Contrasting habitats shape the root mycobiome of pedunculate oak seedlings

Understanding the role of fungi in tree regeneration is crucial. It is known that fungal communities can influence seedling establishment, health, and consequently survival. This study analysed fungal communities associated with oak (\textit{Quercus robur} L.) seedling roots in three habitat types: forests (hereinafter ``Forest''), tree groves (hereinafter ``Grove''), and treeless or sparsely wooded herbaceous communities (hereinafter ``Herb''). The study aimed to understand how the diversity and functional structure of fungal communities vary across habitat types and how these changes may impact oak regeneration success. The analysis included mycorrhizal, saprotrophic, endophytic, and pathogenic fungi, which play a key role in young plant development. A total of 11\,739\,000 operational taxonomic units (OTUs) and 1 \, 050 distinct taxa were identified. The results showed the highest species diversity in the Herb communities, while the lowest was in the Forest samples. Furthermore, the structure of the fungal communities was site-specific — Forest was characterised by a higher proportion of mycorrhizal fungi, whereas pathogens, endophytes, and mycoparasites dominated in Grove and Herb. Moreover, beta-diversity and similarity analyses showed that differences in the composition of seedling root fungal communities between sites were strongly influenced by microenvironmental conditions. These conditions appeared to shape trophic structure by promoting the dominance of mycorrhizal fungi in more stable, shaded forest habitats, while favouring saprotrophic, pathogenic, and mycoparasitic taxa in open habitats with higher organic matter turnover and greater environmental variability