Fungi are important ecological agents in forests that contribute to increase the resilience of the whole ecosystem against environmental challenges. Mediterranean forests rank among the habitats most threatened by climate change and the spread of pests and diseases, which ultimately lead them into a spiral of decline. As such, changes in the composition of soil and trees’ mycobiota might correlate with health status of the forest and has been scarcely addressed in Mediterranean tree species. In this work, rhizosphere and bark-wood samples from declining Spanish forests of Castanea sativa Mill. (chestnut), Quercus ilex L. (holm oak), Q. suber L. (cork oak) and Q. pyrenaica Willd. (Pyrenean oak) were compared. Fungal communities were characterised by means of ITS metabarcoding. Higher diversity in terms of richness was found in soil, with 674 genera belonging to 15 phyla in soil vs 420 genera and 6 phyla in trees. Fungal genera exclusive to declining forests’ soils and trees didn’t include pathogenic organisms, thus preventing the association of certain genera with forest decline. Alpha diversity didn’t correlate with health status or sample type either, as it only increased in soils of asymptomatic chestnuts and not in any of the other analysed tree species. Some differentially abundant genera found in asymptomatic trees, such as Metarhizium, Aspergillus, Russula, Chaetomium, Mortierella or Cladophialophora, may be related to the biological control of decline-contributing pathogens. Finally, no relationship was found between health status and the primary lifestyles of fungi in soil and bark, which can be interpreted as a sign of resilience against adversities following cross-talk between soil and plant fungal communities. Keywords: Global change; Forest declines; Metabarcoding; Biodiversity; Forest pathology; Quercus Castanea
