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

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