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Since 1999Since -2002 the European Commission has funded and the European Forest Institute has coordinated the interdisciplinary RECOGNITION project aiming at elucidating the causes of the growth acceleration which has been observed in some forest tree species in several parts of Europe. Within this project, it was our task to identify and quantify long-term changes in the nutritional status of representative forest stands that potentially could explain this growth increase, using available long-term series of foliar analyses. An inquiry among 25 forest research institutions in Europe resulted in 28 Scots pine (Pinus sylvestris L.) and 21 Norway spruce (Picea abies [L.] Karst.) stands for this historical development investigation (HDI). The stands generally are control plots of fertilization experiments and are located mainly in Central Europe and in Scandinavia. The monitoring periods vary from 15-40 years. The foliar data were given to us by our partner organisations, subjected to rigorous tests for plausibility and comprehensively evaluated using mainly single linear regression approaches. Most Scots pine stands under study in Central Europe, which grow predominantly on naturally poor or devastated soils in regions with relatively high atmospheric N deposition, suffered from N deficiency at the start of the monitoring period, but exhibited a considerable improvement in N nutrition over the past decades. The increase in N levels was usually associated with an increase in the ratios of N and P, K, Ca, and Mg, although critical values of these ratios are far from being reached. For the majority of the investigated Scots pine stands in Central Europe, growth acceleration due to a better N supply is highly probable.In contrast, N seems to be far less important as a potential driving factor for long-term growth changes in Scots pine in Scandinavia. Norway spruce stands examined in Central Europe were, in general, initially well supplied with N and characterised by a decreasing trend in foliar N levels although they received considerable N deposition from the atmosphere. This decrease in concentrations of N and other macronutrients is believed to be mainly a dilution effect as indicated by a simultaneous increase in needle weights. Spruce in Finland also turned out to be adequately provided with N at most sample sites, and no general trend of improving N nutrition was detected.

Changes of the soil chemical status during the recent 22-30 years at two historically degraded forest sites in southern Germany (Pfaffenwinkel, Pustert) stocked with mature Scots pine (Pinus sylvestris L.) stands were studied by repeated soil inventories conducted in 1974, 1982-1984, 1994, and 2004 on replicated control plots of fertilization experiments, allowing a statistical analysis. Additionally, the nutritional status of the stands at all plots was monitored from 1964 until 2004 by annual or bi-annual analysis of current-year foliage, and stand growth was assessed by repeated stand inventories carried out in 3-to 9-year intervals. For both sites, a statistically significant systematic decrease of the forest floor C/N ratio between 1974 and 2004 from 35.4 to 29.2 (Pfaffenwinkel) and from 36.5 to 23.0 (Pustert) was observed. The soils at both sites also showed a considerable accumulation of organic carbon (210 and 400 kg C ha -1 year -1 for Pfaffenwinkel and Pustert, respectively) and nitrogen (13 and 18 kg N ha -1 year -1 ). In addition, the mineral topsoil at both sites has acidified considerably, indicated by significantly decreased pH values (Pustert only; mean decrease 0.1 units per decade), base saturation, and base cation stocks. The trend of N enrichment and base cation loss in the soils is mirrored by the trends of stand nutrition at both sites, which are characterized by improved N nutrition and reduced supply with K, Mg (Pustert only), and Ca. The results confirm findings of other studies indicating a recent N eutrophication and acidification of forest soils in Central Europe and southern Scandinavia. Since soils with historic degradation due to earlier non-sustainable forest utilization are widespread in Central Europe, the results obtained on our study sites probably apply for large forested areas, suggesting a significant potential of Central European forests to sequester atmospheric carbon and nitrogen not only in stand biomass, but also in the soil.

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