Kritisk belastning av aciditet för skogsmark i norra Tjeckien

Forest decline is an increasing problem in Europe. "The Black Triangle" area, situated in the mountainous region where the borders of Germany, the Czech Republic and Poland meet, is one of the worst damaged areas. The reasons for the forest dieback are not fully understood, but the soil acidification caused by the burning of sulphur rich coal in the region is considered a major cause. To reduce the soil acidification the emissions of acidifying compounds must decrease. For this to be possible the decision makers must have some kind of concrete information about the acidification status in different regions. This is the background of the construction of the concept "Critical load of acidity", which is the highest deposition of acidifying compounds that does not cause long-term harmful effects on specified elements of an ecosystem. By comparing the critical load with the acid deposition, areas where the critical load is exceeded can be identified. Critical load maps give useful information when it comes to decisions to forest soils in the Czech part of "the Black Triangle", have been calculated using the Simple Mass Balance model (SMB). The SMB model is based on a long-term steady state approach using long-term sources and sinks of acidity. The main source of acidity is the acid deposition, and one of the dominating sinks is the weathering of base cations in the soil. The harmful effects on the forest ecosystem can be linked to the acid deposition by specifying a critical limit on the molar ratio between the concentrations of base cations [BC] and aluminium ions [Al] in the soil solution. The critical limit used in this study is [BC]/[Al]=1.0. The critical load is the deposition which ensures that the [BC]/[Al] ratio does not decrease below 1.0 in a long-term perspective. The critical load was exceeded in 76% of the forest area according to the model calculations, using deposition data from 1992. The highest exceedances were found in the mountains along the Czech border, northwest of Prague. Non-exceedance areas were found in regions with rich soils. The exceedance of the critical load was compared with forest damages in 13 regions. The coverage fraction of forest with severe damages, expressed as needle loss, in the different regions were plotted against the mean exceedance, the maximum exceedance and the median of the exceedance in the regions. The same type of analysis was performed using the coverage fraction of forest damage related c1earcuts. Relationships were found between the calculated exceedance and forest damage. This was most obvious when the maximum value of the exceedance was used. The magnitude of forest damage degree on 51 permanent plots was compared with the exceedance, but no clear relations were found. Many parameters are included in the SMB model, and since all parameters inherit uncertainties, the uncertainties in the result can be expected to be substantial. Improvements can be made continuously, when new data, with less uncertainties, become available. Of highest priority are improvements of the weathering data and the deposition data. However, although large uncertainties are accounted for in the calculations, the trends are considered to be reliable.