Pinus hartwegii is the only ectomycorrhizal host distributing at 4000 m asl in the Neotropics forming monospecific sky-island forests on the treeline. These ecosystems have unique environmental conditions like high solar radiation, dozens of soil freeze–thaw cycles per year, and high variability in daily temperatures; additionally their soils, as vitric andosols, are poor in nutrients and organic matter. In this extreme environment ectomycorrhizal fungi transform tree fine roots providing the host plant, through the plant-fungal symbiosis, with novel functions (enhanced nutrient acquisition, higher radiation and desiccation tolerance, etc.) improving tree establishment, growth, and survival. We studied the ectomycorrhizal community associated with P. hartwegii from three sites around 3900 m asl in the treeline of Cofre de Perote Volcano, Mexico. We dissected and characterized the mycorrhizal exploration type and morphological traits of mycorrhizae. Fungal identity and distribution were inferred by DNA sequence analysis of the ITS region. Soil conditions were determined by chemistry and macro and micronutrients contents. These fungal communities are characterized by low alpha diversity (less than ten species per site), high beta diversity (only two species shared between sites), and high dominance of Basidiomycota. Dominating genera Tricholoma, Piloderma, Cortinarius and Gautieria ectomycorrhizae belonged to the Medium-Distance exploration type, characterized by abundant external mycelia that often aggregates in mycelial cords and rhizomorphs. These forests constitute Holarctic sky-island refuges in the Neotropic where several potential endemic ectomycorrhizal fungi have evolved. Dominant ectomycorrhizal fungi are specialist soil foragers providing roots with new functions adapted to poor and harsh soil conditions.
