Recently introduced Eucalyptus nitens shows lower root colonization, reduced ectomycorrhizal diversity, and less reliance on Australian symbionts than long-established E. globulus

Mycorrhizal symbioses play a critical role in the establishment and spread of introduced plant species, shaped by plant–fungus specificity, mutual dependence, and the availability of compatible fungi. This study investigates ectomycorrhizal (ECM) fungal communities associated with two exotic tree species, Eucalyptus nitens and E. globulus , in plantations and native oak forests in NW Spain. We assessed root colonization, fungal richness, and community composition using rDNA sequencing of mycorrhizal root tips. Given the more recent introduction of E. nitens , we hypothesized it would support less diverse ECM communities than E. globulus , or alternatively, that habitat would be the main driver of community structure. Both host identity and habitat significantly influenced ECM communities. Eucalyptus globulus showed higher colonization rates and fungal richness than E. nitens , especially in native oak forests. Fungal composition differed between host species in oak forests but was more similar in plantations. Most ECM taxa represented novel interactions, particularly in native forests. Co-introduced Australian fungi were a minority but played a significant role; notably, Descolea maculata was prevalent across all treatments except for E. nitens in oak forests, where its absence may represent a colonization barrier. These findings suggest that recently introduced trees can still host relatively rich ectomycorrhizal communities, establishing numerous and significant novel interactions with fungi absent from their native range. Such associations may facilitate the long-term spread of E. nitens outside plantations. The results highlight the complex interplay between host species, habitat type, and fungal partners in shaping ECM communities. • Host species and habitat shaped ectomycorrhizal fungal community structure. • E. globulus showed higher colonization and richness than E. nitens . • Most ECM fungi formed novel interactions, especially in native oak forests. • Some Australian fungi showed strong dominance, suggesting host preference. • Australian D. maculata was absent in E. nitens in forests, possibly limiting spread.