The influence of tree species diversity gradients on foliar fungal pathogens and endophytes in European forests

Diem T. Nguyen, Johanna Boberg, Michelle Cleary, Katarina Ihrmark, Elna Stenström, Jan Stenlid

Biodiversity is an important determinant of ecosystem processes, functioning and the provision of services. The effects of biodiversity have been studied predominately in artificial grassland systems, with little attention devoted to woody species. FunDivEUROPE, a European Union funded study, aims to quantify the effects of forest biodiversity on ecosystem function and services in major European forest types, which aims to improve our understanding of the functional significance of biodiversity in a forest setting.

While fungi are important drivers of ecosystem dynamics, fungal pathogens can negatively affect forest stability and productivity. Our study aims to determine the effects of tree species along a diversity gradient on foliar fungal pathogens and the fungal community in general. The study sites include six mature forests throughout Europe and two, approximately twelve-year-old experimental plantations in central Germany and southwestern Finland. Visual pathogen damage assessment is conducted on foliage (leaves or needles) from two layers of the canopy, one of which comprises the sun-exposed top part of the crown from trees grown in various diversity compositions, ranging from monoculture to mixtures of three to five tree species. Foliar fungal damage categories considered include powdery mildew, leaf spots, rust and needle cast. Additionally, a subsample of the foliage is randomly collected from the same two layers of the crown to examine the fungal community composition within and on the leaves and needles by 454 pyrosequencing the fungal ITS 2 region of the nuclear ribosomal DNA (rDNA). Specifically, the fungal pathogen Hymenoscyphus pseudoalbidus, the causative agent of ash dieback in Fraxinus excelsior in Europe, will be examined closely from previous year’s ash petioles and those from current year leaves, with species-specific primers followed sequencing of microsatellite markers.

We hypothesize that 1) at the tree level, lower branches have more pathogen damage than top, sun-exposed branches based on the damage assessment data, 2) mixed tree species plots have lower incidence of foliar fungal pathogens than monocultures, indicating that trees in mixtures can buffer or dilute the effects of propagule dispersal, and 3) there is a correlation between the diversity and abundance of foliar endophytes and reduction in the occurrence of pathogens.

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