Rene Graf

European white oaks grow from the Mediterranean coast to Southern Scandinavia, a huge environmental gradient that has likely fostered environmental adaptation. In the face of climate change, leveraging adaptations through assisted gene flow could help improve drought tolerance and maintain forest health, but requires an understanding of the species-specific patterns of adaptation to be successful at the target location. In this study, three common gardens were established in Switzerland, Tuerkiye, and Austria for two European white oak species (Quercus robur, and Q. pubescens) using provenances from Central and Southern Europe. Almost 900 oak seedlings were measured at key water-use efficiency and life history traits for their first two year of life and genotyped with low coverage whole-genome sequencing. Trait heritability and environmental adaptation were then explored through pedigree-free animal models, while the genomic architecture of traits was mapped using a genome wide association study ("GWAS"). Across the species, the heritability of measured traits was moderate to high, but common garden had a strong impact, signalling an environmental effect on the phenotype. Adaptation to precipitation seasonality was detected in key productivity and growth traits for both species, but had a small effect on absolute trait values. The GWAS identified a striking 150 kbp association in the Cyclic Nucleotide-Gated Ion Channel gene family with leaf d13C values. This gene family is involved in stomata opening and likely impacts the intrinsic water use efficiency under stress. Together, the strong signals of phenotypic plasticity and rather weak signals of climatic adaptation in seedlings suggest that assisted gene flow in these two white oaks is relevant only for highly drought-sensitive populations, if conducted managers should focus on seeds sources with high precipitation seasonality and smaller leaf sizes.

Pedunculate oak ( Quercus robur L.), a long-lived forest tree species, forms symbiotic relationships with ectomycorrhizal (ECM) fungi, which can promote nutrient uptake, stress resilience, and growth. Like other tropical and temperate tree species, pedunculate oak exhibits endogenous rhythmic growth (ERG), a trait conferring the ability to repeatedly alternate root and shoot flushes as well as growth cessation as response to changing environmental conditions. However, the effects of different ECM fungal species on the ERG dynamics remain largely unknown. Here, we investigated the impact of two ECM fungi— Piloderma croceum , a basidiomycete previously shown to promote growth while not found in natural oak stands, and Cenococcum geophilum , an oak-native ascomycete with broad ecological range—on growth performance, biomass partitioning, and ERG patterns in a clonal oak system (clone DF159). By combining in vitro experiments with Bayesian modelling, we show that P. croceum promotes tree growth among treatments, without disrupting the endogenous growth rhythm. In contrast, C. geophilum , while showing high mycorrhization rates, led to reduced biomass accumulation and altered developmental progression through the ERG stages, especially by prolonging the steady state development stage—part of the root flush and characterized by peak net carbon assimilation. Co-inoculation revealed a competitive advantage of C. geophilum in root colonization, yet growth responses resembled those of the control. Our findings demonstrate that ECM species exert species-specific effects on biomass production and temporal development of plants, underscoring the functional importance of ECM fungi in shaping host development. Assessing these interactions provides new insights into the functional diversity of ectomycorrhizal symbiosis and can inform forest management strategies aimed at enhanced resilience in oak-dominated ecosystems under rapidly changing climatic conditions.

Background and aims: In forest ecosystems, trees interact with a broad range of soil microorganisms, such as ectomycorrhizal fungi, improving nutrition and water uptake and mitigating biotic and abiotic stress. In the context of the predicted more frequent and more severe droughts, it is critical to characterise how trees and their associated fungal partners respond to water shortage to improve future forest management. Methods: We investigated the importance of diverse environmental predictors (e.g., edaphic, climatic, topographic) on the fungal communities associated with the root systems of decaying and healthy beech trees in natural beech forests. In parallel, we identified specific fungal taxa linked with their host vitality and water stress gradient. Results: We observed that soil water shortage had a greater effect on the structuring of fungal communities than beech vitality or other environmental factors. We identified a core group of fungi that remained unaffected by water availability among all study sites, while 17 fungal species (i.e. Lactarius blennius ) were more abundant in sampling sites more prone to water shortage. Finally, we showed that the higher abundance of ectomycorrhizal fungi was significantly associated with healthy beech root systems, while three saprotrophic fungi (i.e. Pezicula radicicola ) prevailed in the roots of trees exhibiting severe decay. Conclusions: Overall, our study highlights the major role of drought in structuring fungal communities associated with beech root systems and pinpoints the key fungal taxonomic groups found in healthy beech trees.

Background and aims In forest ecosystems, trees interact with a broad range of soil microorganisms, such as ectomycorrhizal fungi, improving nutrition and water uptake and mitigating biotic and abiotic stress. In the context of the predicted more frequent and more severe droughts, it is critical to characterise how trees and their associated fungal partners respond to water shortage to improve future forest management. Methods We investigated the importance of diverse environmental predictors (e.g., edaphic, climatic, topographic) on the fungal communities associated with the root systems of decaying and healthy beech trees in natural beech forests. In parallel, we identified specific fungal taxa linked with their host vitality and water stress gradient. Results We observed that soil water shortage had a greater effect on the structuring of fungal communities than beech vitality or other environmental factors. We identified a core group of fungi that remained unaffected by water availability among all study sites, while other fungal species were more abundant in sampling sites more prone to water shortage. Finally, we showed that the abundance of ectomycorrhizal fungi was significantly associated with healthy beech root systems, while saprotrophic fungi prevailed in the roots of trees exhibiting severe decay. Conclusions Overall, our study highlights the major role of drought in structuring fungal communities associated with beech root systems and pinpoints the key fungal taxonomic groups found in healthy beech trees.