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

Abstract

AbstractCarbon isotope composition of tree‐ring (δ13CRing) is a commonly used proxy for environmental change and ecophysiology. δ13CRing reconstructions are based on a solid knowledge of isotope fractionations during formation of primary photosynthates (δ13CP), such as sucrose. However, δ13CRing is not merely a record of δ13CP. Isotope fractionation processes, which are not yet fully understood, modify δ13CP during sucrose transport. We traced, how the environmental intra‐seasonal δ13CP signal changes from leaves to phloem, tree‐ring and roots, for 7 year old Pinus sylvestris, using δ13C analysis of individual carbohydrates, δ13CRing laser ablation, leaf gas exchange and enzyme activity measurements. The intra‐seasonal δ13CP dynamics was clearly reflected by δ13CRing, suggesting negligible impact of reserve use on δ13CRing. However, δ13CP became increasingly 13C‐enriched during down‐stem transport, probably due to post‐photosynthetic fractionations such as sink organ catabolism. In contrast, δ13C of water‐soluble carbohydrates, analysed for the same extracts, did not reflect the same isotope dynamics and fractionations as δ13CP, but recorded intra‐seasonal δ13CP variability. The impact of environmental signals on δ13CRing, 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 δ13CRing.