Tree organ growth and carbon allocation dynamics impact the magnitude and δ(13)C signal of stem and soil CO(2) fluxes

Incomplete knowledge of carbon (C) allocation dynamics in trees hinders accurate modeling and future predictions of tree growth. We studied C allocation dynamics in a mature Pinus sylvestris L. dominated forest with a novel analytical approach, allowing the first comparison of: (i) magnitude and δ(13)C of shoot, stem and soil CO(2) fluxes (A(shoot), R(stem) and R(soil)), (ii) concentration and δ(13)C of compound-specific and/or bulk non-structural carbohydrates (NSCs) in phloem and roots and (iii) growth of stem and fine roots. Results showed a significant effect of phloem NSC concentrations on tracheid growth, and both variables significantly impacted R(stem). Also, concentrations of root NSCs, especially starch, had a significant effect on fine root growth, although no effect of root NSC concentrations or root growth was detected on R(soil). Time series analysis between δ(13)C of A(shoot) and δ(13)C of R(stem) or δ(13)C of R(soil) revealed strengthened C allocation to stem or roots under high C demands. Furthermore, we detected a significant correlation between δ(13)C of R(stem) and δ(13)C of phloem sucrose and glucose, but not for starch or water-soluble carbohydrates. Our results indicate the need to include C allocation dynamics into tree growth models. We recommend using compound-specific concentration and δ(13)C analysis to reveal C allocation processes that may not be detected by the conventional approach that utilizes bulk organic matter.