Effects of tree species diversity on foliar fungal distribution

Abstract: European forest ecosystems span many different ecological zones and are rich in tree species. The environment in which the trees grow similarly affects fungal communities that interact with these trees. Fungal pathogens can cause severe damage to trees and potentially impair forest stability. In particular, pathogens that damage the foliage will affect the tree’s photosynthetic ability, partly or completely. At the same time, pathogens can create niches for different plants by removing dominant species. The foliage community also comprises fungal species whose ecological functions are not entirely known and may either positively or negatively impact the tree’s health status. The aim of this thesis was to understand the effect of tree species diversity in mitigating fungal pathogen damage and in affecting the fungal community distribution. To achieve this, visual assessment of leaves for pathogen damages was carried out on 16 different tree species from six European forests. The fungal communities of Norway spruce needles from four European forests were studied by using next generation sequencing technology, and fungal communities of birch leaves by sequencing and morphological assessment. In this thesis, foliar fungal pathogen damages were positively correlated with tree species richness – latitude interaction, suggesting that tree species diversity may regulate pathogens but was dependent on the forest. Additionally, foliar fungal community composition was found to differ significantly in different forests, which may be attributable to local environmental effects or reflect the evolutionary history of the host tree, and thereby this study contributes to the understanding of biogeographic patterns of microorganisms. Finally, methods used to study fungal communities revealed that the sequencing-based method provided a richer picture of the fungal community than morphological assessment of fungal structures and symptoms, though neither method informed the distribution patterns as it relates to tree species diversity. Overall, impact of tree species diversity on foliar fungal distribution may not be strong, but it invites us to consider other factors that interact with fungal communities and how fungi may in turn shape their environment.

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