Genetic variation in productivity, leaf traits and carbon isotope discrimination in hybrid poplars cultivated on contrasting sites

• We examined the relationships between productivity, leaf traits and carbon isotope discrimination in bulk leaf matter (Δ 1 ) and in phloem sap (Δ s ) from more than 5-year-old trees belonging to Populus deltoides × P. nigra and Populus trichocarpa × P. deltoides ; trees were grown in alluvial and non alluvial sites in a commercial poplar plantation. • On both sites, a large genetic variability was evidenced for all variables. The genotypic ranking remained stable between years for all variables, while it differed between sites. Δ 1 scaled positively with Δ s and neither Δ 1 nor Δ s were correlated with productivity. A significant genotype by site interaction was evident for all variables. The non alluvial site resulted in lower productivity, and in thicker/denser leaves with lower nitrogen and carbon contents. Noteworthy, the genotypic ranking for Δ 1 measured at the alluvial site was similar to that previously established in a glasshouse. • As observed in previous studies from younger trees, there is a potential to select genotypes, combining high productivity and high water-use efficiency, for growth in moderately drought-prone areas.