Growth Performance Is Driven by Site Conditions and Moderated by Functional Trait Plasticity in Quercus robur and Prunus avium

ABSTRACT 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 Quercus robur L. and Prunus avium L. 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, P. avium showed modest provenance‐level variation in both traits ( R 2 ~ 7%), while in Q. robur only leaf‐out varied ( R 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 P. avium , however, leaf‐out timing was strongly climate‐sensitive, consistent with its role as a short‐lived, pioneer species, whereas Q. robur (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 ( R 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.