Genetic and plastic effects on trait variability in two major tree species: Insights from common garden experiments across Europe

Phenotypic plasticity and genetic adaptation are key mechanisms that enable species to respond to changing environments. Tree traits do not vary independently but rather in coordination. However, our understanding of whether functional traits are governed by the same mechanism is still uncomplete. Thus, we aim at assessing the drivers of trait variability of sessile oak and European beech provenances across their distribution ranges. We estimated growth-related and leaf morphological traits from 9 and 11 provenances of oak (Quercus petraea) and beech (Fagus sylvatica), respectively, grown in four different common gardens distributed across their respective distribution areas. Overall, phenotypic plasticity played a dominant role in explaining individual trait variability. For most oak traits, variation among provenances and genetically based plasticity were correlated with the climate of origin, whereas for beech both provenance-related variation and plasticity showed fewer significant associations with the climate of origin. In oak, climate-transfer distance analyses revealed that some trait measures decreased when provenances were moved away from their local precipitation regime. In beech, significant climate-transfer distances were fewer and primarily related to temperature-related parameters. The pattern of multi-trait phenotypes indicates that resource-use strategies among provenances covary with the temperatures of origin in both species. Although beech shows adaptive potential through genetic differentiation among populations, most trait variation is plastic, which may not suffice long term to cope with extreme climatic events. Oak, by contrast, appears more responsive through adaptive mechanisms. Our study enhances understanding of the interplay between genetic adaptation and phenotypic plasticity in long-lived forest trees.