Ten Years of Monitoring Forest Condition in Europe. Technical Background Report : Studies on Temporal Development, Spatial Distribution and Impacts of Natural and Anthropogenic Stress Factors
The monitoring programme for the crown condition of European forests, which is based on a systematic grid (Level I) of sample plots, now has 10 years of monitoring data available. Data from recent surveys of soil condition in the plots are also available. The major objectives of this study was to use the database derived from 10 years of monitoring of the forest condition for an extended evaluation of the crown condition in Europe. In addition, it was intended to try to find cooelations between crown condition and known stress parameters. Parts of the recently completed database on forest soil condition were used together with external databases on meteorology and air pollution. Interpolation methods and models were applied to estimate parameter values for individual plots. In-depth studies of the quality of the crown data revealed that the assessment of the four damage classes of crown condition within countries is reliable. Assessments showed that an agreement of 10% is reached for about 80% of trees, while an exact agreement for the 5% defoliation classes is obtained in only 30% of the trees. However, results from comparisons made between countries indicate that direct comparison of the defoliation data from different countries should be made with great care. During the considered period a distinct deterioration of forest condition has been identified for the seven most frequent tree species (Norway spruce, Scots pine, Maritime pine, Common beech, Holm oak, Sessile and European oak). Distinct regiona differences were observed at the European scale. In most case these spatial differences could be explained by location, with both altitude and climatic type being important. For many Level I plots, however, the deterioration of forest condition was not sufficiently explained by these two factors. A remarkable clustering of plots was found in parts of central and Eastern Europe, and in the countries in the Iberian peninsula, known stand characteristics could not sufficiently explain deterioration of crown condition. Critical threshold values and various stress parameters affecting the forest condition were estimated by modelling, e.g. air pollution and drought in the plots. Calculated critical levels for air pollutants were exceeded and at approximatey 20% of the plots for SO2 at approximately 90% of the plots for ozone. Results for ozone however refer to 1990 exclusively, a year with particularly high ozone concentrations in Europe. Critical threshold values for acidity and nitrogen were exceeded at approximately 10-25% of the plots. The exceedance values for N and acidity are, however, related to effects on ground vegetation rather than crown condition. Regression techniques were used to trace possible correlations between the stress factors and defoliation. The low transboundary comparability of the defoliation data and the limites reliability of the derived stress factors seriously hampered the clear revelation of cause-effect relationships. Several modelled stress factors (the concentrations of SO2, NOx and O3) showed a significant relationship with forest condition although the explanation of the variability in the vurrent defoliation and changes and trends in defoliation was low. Anther stress factor that sometimes appeared to be significant, depending on the tree species, were relative transpiration (drought). The work on the data quality improvement and the models needs to be continued. This first attempt has again clearly revealed the most important uncertainties in the quality of the data and methods. With the completion of the nutritional status of the needles and leaves and the improvement of the environmental databases e.g. for ozone and meteorological data, transboundary correlative studies may have a better basis. However, at the same time it has to be realised that the Level I grid was not setz-up for correlative studies. Instead, the intensive monitoring (Level II) was established, with detailed surveys being carried out, including soil, foliage and deposition. Combinations of the results from the intensive monitiring will lead to possibilities to trace correlations between forest condition data and stress factors and to validate model calculations.
