The effect of tree species diversity on foliar fungal pathogens

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Foliar fungal pathogens affect ecosystem processes in forests through highly speciesspecific effects of trees on growth and survival. Thus, fungal pathogens can contribute to maintain tree diversity. As host-specialized pathogenic organisms such as foliar fungal pathogens are both response to and effect of tree diversity in forests, both can be disentangled with the experimental design of BEF-China. In my subproject I aim to test the hypotheses: (1) Tree species diversity decreases disease risk and pathogen load of pathogenic fungi on a particular host species because host species density and thus, fungal habitat availability, is diluted with increasing tree species diversity. (2) Tree species diversity increases foliar fungal species diversity. (3) The phylogenetic similarity of host species increases the phylogenetic similarity of foliar fungal communities on different tree species, as a consequence of the co-evolution of the pathogens with their hosts. (4) The degree of herbivory leads to increased fungal pathogen load. (5) Increasing pathogen load decreases the growth and the fitness of tree individuals and species. I will address these hypotheses with three different experimental set ups of the BEF-China project. WP1 will quantify pathogen load as percentage of leaf area infected by foliar fungal pathogens across all tree species in all plots of the Main Experiment. WP2 will identify for a total of 16 selected tree species all fungus species that occur on leaves of a certain tree species and construct tree species-specific phylogenies and trophobiotic networks. This work package will be complemented by molecular identification of fungi by the Chinese partner project of Jiye Yan, who will also construct molecular fungal phylogenies. WP3 will assess the effect of foliar fungal pathogens on plant performance in the BEFmod approach as well as the effects of P fertilization, competition of the herb layer and herbivory on pathogen load (in particular with SP1, 2, 8, 9, 11). Combining information from the two observational levels and the experimental approach of BEFmod will allow me to address one of the key questions in functional biodiversity research: how pathogens mediate plant species richness-growth relationships in response to herbivory and tree species loss with a focus on trophobiotic interactions.