Drivers of intra-seasonal δ13C signal in tree-rings of Pinus sylvestris as indicated by compound-specific and laser ablation isotope analysis

Carbon isotope composition of tree-ring (delta C-13(Ring)) is a commonly used proxy for environmental change and ecophysiology. delta C-13(Ring) reconstructions are based on a solid knowledge of isotope fractionations during formation of primary photosynthates (delta C-13(P)), such as sucrose. However, delta C-13(Ring) is not merely a record of delta C-13(P). Isotope fractionation processes, which are not yet fully understood, modify delta C-13(P) during sucrose transport. We traced, how the environmental intra-seasonal delta C-13(P) signal changes from leaves to phloem, tree-ring and roots, for 7 year old Pinus sylvestris, using delta C-13 analysis of individual carbohydrates, delta C-13(Ring) laser ablation, leaf gas exchange and enzyme activity measurements. The intra-seasonal delta C-13(P) dynamics was clearly reflected by delta C-13(Ring), suggesting negligible impact of reserve use on delta C-13(Ring). However, delta C-13(P) became increasingly C-13-enriched during down-stem transport, probably due to post-photosynthetic fractionations such as sink organ catabolism. In contrast, delta C-13 of water-soluble carbohydrates, analysed for the same extracts, did not reflect the same isotope dynamics and fractionations as delta C-13(P), but recorded intra-seasonal delta C-13(P) variability. The impact of environmental signals on delta C-13(Ring), and the 0.5 and 1.7 parts per thousand depletion in photosynthates compared ring organic matter and tree-ring cellulose, respectively, are useful pieces of information for studies exploiting delta C-13(Ring).