Fungi and tree growth facilitation in European forests under drought conditions.

Droughts in forest ecosystems are a central concern for current and future biodiversity loss, carbon sequestration, and ecosystem functioning. Trees rely on symbiotic relationships with fungi to enhance nutrient uptake and improve stress tolerance, but the impacts of drought on plant-fungal relationships remain unclear and vary across different tree species compositions. This study investigated how inter- and intraspecific interactions among three prominent tree species in European forests—Fagus sylvatica (European beech), Quercus petraea (Sessile oak), and Tilia cordata (small-leaved lime)—shift under simulated drought conditions in relation to their rhizosphere fungal communities. We hypothesized that drought would shift the diversity and functional capacity of fungal communities, with these effects being dependent on the tree species and competitive context. To test this, we set up raised-bed experiments with seedlings of the three species as mono- or polycultures, exposing them to ambient rainfall conditions or two years of reduced precipitation using plastic roofing. We assessed tree seedling growth and development, and at the end of the experiment, we sampled rhizosphere soils from individual trees to characterize fungal diversity using full-length ITS DNA metabarcoding on an Oxford Nanopore Technology PromethION platform. Intraspecific versus interspecific competition provided more favourable conditions for tree growth under drought conditions. Our results show that fungal communities were responsive to variations in plant species, competitive context, and drought, and that fungal biodiversity explained unique patterns in plant growth responses to drought and competition, particularly for plant-symbiotic ectomycorrhizal fungi. This study highlights the variable effects of drought on fungal communities and underscores the importance of species-specific interactions in forest ecosystem responses to climate stress. These findings contribute to our understanding of the ecological role of fungi in forest species' resilience to climate change and may inform future forest management strategies aimed at mitigating the effects of drought in temperate regions.