EDNA Study of the soil fungal populations associated with C. robustum stands on the island of St. Helena

The island of St Helena is small and over 1900 km from the nearest continental landmass. A unique flora, including five endemic genera of Asteraceae, has developed on the island. Plant communities have changed radically since the island's discovery by Europeans ca. 1550, with deforestation and introduction of continental species leading to extinctions and large reductions in the range and abundance of endemic species. One of these species, Commidendrum robustum, an arborescent Asteraceae, persists in small relict populations where coverage has been continual, and is now the subject of a restoration programme. Here we analysed the fungal community, via eDNA metabarcoding (ITS2 and LSU loci), to test if the relict populations of C. robustum harbour an indigenous fungal community and whether communities under plantings of different ages resemble those on relict sites. In addition plant community was analysed using eDNA metabarcoding (ITS2 locus). Fungal community composition showed significant differences in C. robustum stands of different ages (PERMANOVA analysis). Fungal communities associated with C. robustum soils were unusual in having very low abundances of Basidiomycota and being dominated by Capnodiales, an order of Ascomycota more commonly inhabiting leaf surfaces. SIMPER analysis revealed the differences between sites to be mainly due to variations in species of Capnodiales present. Use of plant community data as a predictor of fungal community highlighted the importance of ground flora and soil factors as drivers, with the relative abundance of the introduced grass species Ehrharta erecta, organic matter and pH significantly associated with fungal community structure. Arbuscular mycorrhizal fungi (AMF) were present at low levels but we found no link between C. robustum sequence abundance and AMF. However, AMF abundance was correlated with abundance of Poales hosts; Capnodiales abundance linked to soil organic content. Findings raise the possibility that C. robustum may not be mycorrhizal.