Defoliation

The variation of crown condition is mainly the result of intrinsic factors, age, and site conditions. Moreover, defoliation may be caused by a number of biotic and abiotic stressors. Defoliation assessment attempts to quantify foliage missing as an effect of stressors including air pollutants and not as an effect of long lasting site conditions. In order to compensate for site conditions, local reference trees are used, defined as the best tree with full foliage that could grow at the particular site. Alternatively, absolute references are used, defined as the best possible tree of a genus or a species, regardless of site conditions, tree age etc. depicted on regionally applicable photos (photo guides). Changes in defoliation and discolouration attributable to air pollution cannot be differentiated from those caused by other factors. Consequently, defoliation due to factors other than air pollution is included in the assessment results, but trees showing mechanical damage are not included in the sample.

The true influence of site conditions and the share of tolerable defoliation cannot be quantified precisely, damaged trees cannot be distinguished from healthy ones only by means of a certain defoliation threshold. Some differences in the level of damage across national borders may be at least partly due to differences in standards used. This restriction, however, does not affect the reliability of trends over time.

Further information and discussion of results are available in
ICP Forests' Technical Reports