Contrasting Genomic Signatures of Climate Adaptation and Adaptive Plasticity Across the Distribution Ranges of Sessile Oak and European Beech

The persistence of organisms in changing climates depends on both phenotypic plasticity and adaptation. However, despite extensive research, it remains largely unclear how forest trees will genetically adapt or phenotypically acclimate to future climates. In this study, we investigated the genetic architecture of tree-ring and leaf traits in sessile oak (Quercus petraea) and European beech (Fagus sylvatica) and tested for signals of climate adaptation at loci controlling these traits. We combined trait data from common gardens established in the 1990s across the species' European ranges with climatic and genomic data. We conducted genome-wide association (GWA) analyses and tested the effects of temperature and precipitation transfer distances on associated loci. Our GWA analyses revealed a complex, multi-locus genetic architecture underlying functional traits in both species. Sessile oak displayed a greater number of trait associations than European beech and we found indications of pleiotropy, particularly in sessile oak. European beech showed limited plastic and adaptive responses to climate at the genomic level. In contrast, sessile oak showed signals of both adaptive plasticity and climate adaptation, particularly at loci associated with growth responses to extreme droughts and to long-term summer temperature. Our findings suggest that sessile oak possesses a stronger capacity to respond to climate change than European beech. This capacity may facilitate range expansion in sessile oak, while raising concerns about the sustainability of beech in European lowland forests under future climates.