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.