From pathogens to partners: temporal and biogeographical patterns in fungal associations of alien trees

Alien trees reshape belowground fungal communities, but the factors governing the balance between mutualists and pathogens remain unclear. We tested whether residence time, mycorrhizal type, and biogeographical origin shape this balance, and whether alien stands differ from native vegetation. We sampled soils beneath 73 alien tree species in 48 chateau parks and native stands. Using ITS2 metabarcoding with guild assignment, we quantified ectomycorrhizal (ECM) and pathogen fungi and analysed predictors with multivariate models and binomial GLMMs, accounting for spatial structure and covariates. Symbiotic fungal community composition varied with origin, phylogenetic group and mycorrhizal type. With increasing residence time, ECM alien trees showed higher ECM fungal richness and relative abundance; whereas, contrary to enemy accumulation expectations, pathogen richness and relative abundance declined. Alien arbuscular mycorrhizal (AM) trees harboured more pathogens than ECM trees. Alien tree assemblages had a lower ECM fungal share, twice the pathogen relative richness, and threefold higher pathogen relative abundance than native assemblages. Residence time and mycorrhizal type are primary filters shaping belowground trajectories of alien trees, with biogeographical origin patterning community composition. Elevated pathogen loads in alien stands highlight spillover risks to neighbouring vegetation, informing risk assessment and monitoring of alien tree plantings.