Fungal disease incidence varies along tree diversity gradients and depends on latitude in European forests

Fungal pathogens increasingly threaten European forests that host a diversity of tree species. Damages to trees may have cascading consequences for forest ecosystems and their functioning. Previous experimental studies suggest that foliar and root pathogen abundance and disease severity decrease with increasing tree species diversity, but evidences from natural forests are rare. In our publication "Fungal disease incidence along tree diversity gradients depends on latitude in European forests" (Ecology and Evolution, 2016, DOI: 10.1002/ece3.2056), we tested whether foliar fungal disease incidence was indeed negatively affected by tree species diversity in different forest types across Europe.

To that end, we measured the foliar fungal disease incidence on 16 different tree species in 209 plots in six European countries, representing a forest-type gradient from the Mediterranean to boreal forests. Damage symptoms included: leaf spots, powdery mildew, rust and needle casts. Forest plots of single species (monoculture plots) and those with different combinations of two to five tree species (mixed species plots) were compared. Specifically, we analyzed the influence of tree species richness, functional type (conifer vs. broadleaved) and phylogenetic diversity on overall fungal disease incidence.

Contrary to expectation, we found that the effect of tree species richness on disease incidence varied with latitude and functional type. Disease incidence tended to 1) increase with tree diversity, in particular in northern latitudes, and 2) decrease with tree species richness in conifers, but not in broadleaved trees. However, for specific damage symptoms, no tree species richness effects were observed. Although the patterns were weak, susceptibility of forests to disease appears to depend on the forest site and tree type.

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