Since the mid 1980s the effects of environmental changes on forest condition have been monitored in Europe. First, national surveys were conducted, later long-term research plots were installed, such as the LWF plots of the WSL. All the indicators of forest condition, including crown defoliation, tree mortality and growth, are influenced by site conditions, inter-tree competition and stand age. This makes it difficult to find simple cause-effect relationships with environmental factors, in particular with air pollution.
On the LWF plots, tree growth and defoliation vary highly between forests. Annual mortality rates, on the other hand, were mostly below 1 %. Higher values were linked to diseases, pathogens or extreme summer drought. Defoliation was subject to annual perturbations caused by cyclic outbreaks of defoliators, drought, storms or frost. The same is true for tree growth. Within a forest, trees with higher defoliation tend to grow slower and have a higher mortality rate as compared to trees with low defoliation. Between stand comparisons are more complex and do not allow to draw simple conclusions on forest condition based on one indicator alone.