Forest condition in Europe has been surveyed on large scale for 16 years under the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (IPC Forests) of the United Nations Economic Commission for Europe (UN/ECE) and under the Scheme on the Protection of Forests against Atmospheric Pollution of the European Union (EU). In the year 2001, crown condition was assessed on 5 942 plots, constituting a transnational grid of 16 x 16 km in 30 countries. On these plots, defoliation of 132 350 sample trees was assessed for the calculation of results at the European scale. The transnational plots constitute a subsample of 340 903 plots from denser national grids of 34 countries. The mean defoliation of the 2001 transnational tree sample was 20.1%. Of the main species, Quercus robur and Q. petraea had by far the highest mean defoliation (24.9%), followed by Fagus sylvatica (20.7%), Picea abies (19.4%) and Pinus sylvestris (19.1%). The development of defoliation has been traced on subsample of those trees continuously observed since 1989. This reveals increasing defoliation for the most common six species with a high spatial variation. The participating countries reported different causes for important changes in defoliation. The recovery of Pinus sylvestris is attributed to a decrease of acid deposition, but insect attacks or fungi infestations are responsible for regional increases in defoliation. The constant increase in defoliation of Picea abies is attributed to storms and subsequent insect attacks. The defoliation of Quercus robur and Q. petraea remained almost unchanged. Fagus sylvatica in 2001 showed the highest increase in defoliation among the main species, which was particularly pronounced in the Mountainous (south) region and partly explained by hailstorms. Over the total period of observation Quercus ilex and Q. rotundifolia and Pinus pinaster experienced the most severe deterioration of crown condition, which was attributed mainly to summer heat and drought. Crown condition data of Pinus sylvestris and Fagus sylvatica were evaluated in relation to biotic agents, soil condition, deposition and meterology. The results confirm that defoliation varies with stand age and geographic region. Besides this variation there is a small but statistically significant relationship between sulphur deposition and defoliation. High defoliation figures are found in areas with high sulphur deposition and a decrease in defoliation is observed in areas where sucessful abatement strategies have reduced sulphur deposition. The correlations between nitrogen inputs and forest condition are not statistically significant and eveal ambiguous conditions. This might confirm current knowledge that nitrogen inputs on one hand fertilise forest ecosystems, but on the other may have acidifying and thus damaging effects. The analysis of the spatial variation of defoliation for Fagus sylvatica showed a statistically significant positive correlation with the index for fungi infestations. The index for insect pests was positively correlated with defoliation for both tree species. Only in the case of spatial variation of defoliation for Pinus sylvestris was this correlation statistically significant. Defoliation was correlated to the interaction between precipitation and soil properties indicating a benefit of precipitation on soils with limited water availability.
