Crown condition has been monitored annually for 14 years on the European scale within the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP 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 1999, crown condition was assessed on 374 238 sample trees distributed on 18 717 plots on the different national grids in 31 of the participating 38 countries. 128 977 sample trees on 5 764 of these plots, constituting a transnational grid of 16 x 16 km in 30 countries, were evaluated for the calculation of results at the European scale. In all parts of Europe, defoliation of various extent is observed. Of the 1999 total transnational tree sample, mean defoliation is 19.7 %. Of the main tree species, Quercus robur has the highest mean defoliation with 25.1 %, followed by Picea abies (19.7%), Fagus sylvatica (19.6%) and Pinus sylvestris (18.9%). These results are not directly comparable with those of previous years due to differences in the sample sizes and changes in methods in some countries. The development of defoliation, however, can be traced by means of a selection of those plots continuously observed. In recent years, the sharpest deterioration has occurred on Pinus pinaster and Quercus ilex in southern Europe. Fagus sylvatica deteriorated in the Subatlantic, Mountainous (south) and Continental regions. Picea abies deteriorated in several parts of Europe, but improved particularly in the main damage areas of central Europe. Pinus sylvestris continued its recuperation observed in the main damage areas of central Europe since the mid 1990s. As there are not any specific symptoms of individual types of damage, defoliation reflects the impact of many different natural and anthropogenic factors. Weather conditions and biotic stressors are the ones mentioned most frequently by the countries. Several countries refer to air pollution as a-predisposing, accompanying or triggering factor, but the degree to which air pollution explains the spatial and temporal variation of defoliation at the large scale can not be derived from crown condition assessments alone. It is noteworthy that defoliation has been increasing in many regions and that regions of high defoliation coincide with areas of highest depositions in central Europe. However, a substantiation of cause-effect relationships needs holistic studies utilizing the wealth of parameters available today, as they have been launched (UN/ECE and EC, 2000). In connection with other parameters assessed within the monitoring of forest condition in Europe, crown condition assessments constitute an indispensable contribution to the analysis of the effects of air pollution and other stressors on forests.
