Black poplar (Populus nigra L.) is a major tree species of riparian ecosystems in Europe that covers a wide range of pedoclimatic conditions and river dynamics, which may have promoted the development of adaptive strategies in natural populations. This species is of economic importance as a parental pool used in many breeding programs for the development of P. deltoides × P. nigra hybrids cultivated for biomass and wood production. The present study aimed at deciphering the components of phenotypic variation for a set of morphological, structural and functional leaf traits related to growth and water-use efficiency. Twenty-four to 62 P. nigra individuals sampled in nine European natural metapopulations were established in replicated clonal tests in two experimental sites contrasted for soil texture and fertility. The relative importance of site, genetic and Genotype × Site (G × S) interaction effects on total phenotypic variation of leaf traits was estimated. In the most favourable site for growth, individual area of mature and fully illuminated leaves increased by an average of 150 % while bulk leaf carbon isotope discrimination (Δ13C) decreased by an average of 2.5 ‰. In the two sites, substantial genetic variation was expressed within the nine P. nigra metapopulations for leaf traits. As indicated by G × S interaction variance comparable or even greater than genetic effects, the expression of genetic variation was significantly modulated by the site for all metapopulations. For individual leaf area, G × S interactions were explained by both changes in genotype ranking and increased genetic variation in the most favourable site while these interactions were mainly attributed to changes of genotype ranking across sites for Δ13C. Despite high intra-metapopulation genetic variation, the genetic differentiation among the nine metapopulations was high (0.42 ≤ QST ≤ 0.51) for all traits related to leaf morphology, such as individual leaf area, petiole length and leaf shape, and its pattern was conserved in the two experimental sites. In contrast, Δ13C exhibited a moderate level of metapopulation differentiation (QST ≈ 0.25) with significant changes of metapopulation clustering between the two sites. These results will be discussed considering the adaptive potential of black poplar to variations in environmental conditions.