Growth Performance Is Driven by Site Conditions and Moderated by Functional Trait Plasticity in <scp> <i>Quercus robur</i> </scp> and <scp> <i>Prunus avium</i> </scp>

Functional traits mediate plant-environment interactions, yet their plasticity and genetic variability remain poorly quantified in long-lived tree species. We examined provenance trial (common garden) data from one growing season of <i>Quercus robur L.</i> and <i>Prunus avium L.</i> across six sites spanning a ~2°C climate gradient to evaluate phenotypic plasticity and genetic differentiation in specific leaf area (SLA) and spring leaf-out, and their effects on growth. We applied mixed-effects models to separate provenance, family, and site effects, and to test trait-growth relationships under contrasting water availability and temperature conditions. Growth performance from planting to between 16 and 8 years of age, respectively, was primarily determined by site conditions (modeled as a trial random effect, which includes climate, soil, and other local factors), with SLA emerging as the most significant functional trait across both species. As for the functional traits themselves, <i>P. avium</i> showed modest provenance-level variation in both traits (<i>R</i> ² ~ 7%), while in <i>Q. robur</i> only leaf-out varied (<i>R</i> ² ~ 19%) across provenances and families, indicating in both cases the presence of genetic differentiation. Plasticity in SLA and leaf-out was detectable in response to climate variables but was generally small relative to inter-individual variability (plasticity explained < 10% of trait variance). In <i>P. avium</i>, however, leaf-out timing was strongly climate-sensitive, consistent with its role as a short-lived, pioneer species, whereas <i>Q. robur</i> (a long-lived, dominant species) showed weaker plastic responses but slightly higher genetic structuring of traits. For all traits studied, a large proportion of the observed trait differences cannot be explained by climate or genetics, indicating high levels of individual variability (<i>R</i> ² ~ 70%). This high inter-individual variability, together with the modest but significant plasticity observed, suggests that the studied species possess the genetic potential (aided by plasticity) needed for acclimation and adaptation to climate change.