Resin acid δ¹³C and δ¹⁸O as indicators of intra-seasonal physiological and environmental variability

Understanding the dynamics of δ¹³C and δ¹⁸O in modern resin is crucial for interpreting (sub)fossilized resin records and resin production dynamics. We measured the δ¹³C and δ¹⁸O offsets between resin acids and their precursor molecules in the top-canopy twigs and breast-height stems of mature Pinus sylvestris trees. We also investigated the physiological and environmental signals imprinted in resin δ¹³C and δ¹⁸O at an intra-seasonal scale. Resin δ¹³C was c. 2‰ lower than sucrose δ¹³C, in both twigs and stems, likely due to the loss of ¹³C-enriched C-1 atoms of pyruvate during isoprene formation and kinetic isotope effects during diterpene synthesis. Resin δ¹⁸O was c. 20‰ higher than xylem water δ¹⁸O and c. 20‰ lower than δ¹⁸O of water-soluble carbohydrates, possibly caused by discrimination against ¹⁸O during O₂-based diterpene oxidation and 35%-50% oxygen atom exchange with water. Resin δ¹³C and δ¹⁸O recorded a strong signal of soil water potential; however, their overall capacity to infer intraseasonal environmental changes was limited by their temporal, within-tree and among-tree variations. Future studies should validate the potential isotope fractionation mechanisms associated with resin synthesis and explore the use of resin δ¹³C and δ¹⁸O as a long-term proxy for physiological and environmental changes.