Time course of δ13C in poplar wood: genotype ranking remains stable over the life cycle in plantations despite some differences between cellulose and bulk wood

Genetic differences in delta C-13 (isotopic composition of dry matter carbon) have been evidenced among poplar genotypes at juvenile stages. To check whether such differences were maintained with age in trees growing in plantations, we investigated the time course of delta C-13 as recorded in annual tree rings from different genotypes growing at three sites in southwestern France and felled at similar to 15-17 years. Wood cores were cut from tree discs to record the time course of annual basal area increment (BAI). The isotopic ratio delta C-13 was recorded in bulk wood and in extracted cellulose from the annual rings corresponding to the period 1996-2005. Discrimination against C-13 between atmosphere and tissues (delta C-13) was computed by taking into account the inter-annual time course of delta C-13 in the atmosphere. Annual BAI increased steadily and stabilized at about 8 years. An offset in delta C-13 of similar to 1 parts per thousand was recorded between extracted cellulose and bulk wood. It was relatively stable among genotypes within sites but varied among sites and increased slightly with age. Site effects as well as genotype differences were detected in delta C-13 recorded from the cellulose fraction. Absolute values as well as the genotype ranking of delta C-13 remained stable with age in the three sites. Genotype means of delta C-13 were not correlated to annual BAI. We conclude that genotypic differences of delta C-13 occur in older poplar trees in plantations, and that the differences as well as the genotype ranking remain stable while trees age until harvest.