Drought and simulated pathogen attack shape ectomycorrhizal communities and soil nutrient dynamics in semi-mature sessile oak

Symbiotic ectomycorrhizal fungi (EMF) are central to oak tree nutrition and stress tolerance, contributing to nutrient acquisition in exchange for host-derived photosynthates. Trees increasingly experience interacting abiotic and biotic stress events, such as drought and pest or pathogen attack, which may reduce C reserves, disrupt physiological functioning, and contribute to a progressive decline in tree vitality and resilience. As part of novel field manipulation experiment on semi-mature oak ( Quercus petraea (Matt.) Liebl.), we studied the effects of severe drought, simulated by installing rain exclusion shelters, and pathogen attack, simulated by partial ringbarking, on soil fungal communities. Altered community composition with increased relative presence of saprotrophic fungi compared with untreated controls was seen in both treatments but EMF communities were more strongly affected by 20 months of simulated drought than by partial ringbarking. The reduced number of EMF species under drought were likely associated with a combination of low soil moisture and drought-induced changes in soil nutrient dynamics, including increased inorganic nitrogen availability. Partial ringbarking did not affect overall EMF community richness compared to the control, but a specific nutrient stress indicator species, Cortinarius sp., was detected. These patterns suggest that reduced belowground carbon allocation and impaired root function under ringbarking may have altered host nutrient demand. This may have favoured the maintenance of nutrient-acquisitive ectomycorrhizal symbionts despite their relatively high carbon requirements. Understanding how EMF communities respond to prolonged environmental stress will be increasingly important for predicting woodland resilience and the progression of oak decline under climate change.