Relationships between crown defoliation and tree diversity depend on the environmental context

Tree health is positively influenced by forest diversity, especially in relation to insect and pathogen attacks, but there is no clear general evidence about the possible relationships between tree diversity and crown defoliation as assessed in the ICP Forests programme. It is recognized that the influence of tree diversity on forest health may be variable in relation to the environmental context, and it can be expected that different patterns of crown defoliation occur on different tree species in different eco-regions. Italy is an excellent case of study because of the ecological heterogeneity of the forest area, which includes alpine, mountain and Mediterranean environmental conditions. The 250 plots of the Italian Level I ICP Forests network were subdivided in four groups of plots by mean of a multivariate cluster analysis, considering ecological and structural parameters (geographical, climate and soil characteristics, diversity indices) of stands. Relationships between crown defoliation and diversity were analysed on the whole dataset and in homogeneous clusters for the most important tree species (beech, Norway spruce, Turkey oak, downy oak, chestnut). Tree diversity was determined by means of taxonomic (Shannon) and structural (Mingling) diversity indices. The main results evidenced a general positive correlation between defoliation and diversity (i.e. increasing defoliation with diversity) in the whole population (i.e. all tree species together) and singularly in the main species. This unexpected result can be explained by the fact that, because of their ecological behaviour and past management, the main species tend to constitute monospecific forests in the most favourable environmental conditions. Analysing individual clusters of plots (i.e., with similar ecological conditions), species-specific correlations between crown defoliation and tree diversity (both negative and positive) were found in some cases.