Leaf traits are related to functional interactions between trees in mixed forests. Silviculture of mixed forests in Europe.

Mixed forests are assumed to be more productive and resilient to environmental stress than monocultures, thank to reciprocal beneficial interactions and the ability of different species to exploit more efficiently the resources of the ecosystem. The effects of such interactions on the physiology of singular trees can be assessed by analyzing the overall crown conditions and key foliar features. This contribute reports some experiences carried out within the project FunDivEurope (Functional Significance of Forest Diversity in Europe) in different European mixed forests. Tree diversity can be either beneficial or detrimental for photosynthetic efficiency, as measured by mean of chlorophyll fluorescence analysis. Different behaviors may depend from belowground and aboveground completion for water, nutrient and light, from the development of the forest stand and from the overall ecological conditions. This presentation proposes special focus of the explorative plots in Tuscany (Colline Metallifere), where a comprehensive analysis of crown conditions and foliar features (including the analysis o nitrogen and stable carbon isotopes content) was carried out. The survey took in account structural and vegetational characteristics of the forest stands, such as leaf area index (LAI), basal area (BA) and tree diversity expressed by means of the Shannon Index. The five tree species examined (Quercus ilex; Quercus cerris; Quercus petraea; Ostrya carpinifolia; Castanea sativa) showed species-specific crown conditions and physiological characteristics in relation to the stand structure and diversity. Tree diversity reduced defoliation on C. sativa, as consequence of decreased Dryocosmus kuriphilus attacks on chestnut tree crowns. This behaviour is to be connected to the mixture with oak species that houses cynipid galls and associated parasitoids. In C. sativa the diversity is also associated to higher levels of foliar nitrogen and lower content of the stable carbon isotope 13C. The content of 13C is lower also in deciduous oaks (Q. cerris and Q. petraea) growing in increasing levels of diversity, so suggesting a better ability to take up water in mixed stands. Defoliation decreased in Quercus ilex with increasing Shannon Index. This behavior is probably due to the smaller size of this species in comparison with the tallest deciduous oaks, thanks to it, Quercus ilex it profits of the physical protection of dominant canopies. Leaf area index was positively related to diversity (in relation to a better exploitation of the space in mixed forests), and negatively associated to defoliation, whereas the crown compression produced the opposite effect. These results suggest the combined analysis of defoliation with foliar features can provide insights for forest management