Shining a new light on the classical concepts of carbon-isotope dendrochronology.

Retrospective information about plant ecophysiology and the climate system are key inputs in Earth system and vegetation models. Dendrochronology provides such information with large spatiotemporal coverage, and carbon-isotope analysis across tree-ring series is among the most advanced dendrochronological tools. For the past 70 years, this analysis was performed on whole molecules and, to this day, C discrimination during carbon assimilation is invoked to explain isotope variation and associated climate signals. However, recently it was reported that tree-ring glucose exhibits multiple isotope signals at the intramolecular level (see Wieloch et al., 2025). Here, I estimated the signals' contribution to whole-molecule isotope variation and found that downstream processes in leaf and stem metabolism each introduce more variation than carbon assimilation. Moreover, downstream processes introduce most of the climate information. These findings are inconsistent with the classical concepts/practices of carbon-isotope dendrochronology. More importantly, intramolecular tree-ring isotope analysis promises novel insights into forest metabolism and the climate of the past.