The elasticity (nutrient storage, litter decomposition, bioturbation of soil) and diversity of central European forest ecosystems has been reduced by centuries of overutilization. Since the middle of the nineteenth century, their development has been influenced by silvicultural measures, as well as by the deposition of acids and nutrients, especially nitrogen from anthropogenic sources, i.e., by a mixture of stabilizing and destabilizing external influences. During recent decades, most forest soils have been acidified by acid deposition resulting in low levels of nutrient cations and negative alkalinity in the soil solution. Widespread acute acidification of soil in the rooting zone is indicated by extremely high manganese (Mn) contents in leaves (fingerprint). Soil acidification has caused drastic losses of fine roots in subsoil, indicated by denuded structural root systems where adventitious fine root complexes exist only sporadically. Research at the organ (leaf, fine root, mycorrhiza) and cellular levels has provided much information on the effects of air pollutants and soil acidification on leaves and roots. There are considerable uncertainties, however, as to how changes in the status of leaves or roots are processed within the tree and ecosystem from one level of hierarchy to the next on an increasing spatial and time scale, and how these lead to decline symptoms like crown thinning, stand opening (as a consequence of dieback or perturbations), and changes in species composition (soil biota, ground vegetation, tree regeneration). At the tree level, nutrient imbalances (due to cation losses from soil, changes in the acid/base status of the soil, proton buffering in leaves, and N deposition), as well as disturbances in the transport system of assimilates and water, are suspected of causing the decline symptoms. Information on the filtering mechanisms at various hierarchical levels, especially in the case of a break in the hierarchy, is missing. The null hypothesis (no effects of air pollutants on forest ecosystems) can be considered to be falsified. Forest ecosystems are in transition. The current state of knowledge is not sufficient to define precisely the final state that will be reached, given continuously changing environmental conditions and human impacts. The hypothesis, however, of large-scale forest dieback in the near future is not backed by data and can be discarded.Key words: forest ecosystem, process hierarchy, air pollution, deposition, acidity, nitrogen.