Species richness and species identity effects on occurrence of foliar fungal pathogens in a tree diversity experiment

Current theory on transmission rates of plant pathogens predicts a strong influence of host richness on the degree of infection. In addition, identity effects, caused by the presence of particular species in a community, may also drive biodiversity and ecosystem functioning relationships, with "selection'' or "sampling effects'' being particularly important. We tested the effect of tree species richness and tree species identity effects on foliar fungal pathogens on four forest tree species of the temperate zone making use of the BIOTREE tree diversity experiment in Germany. We hypothesized that fungal species richness is positively and fungal pathogen load negatively related to tree species richness. In addition, we tested whether species number of foliar biotrophic fungi and pathogen load depend on tree community composition and on the presence or absence of particular disease-prone tree species. All foliar fungi were identified macro-and microscopically and subjected to statistical analyses at three hierarchical levels, at the plot level, the level of single tree species and the level of individual fungus species. There was a negative effect of tree richness on the pathogen load of common powdery mildew species. Moreover, we found strong tree species identity effects at the plot level as the presence of Quercus resulted in a high pathogen load. Thus, for the first time we experimentally showed that disease risk and pathogen transmission of foliar fungal pathogens in temperate forest tree ecosystems may depend on tree richness and on the presence of particular disease-prone species.