Arbuscular mycorrhiza understory alters AM fungal community composition with little effect on other soil fungi in an ectomycorrhizal-dominated forest

Temperate forests in Central Europe are largely dominated by ectomycorrhizal (ECM) trees, with arbuscular mycorrhizal (AM) species currently representing a lesser proportion. Climate-driven forest dieback has prompted management strategies to diversify species composition and foster structural complexity to enhance ecosystem resilience. In Brandenburg, eastern Germany, forest transformation efforts primarily promote native ECM-associated trees to increase structural and compositional diversity, while concurrently the AM tree species Prunus serotina has expanded in the understory through natural regeneration. Rising temperatures may increase the prevalence of AM symbiotic systems, yet the consequences for belowground fungal communities in ECM-dominated forests, particularly for symbiotic fungi, remain poorly understood.We asked whether AM-dominated understory reorganizes communities of AM fungi in ECM-dominated forests, and to what extent forest structure and microclimate explain variation in biomass and community composition of AM fungi relative to other fungal groups. We collected soil samples from 40 plots in a managed forest in Brandenburg, Germany, in Scots pine and mixed conifer–deciduous stands, each with or without AM-dominated understory. AM fungal and total fungal biomass were quantified using neutral and phospholipid fatty acid analysis. Fungal diversity and community composition were assessed by DNA metabarcoding targeting the ITS2 region for the total fungal community and an 18S rRNA region specific for AM fungi. Stand structural metrics derived from laser scanning and microclimatic variables were included as continuous explanatory factors.Preliminary results indicate that AM-dominated understory increased biomass and beta diversity of AM fungi across pine and mixed conifer-deciduous stands, while alpha and gamma diversity of AM fungi declined. That suggests dominance-driven community reorganization rather than a net increase in diversity. Forest structure and microclimate explained little variation. The total fungal community composition and biomass remained largely unaffected by the presence of AM-dominated understory. In these dry and nutrient-poor Podzol soils, a higher proportion of AM symbiotic systems may add complementary pathways of water and nutrient acquisition to those provided by ECM fungi. This functional diversification could contribute to forest resilience and may become increasingly important under a changing climate.