to an increased risk in forest vitality decrease. The uncertainty in the area exceeding critical loads was estimated to be about +_ 50% of the given value. This is mainly due to uncertainties in the chemical criteria that have been used. However, despite the uncertainties involved it is clear that large exceedances in critical N and S loads occur in Western and Central Europe. This coincides with the area where a decrease in forest vitality has been reported. 358 w. DE VRIES~ G. J. RE[NDS AND M. POSCHmobilization of A1 in the soil in response to S and N inputs. Still, the relationship between forest vitality and air pollution is not clear due to a large number of natural stress factors such as unsuitable soil conditions, seasalt deposition, drought, frost, pests and fungal diseases. However, numerous studies, both in the laboratory and in the field, have shown that nutrient imbalances resulting from high concentrations of A1 and N(NH4) as compared to (divalent) base cations (BC) such as ca and Mg, have a negative influence on root elongation and root uptake (cf Sverdrup et al., 1990;Heij et al., 1991 andDe Vries, 1991, for summarizing overviews). Such unfavorable conditions (sometimes called predisposing stress) do increase the vulnerability of forest stands to frost and drought and to insect and pathogen damage (sometimes called triggering stress).Information on critical deposition levels (loads) for N and S are a prerequisite for political decisions on emission reductions. An overview of average critical loads on forests, based on critical values for the A1 concentration and A1/BC ratio, is given in De Vries and Gregor (1991). To gain more insight in the magnitude and the spatial variation in critical loads, the Working Group on Effects of the UN-ECE Executive Body on Longe Range Transboundary Air Pollution has installed a Task Force on Mapping. This Task Force is assisted by a Coordination Centre on Effects (CCE). The Task Force and the Coordination Centre have produced a Manual on Mapping Critical Loads ) and a Mapping Vademecum (Hettelingh and De Vries, 1992) respectively, containing guidelines for the preparation of critical load maps. At present, such maps have been produced by...
