Regional species turnover and local host and forest stand characteristics jointly structure early foliar mycobiomes of Scots pine seedlings.

The foliar mycobiome plays an important role in tree health and stress resilience, yet factors shaping foliar fungal communities in boreal conifer seedlings remain poorly understood. We investigated how geographic region, seedling origin (planted vs. naturally regenerated), needle nitrogen status, and local stand structure influence needle-associated fungal communities of Scots pine (Pinus sylvestris) seedlings. Needle samples were collected from seedlings without visible disease symptoms across 23 forest stands in southern Finland. Fungal communities were characterized using Illumina ITS2 amplicon sequencing and clustered into operational taxonomical units (OTUs). Alpha diversity was analyzed via linear mixed models (LMMs), and community composition via PERMANOVA and NMDS using Bray-Curtis dissimilarity. Geographic region was the strongest driver of fungal community composition among stands, whereas within stands, community composition was primarily associated with seedling height and birch abundance. Alpha diversity was higher in the Southeastern region compared to the Southwest, and tended to be lower in planted than naturally regenerated seedlings. Moreover, diversity decreased with higher proportions of birch and spruce but increased with higher needle C: N ratios. These findings suggest that early foliar mycobiomes reflect regional conditions and local forest structure, highlighting potential forest management impacts on fungal communities.