Responses of root-associated fungal communities of mature beech and spruce during five years of experimental drought

Drought affects the fine-root systems of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] KARST) in different ways, but little is known about how this impacts their fine-root-associated fungal communities. In a five-year throughfall exclusion experiment (KROOF) in a mature stand, we investigated whether recurrent drought periods progressively alter fine-root associated fungal communities, fine-root vitality, and ectomycorrhizal functionality in relation to the tree root zone (pure beech, pure spruce, or their mixture) and abiotic soil parameters. We found that the influence of recurrent droughts on root fungal communities peaked in the third year of the experiment and affected fungal functional groups in different ways. The root zone was the predominant factor in structuring all functional groups of root-associated fungi, while we did not find a prominent effect of root mixture. The importance of other factors (year of sampling, soil depth) varied among fungal functional groups. Our results indicate a robust biotrophic root-fungal system relying mainly on surviving root tips, complemented by a fluctuating saprotrophic fungal assembly.