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Carbon isotope composition (delta C-13) 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 delta C-13 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.

delta C-13 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 delta C-13 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 (delta C-13) in both, mesic and xeric trial sites.

Aridity and continentality enhanced population differences in delta C-13, 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 delta C-13 and h between families allows the possibility of a selection for growth and water use efficiency within populations in sub-humid sites.