The core taxa connectivity in leaf microbiomes in ash trees stands is modulated by host-related and environmental...

Forests cover large areas of the terrestrial land surface and play a key role in climate regulation and biodiversity conservation. Tree-associated microbial communities are integral components of forest ecosystems and contribute to plant functioning, stress tolerance, and interactions with the environment. However, the factors shaping the phyllosphere microbiome of forest trees across large environmental gradients remain less explored. Here, we investigated the bacterial leaf microbiome of Fraxinus excelsior across different climatic regions in Germany to identify its core microbiome and determine the relative importance of environmental and host-related factors shaping the phyllosphere microbiome of forest trees across large environmental gradients assembly. Therefore, 30 ash trees were sampled at each of the 12 sites across Germany, at which we assessed their leaf microbiome, leaf physio-chemical properties and a set of site parameters.

In total, 755 genera were detected. Only three genera Hymenobacter, Massilia and Sphingomonas were predominant across all sites. Geographical position, precipitation, tree age and the leaf and soil chemistry parameters were significant factors that explained 22.1% of variance between microbiome structures between the different sites. Seven genera were identified as the core leave microbiome of F. excelsior. The core microbiome assessed for each sampling site was mostly more diverse and differed between sites. Complex co-occurrence networks separated by sites with the identified six hub bacterial taxa proved that they were essential for the local core microbiomes. However, the network complexity differed significantly between the sampling sites.

Our findings clearly indicate the existence of an ash core microbiome on leaves that occurred at all sites but was locally differently interconnected to further microbiome members. The network analyses revealed that less frequent genera play important roles as hub taxa. Furthermore, we were also able to prove the modulating role of biotic and abiotic factors on the ash phyllosphere microbiome. An optimized core microbiome may be considered crucial for improving tolerance against plant diseases, such as ash dieback, in future.