Fungal symbionts associate with holm oak tree health in declining oak savannas of the southwest of the Iberian Peninsula

Despite being well adapted to the long seasonal droughts of the Mediterranean Basin, the holm oak (Quercus ilex L. subsp. ballota (Desf.) Samp.) has undergone a gradual decline during the last decades. The reasons of this decline are not fully understood, but evidence suggests that the quality and quantity of the interactions of holm oaks with the soil microbiota are playing a central role in the resilience of this species. With the aim to test the previous statement as our hypothesis, 9 oak savannah sites (called dehesas from here on) were sampled, in which asymptomatic (healthy) and symptomatic (early stage of decline) trees were interspersed. Using high throughput amplicon sequencing, soil bacterial and fungal biota (called soil microbiota from here on) were sequenced. Our results showed that soil fungal community composition differed between healthy and declining trees whereas bacterial communities did not show significant differences. Specifically, the degree of holm-oak defoliation was negatively correlated to the relative abundance of ectomycorrhizal (ECM) fungi and fungal endophytes, suggesting a relation between trees' health and soil fungal symbionts. Although our observational study cannot give a final answer to the directionality of the relationships observed, our results support the idea that small changes in symbiont abundance might be causing holm oaks to lose their ability to withstand the strong environmental pressures.