Forested catchments in Europe are studied within the ICP Integrated Monitoring (IM). These sites are located in undisturbed areas, such as natural parks or comparable areas. The network currently covers
50 sites in 22 countries. Fluxes and trends of sulfur and nitrogen compounds were recently evaluated for 22 sites. The site-specific trends were calculated for deposition and runoff water fluxes and concentrations using monthly data and nonparametric methods. Statistically significant downward trends of SO4 and NO3 bulk deposition (fluxes and concentrations) were observed at 50% of the sites. Sites with higher nitrogen deposition and lower carbon/nitrogen ratios clearly showed an increased risk of elevated nitrogen leaching. Decreasing SO4 and base cation trends in output fluxes and/or concentrations of surface/soil water were commonly observed at the ICP IM sites. At several sites in Nordic countries, decreasing NO3 and H+ trends (increasing pH) were also observed. These results partly confirm the effective implementation of emission reduction policy in Europe. However, clear responses were not observed at all sites, showing that recovery at many sensitive sites can be slow and that the response at individual sites may vary greatly.
Defoliation and discoloration of stands have been monitored in Europe on a regular 16 x 16-km grid since 1986—the so-called level 1 program [ICP - Forests (F)]. The network currently covers 374,238 sample trees distributed on 18,717 plots in 31 countries. At approximately 900 sites in Europe, key parameters, such as deposition, growth, soil chemistry, and leaf content, have been monitored since 1994 to study cause-effect relationships and the chemical and physical parameters determining forest ecosystem vitality (level 2). The duration of the level 2 program is too short for trend detection. These data are more relevant for studying cause-effect relationships. Figure 7 shows the annual variation in defoliation for different tree species in Europe as reported in the extended summary report by ICP-F. It is concluded by ICP-F that in all parts of Europe there is defoliation to a various extent. Of the 1999
FIGURE 7 Development of mean defoliation of the six most abundant species in Europe. Data from ICP (2000).
total transnational tree sample, mean defoliation is 19.7%. Of the main tree species, Quercus robur has the highest defoliation (25.1%), followed by Picea abies (19.7%), Fagus sylvatica (19.6%), and Pinus sylvestris (18.9%). The trend in defoliation over 14 years for continuously observed plots shows the sharpest deterioration of Pinus pinaster and Quercus iles in southern Europe. Fagus sylvatica deteriorated in the sub-Atlantic, mountainous (south), and continental regions. Picea abies deteriorated in several parts of Europe but improved in the main damage areas of central Europe since the mid-1990s.
Because there are no specific symptoms of individual types of damage, defoliation reflects the impact of many different natural and anthropogenic factors. Weather conditions and biotic stresses are most frequently cited. 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 on the large scale cannot be derived from crown condition assessment alone. Statistical analysis of 262 of the 860 level 2 plots indicates that defoliation is influenced mainly by stand age, soil type, precipitation, and nitrogen and sulfur deposition. These factors explain 30-50% of the variation. Nitrogen deposition correlated with defoliation of spruce and oak, and sulfur deposition was found to correlate with defoliation of pine, spruce, and oak. Calculated drought stress was found to impact nearly all tree species, whereas calculated ozone mainly impacted broadleaves, specifically the common beech trees.