EFFECTS OF AMMONIA DEPOSITION ON FORESTS IN THE NETHERLANDS

With 40 kg N ha(-1)yr(-1) the Netherlands is the country with the high est nitrogen deposition in Europe. Approximately 2/3 of the nitrogen d eposition is from NHx. Considering defoliation and discoloration of th e foliage, tree vitality of Dutch forest is poor, and declining for so me tree species. Nevertheless the wood production is normal or relativ ely high, except in highly polluted regions. At many locations the N s tatus of the foliage and the Al3+/Ca2+ and NH4+/K+ in the soil solutio n exceed assumed critical levels. But these exceedances are not well c orrelated with the loss and color of Foliage. From laboratory experime nts it is evident that these exceedances cause enhanced risk for damag e 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 drastical ly, from a lichen dominated to a grass-dominated vegetation. Predictio n of the future performance of the forest is complicated by several cr ucial gaps in knowledge. One is that the relation between nitrogen dep osition and risks for damage by secondary stresses is still poorly val idated in the field situation, and another is that the assumption in t he critical load concept of a similarity in effects of the different f orms of nitrogen deposition may be wrong. (C) 1998 Elsevier Science Lt d. All rights reserved.