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 (δ¹³C_Ring) is a commonly used proxy for environmental change and ecophysiology. δ¹³C_Ring reconstructions are based on a solid knowledge of isotope fractionations during formation of primary photosynthates (δ¹³C_P), such as sucrose. However, δ¹³C_Ring is not merely a record of δ¹³C_P. Isotope fractionation processes, which are not yet fully understood, modify δ¹³C_P during sucrose transport. We traced, how the environmental intra‐seasonal δ¹³C_P signal changes from leaves to phloem, tree‐ring and roots, for 7 year old Pinus sylvestris, using δ¹³C analysis of
individual carbohydrates, δ¹³C_Ring laser ablation, leaf gas exchange and enzyme activity measurements. The intra‐seasonal δ¹³C_P dynamics was clearly reflected by δ¹³C_Ring, suggesting negligible impact of reserve use on δ¹³C_Ring. However, δ¹³C_P 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 δ¹³C_P, but recorded intra‐seasonal δ¹³C_P variability. The impact of environmental signals on δ¹³C_Ring, 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 δ¹³C_Ring.