Unenriched xylem water contribution during cellulose synthesis influenced by atmospheric demand governs the intra-annual tree-ring δ 18 O signature.

The oxygen isotope composition (δ 18 O) of tree-ring cellulose is used to evaluate tree physiological responses to climate, but their interpretation is still limited due to the complexity of the isotope fractionation pathways. We assessed the relative contribution of seasonal needle and xylem water δ 18 O variations to the intra-annual tree-ring cellulose δ 18 O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ 18 O measurements of soil water, needle water, and twig xylem water with intra-annual δ 18 O measurements of tree-ring cellulose, xylogenesis analysis, and mechanistic and structural equation modeling. Intra-annual cellulose δ 18 O values resembled source water δ 18 O mean levels better than needle water δ 18 O. Large parts of the rings were formed under high proportional exchange with unenriched xylem water (p ex ). Maximum p ex values were achieved in August and imprinted on sections at 50-75% of the ring. High p ex values were associated with periods of high atmospheric evaporative demand (VPD). While VPD governed needle water δ 18 O variability, we estimated a limited Péclet effect at both sites. Due to a variable p ex , source water has a strong influence over large parts of the intra-annual tree-ring cellulose δ 18 O variations, potentially masking signals coming from needle-level processes.