Crown transparency
 

The extent of crown defoliation is regarded as an indicator for stress impact on a tree. However, it can be caused by different factors: sudden clearing, nutrient deficiency, parasite infestation, root rott, extreme meteorological conditions and air pollution. Most often, several factors are acting together. A crown transparency of >25% is usually regarded as a limit for stress indication. In the graph below the proportion of trees exceeding this limit is shown.

 
   
The proportion of  trees with a crown transparency of >25% has stayed more or less constant during the observation period. 
 
During the past 15-20 years periods of increased crown transparency can be observed both for beech and Norway spruce. They can be explained mainly by meteorological extremes and fructification.