Developing approaches to test for the genetic selection of young beech trees (Fagus sylvatica) subject to increasingly extreme environments

Extreme weather events, such as the 2018 drought in Central Europe, present significant challenges to forest ecosystems and may drive genetic adaptation in tree populations. Preserving genetic diversity is essential for forest resilience under climate change. This study investigates whether young European beech  trees (Fagus sylvatica) germinating after the 2018 drought show genetic differences compared to adult cohorts, potentially reflecting adaptation to drought conditions. We sampled 400 individuals (20 seedlings and their presumed mother trees per site) across ten locations in the Basel (Riehen, Birsfelden, Rünenberg, Hölstein, Bonfol JU) and Zurich (Rafz, Hinwil, Zugerberg, Wangen SZ, Wangen) regions, representing a drought gradient (from dry to moist). Bud tissue was collected in February 2024, and whole-genome sequencing was performed on extracted DNA, yielding 2,476,201 SNPs across 362 samples.

Population genetic analyses revealed that genetic differentiation was primarily driven by site (population) rather than life stage (juvenile or adult trees). We tested whether the site, stage or the interaction could explain the genetic structure using multivariate ANOVA. While the site was clearly significant, there was no significant overall genetic differentiation by stage or the interaction of site and stage.

Genetic variation within sub-populations differed significantly among sites (P = 0.012) but was largely maintained across generations, with only one site (Birsfelden) showing higher variation in juveniles. Variation correlated negatively with foliar potassium and long-term precipitation in juveniles, suggesting potential environmental influences.

Genome-wide association studies identified SNPs linked to phenotypes such as juvenile height, adult growth rate, and crown thinning, mapping to 35–148 genes. However, functional enrichment was limited, and it remains unknown by which mechanisms these genes might play a role.

Overall, our findings indicate that genetic differentiation among beech populations is primarily driven by geography rather than recent drought events, and that genetic variation within sites has remained stable across generations. While we observed site-specific differences and phenotype-associated loci, evidence for rapid genetic shifts following drought was weak.