Unenriched xylem water contribution during cellulose synthesis influenced by atmospheric demand governs the intra‐annual tree‐ring δ18O signature

Abstract

Summary The oxygen isotope composition (δ18O) 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 δ18O variations to the intra‐annual tree‐ring cellulose δ18O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ18O measurements of soil water, needle water, and twig xylem water with intra‐annual δ18O measurements of tree‐ring cellulose, xylogenesis analysis, and mechanistic and structural equation modeling. Intra‐annual cellulose δ18O values resembled source water δ18O mean levels better than needle water δ18O. Large parts of the rings were formed under high proportional exchange with unenriched xylem water (pex). Maximum pex values were achieved in August and imprinted on sections at 50–75% of the ring. High pex values were associated with periods of high atmospheric evaporative demand (VPD). While VPD governed needle water δ18O variability, we estimated a limited Péclet effect at both sites. Due to a variable pex, source water has a strong influence over large parts of the intra‐annual tree‐ring cellulose δ18O variations, potentially masking signals coming from needle‐level processes.