Effects of environmental stress on forest crown condition in europe. Part III: Estimation of critical deposition and concentration levels and their exceedances

The stress by air pollution at the systematic Pan-European 16 x 16 km(2) forest (crown) condition monitoring network, is discussed by comparing site-specific estimates of critical and present concentration and deposition levels for S and N compounds and ozone. Results indicate that the exceedance of critical levels, related to direct above-ground impacts, decrease going from O-3 > SO2 > N compounds. Critical N loads related to effects on the forest understorey are exceeded at approximately 25% of the plots, located mainly in Western and Central Europe. Critical N loads related to effects on trees are hardly ever exceeded, but most likely, this is an under estimate. Critical acid deposition levels are exceeded at approximately 30% of the plots with a low base saturation, where acid inputs may release toxic Al. This is especially the case in Central and Eastern Europe, where present loads are high and in boreal forest in Southern Scandinavia where critical loads are low. Although the uncertainties in the calculated exceedances is large, the spatial pattern, which is most important for a correlative study, seems reliable, implying that the critical load concept is suitable for regional risk assessments.