In the early 1980s, a dramatic deterioration in forest condition was observed in Europe and this initiated the implementation of forest condition monitoring under the Convention on Long-range Transboundary Air Pollution (CLRTAP).
The health status of forest trees in Europe is monitored over large areas by surveys of tree crown condition. Trees that are fully foliated are regarded as healthy. The Ministerial Conference on the Protection of Forests in Europe (MCPFE) uses defoliation as an indicator for forest health and vitality.
The results of the crown condition data are presented in terms of percentages with 5% defoliation steps. A defoliation of >10-25% is considered a warning stage and a defoliation > 25% is taken as a threshold for damage. A distinction can be made between defoliation classes 0 and 1 (0-25% defoliation) on the one hand, and classes 2, 3 and 4 (defoliation > 25%) on the other hand.
In 2009, 20.2 % of all trees assessed had a needle or leaf loss of more than 25 % and were thus classified as either damaged or dead. In 2009, mean defoliation of all assessed trees in Europe was 19.0%. Deciduous trees showed a mean defoliation of 20.1%, slightly higher than that of conifers (18.1%). Plots showing deterioration are scattered across Europe, but their share is particularly high in southern France, at the eastern edge of the Pyrenean Mountains, Czech Republic, and northeastern Italy.
The status and trends in forest condition vary regionally and for different species. Local conditions may differ from the European average.
Defoliation is an indicator of tree health and vitality that can be easily monitored over large areas and reacts to many different factors, including climatic conditions, insect and fungal infestations, deposition of pollutants...
Figure 1: Change in defoliation for all tree species over the period 1998 to 2009 (Fischer and Lorenz, 2011) - FutMon project LIFE07 ENV/D/000218.
Over the long time period, a decrease in the mean defoliation was noticed. In recent years, however, almost no change in crown condition was seen. The share of healthy pines (0-10%) increased and the share of the damaged pine trees (>25%) decreased. Most plots showed no clear trend from 2002 to 2010.
Picea abies is the second most frequently occurring tree species in the large scale tree sample. Since 1991, the share of healthy trees (0-10%) increased slightly. In the same period the share of more damaged spruce (>25%) decreased slightly.
Mediterranean lowland pines
The group of Mediterranean lowland pines is composed of Pinus brutia, Pinus pinaster, Pinus halepensis and Pinus pinea. Their occurrence is limited to the Mediterranean region. Crown condition of this tree species group is characterized by a considerable increase in mean defoliation of the pine trees since 1991. The share of healthy trees (0-10%) has decreased from 72.9% in 1991 to 23.2% in 2010.
Fagus sylvatica is the most common deciduous tree species. Temporal trends of mean defoliation from 2003 to 2010 showed an increase in mean defoliation, especially on plots in France and Croatia. Decreasing trends were detected for plots in Italy and western Germany.
In the 1990s, mean defoliation of evergreen oak trees (Quercus coccifera, Quercus ilex, Quercus rotundifolia and Quercus suber) was relatively low and the share of healthy trees (0-10%) was high. Since 2006, a slight recovery of the crown condition has been recorded, 13.8% of all plots showed an increasing trend of mean defoliation from 2002 to 2010.