Unenriched xylem water contribution during cellulose synthesis influenced by atmospheric demand governs the intra-annual tree-ring δ18 O signature

Abstract

The oxygen isotope composition (δ¹⁸ 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 δ¹⁸ O variations to the intra-annual tree-ring cellulose δ¹⁸ O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ¹⁸ O measurements of soil water, needle water, and twig xylem water with intra-annual δ¹⁸ O measurements of tree-ring cellulose, xylogenesis analysis, and mechanistic and structural equation modeling. Intra-annual cellulose δ¹⁸ O values resembled source water δ¹⁸ O mean levels better than needle water δ¹⁸ 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 δ¹⁸ 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 δ¹⁸ O variations, potentially masking signals coming from needle-level processes.

Keywords: p ex; Péclet; isotopic fractionation; mechanistic modeling; stable oxygen isotopes; tree rings; water; xylogenesis.