Underground allies: Mycorrhizal flexibility facilitates orchid occurrence in post-mining landscapes.

Orchids are generally regarded as ecological specialists because their dependence on obligate mycorrhizal symbioses and pollinators is thought to constrain them to relatively undisturbed habitats. However, the occurrence of some species in highly degraded environments challenges this assumption. Epipactis helleborine, a widespread Eurasian orchid, successfully colonizes heavy metal contaminated mining sites in Italy, where most vegetation fails to establish. We investigated whether the occurrence of E. helleborine in these extreme habitats is facilitated by shifts in fungal associations and examined the genetic structure underlying this ecological novelty. Fungal communities and orchid genomic structure were analysed across paired mining and woodland sites in Sardinia and Tuscany using fungal amplicon sequencing and plant ddRAD sequencing, respectively. While total fungal community composition showed only weak habitat-related structuring, mycorrhizal communities exhibited moderate to strong differentiation between woodland and mining habitats. Woodland and mining populations shared several ectomycorrhizal associates, including Balsamia, while Tuber was markedly more abundant in woodlands and largely absent from mines, which instead harboured higher proportions of stress-tolerant and opportunistic taxa. Despite these ecological differences, genomic analyses revealed little genetic differentiation between habitats, supporting repeated colonization and ongoing gene flow among populations. Our findings add to growing evidence that mycorrhizal flexibility facilitates the establishment and persistence of plants in novel or anthropogenically altered environments. The ability of E. helleborine to associate with diverse mycorrhizal partners may have contributed to its successful colonization of post-mining habitats while maintaining genetic connectivity with nearby woodland populations.