Forest diversity and tree health relationships in Italian forests depend on environmental context

Forest tree diversity plays a relevant role in promoting forest productivity and overall capacity to deliver ecosystem services. Tree health is positively influenced by forest diversity, especially in relation to insect and pathogen attacks. However, there is no clear general evidence about the possible relationships between stand-scale tree diversity and tree health (estimated as degree of crown defoliation), as assessed in the current European forest monitoring programme (the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests, ICP Forests). It is recognized that the influence of stand-level 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 areas, that includes alpine, mountain and Mediterranean climatic regions as well as a broad range of soil types. Aim of this study was to investigate the relationships between forest diversity and tree health conditions of the main forest types in Italy, using the current forest monitoring network. In this study, 250 plots belong to the Italian Level I ICP Forests network were grouped in four groups of plots by means of a multivariate cluster analysis, considering ecological and structural parameters of the stands (geographical, climatic and soil characteristics, taxonomic (Shannon) and structural (mingling) diversity indices), obtained from various datasets. Relationships between crown defoliation and diversity (at tree- and stand-level) were analysed on the whole dataset and in homogeneous clusters for the main forest tree species (Fagus sylvatica L., Picea abies L., Quercus cerris L., Quercus pubescens Willd., Castanea sativa Mill.). The main results show that at country level, tree species richness, diversity and composition of the stands reflect different and often contrasting ecological conditions. The responses of trees (crown defoliation) seems driven more by the environmental context rather than diversity. In homogenous ecological conditions, low levels of stand diversity were more effective to improve the crown conditions (i.e. minor defoliation) than either monospecific or highly-diverse stands. This aspect should be considered to promote and apply forest management options aimed at enhancing stand diversity in monospecific stands in areas where the main tree species are in their ecological optimum.