Effects of environmental stress on forest crown condition in Europe. Part I: Hypotheses and approach to the study

This paper is the first in a series of four, describing the hypothesis and approach of a correlative study between observed data on crown condition in Europe, monitored since 1986 at a systematic 16 x 16 km grid, and site-specific estimations of various natural and anthropogenic stress factors. The study was based on the hypothesis that forests respond to various natural and anthropogenic stress factors, whose contribution depend on the geographic region considered. In view of this hypothesis, major stand and site characteristics, chemical soil composition, meteorological stress factors (temperature and drought stress indices) and air pollution stress (concentrations and/or depositions of SOx, NOy, NHx and O-3) were included as predictor variables. The response variables considered were actual defoliation and changes/trends in defoliation for five major tree species. The spatial distribution of the average defoliation during the period 1986-1995 shows high defoliation in Central Europe and in parts of Scandinavia and of Southern Europe. There are, however, sharp changes at country borders, which are due to methodological differences between countries. The spatial distribution of the calculated trends show a distinct cluster of large deterioration in parts of Central and Eastern Europe and in Spain and a rather scattered pattern of positive and negative trends for most of Europe, indicating that other factors than air pollution only have a strong impact on defoliation. The limitations of the study are discussed in view of the quality of the considered response and predictor variables.