Variation in branchlet δ¹³C in relation to branchlet nitrogen concentration and growth in 8-year-old hoop pine families (Araucaria cunninghamii) in subtropical Australia

Carbon isotope composition (δ¹³C) of branchlet tissue at nine canopy positions, and nitrogen concentration (N(mass)) at four canopy positions, were assessed in 8-year-old hoop pine (Araucaria cunninghamii Ait. ex D. Don) trees from 23 half-sib families, grown in six blocks of a progeny test in southeastern Queensland, Australia. There was considerable variation among sampling positions, families and blocks in both δ¹³C and N(mass). The δ¹³C was positively related to N(mass) only for samples from the upper outer crown (P < 0.005). Phenotypic correlations existed between tree growth and canopy δ¹³C. Branchlet δ¹³C of the inner and lower outer crown was positively related (P < 0.037) to tree height, but δ¹³C in branchlets of the upper outer crown was not related to tree height, or was related negatively (P < 0.045). There were significant differences in δ¹³C between hoop pine families for six canopy positions (upper canopy positions as well as lower canopy positions on the northern side), with heritabilities greater than 0.40. The significance of these findings is discussed in relation to water and light competition within the tree canopy of hoop pine.