The analysis of ~d13C and ~d18O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes. In Pinus sylvestris, we traced the isotopic signals from their origin in the leaf water (~d18O) or the newly assimilated carbon (~d13C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season. Seasonally, variable 13C enrichment of sugars related to phloem loading and transport did lead to uncoupling between ~d13C in the tree-ring, and the ci/ca ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27ppt, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis. This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of ~d13C and ~d18O in tree-ring ecology.
