The history and possible causes of forest decline in central Europe, with particular attention to the German situation

Abstract

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 intluences. 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 acidlbase 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.RCsumC : Au cours de plusieurs siecles d'utilisation excessive, I'elasticitk (accumulation de nutriments, dkcomposition des litieres, perturbation biologique des sols) et la diversite des kcosystkmes forestiers de 1'Europe centrale ont 6te rkduites. Depuis le milieu du 19' siecle, leur diveloppement a it6 intluence par des pratiques sylvicoles ainsi que par les dep6ts d'acides et de nutriments, surtout de I'azote de sources anthropogenes, c.-a-d. par un melange d'influences externes stabilisantes et destabilisantes. Au cours des recentes decades, la plupart des sols forestiers ont it6 acidifies par les dep6ts acides, ce qui a conduit a ...

Monitoring the effects of air pollution on forest condition in Europe: is crown defoliation an adequate indicator?

“…Little evidence has been found for a widespread effect of air pollution on tree health in European forests (Ulrich 1995). Instead, climate conditions seem to have a major effect on crown condi tion and tree growth (Kandler & Innes 1995, Solberg & Torseth 1997.…”

Thirteen-year monitoring of liming and PK fertilization effects on tree vitality in Norway spruce and European beech stands

“…They were impoverished by timber harvesting and by ancestral practices, such as burning and litter raking (Hofmeister et al 2008). Since the beginning of industrialization, they were also subject to increased acid deposition, which promoted the loss of nutrients by deep drainage (Malek et al 2005) and brought about severe problems of forest health (Ulrich 1995). Although emission of acid precursors has been decreased since the 1970s (Mylona 1996), N deposition on forest ecosystems is still elevated and likely to lead to N saturation (Aber et al 1998) and to nutritional imbalances (Prietzel and Stetter 2010;Mellert et al 2004;Weissen et al 1990).…”

Crown condition as a function of soil, site and tree characteristics

“…These processes include for example soil acidification (Ulrich et al 1994;Ulrich 1995), effects of phyto-toxic reaction products of acidification (RostSiebert 1988;Sverdrup and Warvinge 1993;Cronan and Grugal 1995), nutrient shortage, e.g. Mg-deficiency (Zech and Popp 1983;Evers 1994) or nutrient imbalances attributed to nitrogen surplus (Evers et al 1968;Hildebrand 1994;Mohr 1994), which were discussed to be main processes causing forest decline and needle loss.…”

“…The leading hypothesis on the causes of forest decline postulates for the chronic type of the damages the following chain of cause and effect: acidic deposition and a surplus of nitrogen availability cause an accelerated soil acidification and disequilibria in tree nutrition, which lead to yellowing and loss of needles/leaves (Scho¨pfer and Hradetzky 1984;Ulrich et al 1994;Ulrich 1995;Landmann and Bonneau 1995). In order to investigate some aspects of this hypothesis, data from different surveys on crown condition, tree nutrition and soil chemical status were combined in this investigation.…”