Linking forest diversity and tree health： preliminary insights from a large-scale survey in Italy

Forest health is currently assessed in Europe （ICP Forests monitoring program）. Crown defoliation and dieback, tree mortality, and pathogenic damage are the main aspects considered in tree health assessment. The worsening of environmental conditions （i.e., increase of temperature and drought events） may cause large-spatial scale tree mortality and forest decline. However, the role of stand features, including tree species assemblage and diversity as factors that modify environmental impacts, is poorly considered. The present contribution reanalyses the historical dataset of crown conditions in Italian forests from ] 997 to 2014 to identify ecological and structural factors that influence tree crown defoliation, highlighting in a special manner the role of tree diversity. The effects of tree diversity were explored using the entire data set through multivariate cluster analyses and on individual trees, analysing the influence of the neighbouring tree diversity and identity at the local （neighbour） level. Preliminary results suggest that each tree species shows a specific behaviour in relation to crown defoliation, and the distribution of crown defoliation across Italian forests reflects the distribution of the main forest types and their ecological equilibrium with the environment. The potentiality and the problems connected to the possible extension of this analysis at a more general level （European and North American） were discussed.

No plant functional diversity effects on foliar fungal pathogens in experimental tree communities

Foliar fungal pathogens affect forest ecosystem processes by exerting highly species-specific impacts on growth and survival of trees.As many ecosystem processes in forests depend on functional diversity of specific tree species,a close relationship is expected between this and foliar fungal pathogen infestation.Testing for such a relationship in the German tree diversity experiment BIOTREE(Bechstedt),we hypothesized that pathogen richness and pathogen load decline with increasing functional diversity of tree communities.Using macro-and microscopic analyses,we assessed pathogen richness and load on 16 tree species in plots that,although differing in functional diversity,had the same tree species richness.We found no effects of functional diversity on pathogen richness or load.However,we encountered strong species identity effects in plot species composition,as susceptible tree species contributed positively to each community’s pathogen richness and load.Furthermore,testing for effects of particular leaf traits and geographical range size of host species revealed a significant effect of total leaf phenolics,which was unexpected as pathogen richness increased with increasing content in polyphenolics.Our study showed that at the community level,host species’identity was more important for foliar fungal pathogen richness and load than the functional diversity of host trees.The positive relationship between pathogen richness and phenolics in leaves,along with the finding that pathogen richness is very much conserved in tree species,point to an evolutionary arms race between hosts and fungi resulting from fungi increasing their capacity to infect tree leaves and trees boosting their defences.