Effects of ammonia deposition on forests in the Netherlands

With 40 kg N ha-1 yr-1 the Netherlands is the country with the highest nitrogen deposition in Europe. Approximately 2/3 of the nitrogen deposition is from NHx. Considering defoliation and discoloration of the foliage, tree vitality of Dutch forest is poor, and declining for some tree species. Nevertheless the wood production is normal or relatively high, except in highly polluted regions. At many locations the N status of the foliage and the Al3+/Ca2+ and NH+4/K+ in the soil solution exceed assumed critical levels. But these exceedances are not well correlated with the loss and color of foliage. From laboratory experiments it is evident that these exceedances cause enhanced risk for damage by biotic and abiotic stresses, but only few indications exist that this has a severe impact on tree growth in Dutch forests. In contrast, the species composition of the undergrowth has been changed drastically, from a lichen dominated to a grass-dominated vegetation. Prediction of the future performance of the forest is complicated by several crucial gaps in knowledge. One is that the relation between nitrogen deposition and risks for damage by secondary stresses is still poorly validated in the field situation, and another is that the assumption in the critical load concept of a similarity in effects of the different forms of nitrogen deposition may be wrong.