Biotrophic root-fungal systems of beech and spruce acclimatised to five years of repeated experimental drought.

The Kranzberg Roof Experiment investigates the impact of five years of recurrent drought and subsequent recovery in a mature forest of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] KARST).

Within this framework, we studied fine-root-associated fungal communities, fine-root vitality, and ectomycorrhizal functionality in relation to mixed and monospecific tree root zones.

Changes in the fungal community peaked in the third year of drought but later stabilised, indicating a gradual acclimatisation to drought over time that was maintained during early recovery. Thereby, tree species was the dominant factor in structuring root-associated fungal functional groups, suggesting a strong relationship with tree-species-specific fine-root reactions to drought.

However, the trees’ root-fungal systems were functionally resilient, and the system's capabilities were mainly quantitatively affected due to the loss of surviving fine roots. 

This fits well with results that quantitative effects (e.g., fewer leaves – fewer fine roots) may have driven tree acclimation. Beyond that, it suggests that the surviving root-fungal systems (i.e., ectomycorrhizal root tips) functioned as moist islands within dried-out soil, kept alive by an interplay between tree-redistributed water and fungal symbionts. Elucidating this is one of the challenging topics for the final phase of the Kranzberg Roof Experiment, a terminal drought now beginning.