Assessing composition and drivers of phyllosphere microbiota of beech and Scots pine across European forests

Tree associated microbes are an important component of forest biodiversity and they are essential for forest ecosystem functioning. Most plant–microbe research has focused on the rhizosphere, while composition of microbial communities in the phyllosphere remains underexplored. By using 16S rRNA gene sequencing analyses, we investigated differences between beech and Scots pine phyllospheric microbiomes across an environmental gradient from Fennoscandia to the Mediterranean area, map their functional profiles, and elucidate drivers of phyllosphere microbiota assembly. We identified tree species and the associated foliar trait (specifically carbon:nitrogen ratio) as primary drivers of the bacterial communities. Moreover, we also found that temperature and nitrogen deposition played a crucial role in affecting microbial assembly for both tree species. Functions related to ureolysis and methanol oxidation were more represented in beech than Scots pine, whereas Scots pine phyllosphere were richer in microbes able to perform methanotrophy, nitrogen-fixation, nitrate reduction, and hydro-carbon degradation. This study contributes to advancing our understanding on the vast diversity of microbial communities hidden in tree canopies of two of the most common tree species in European forests, and on factors shaping it. Moreover, it highlights the need of broad-scale comparative studies (covering a wide range of foliar traits and environmental conditions) to elucidate how phyllosphere microbiota mediates forest ecosystem responses to global change.