Phenotypic plasticity in Pinus pinaster δ13C: environment modulates genetic variation

• Carbon isotope composition (δ13C) is a complex trait involved in acclimation, adaptive processes and related to water use efficiency (WUE) and/or productivity.• To estimate the genetic variation in δ13C and growth (h), their relationship, and the genotype by environment interaction effect in both variables, we analyzed three Pinus pinaster populations and six to ten families per population, in two experimental trial sites and in two consecutive years.• δ13C increased in the drier site (from −27.15 ± 0.18 to −24.53 ± 0.22) and was more affected by environment (62% of variance) than by genotype.• All populations and families exhibited a high phenotypic plasticity in δ13C and increased WUE in the xeric site.• As expected, significant height differences between sites, years, populations and families were displayed. Smaller trees were associated to higher water use efficiency (δ13C) in both, mesic and xeric trial sites.• Aridity and continentality enhanced population differences in δ13C, therefore, in afforestation programs of arid and continental zones, the selection at the population level would be more efficient than at the family level. In the mesic site, the presence of a high genetic variation in δ13C and h between families allows the possibility of a selection for growth and water use efficiency within populations in sub-humid sites.