Adaptive variation in drought-related traits across southern and central European white oak (Quercus sect. Quercus) populations

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.