Large-diameter trees disproportionately contribute to soil fungal diversity in a coniferous forest with one of oldest living trees on Earth

Abstract Soil fungi play major roles in ecosystem functioning and plant resilience to environmental stresses. However, they are often overlooked in ecosystem restoration plans. In the Coastal Range of southern Chile, the endangered conifer Alerce ( Fitzroya cupressoides ) is the second longest-lived tree species on Earth and one of the largest in South America. These Alerce forests are currently facing major threats due to habitat destruction and climate change. Using ITS2 and SSU soil metabarcoding, we characterized soil and mycorrhizal fungal communities in an Alerce forest comprising a millennial tree (the ‘Alerce Abuelo’) estimated to be over 2400 years old. We also compared the performance of reference DNA sequence databases (i.e. MaarjAM, UNITE and EUKARYOME) for the taxonomic identification of mycorrhizal fungi in metabarcoding data. Beneath the Alerce Abuelo, soil fungal richness was 2.25 higher than the mean richness per sample, harboring 361 unique fungal OTUs. Likewise, arbuscular mycorrhizal (AM) fungal richness was 1.75 times higher than the mean richness per sample. Soil and AM fungal richness positively correlated with diameter at breast height (DBH) and tree biomass, and negatively with available phosphorus, which was the best predictor of fungal community composition. Finally, EUKARYOME detected more mycorrhizal taxa compared to MaarjAM for SSU (AM fungi) and UNITE for ITS2 (ectomycorrhizal fungi). We therefore recommend to carefully evaluate the approaches used for assigning mycorrhizal guilds in comparative studies. Our study illustrates the importance of millennial trees in sustaining fungal diversity in forest soils. By accumulating fungal diversity as they age, large-diameter Alerce trees can act as umbrella species for soil and mycorrhizal fungi, thereby protecting fungal communities for future forest restoration efforts.