Crown condition of Quercus robur in Flanders (Belgium)

In 2005, 23.2% of the oaks (Quercus robur) in the Level I survey were damaged with an average leaf loss of 22.4%. There is a significant relation between defoliation and damage by defoliators. Before 2006 only the extent of the insect damage was taken into account, not the intensity of the damage. Trees were classified into 4 damage classes according to the part of the crown being affected: no damage (0%), slight damage (1-20%), moderate damage (21-40%) and severe damage (>40%). Defoliation scores increased significantly when more then 20% of the crown was affected. A correlation was found between insect damage and fungal infection. In 2005, 80% of the trees with moderate to severe insect damage were also infected by fungi, in particular by powdery mildew (Microsphaera alphitoïdes). The infection has a less important influence on the defoliation score than the insect damage. The international method for biotic damage assessment (Manual ICP Forests - Visual Assessment of Crown Condition), uses more classes and the damage by insects is better correlated with the defoliation score. Data collected with the previously used, regional method are difficult to translate into the new classes and vice versa.