Tracing Oxygen and Hydrogen Isotope Signals From Water Sources to Tree-Ring Compounds.

Stable oxygen (δ18O) and hydrogen (δ2H) isotope compositions of tree-ring compounds preserve information about environmental waters; however, our understanding of their isotopic relationships is hampered by the lack of long-term data sets. We investigated correlations using unique 17-year (2006-2022) δ18O and δ2H time series of bi-weekly measured soil solution, modelled precipitation and xylem water, along with those of tree-ring α-cellulose and lignin methoxy groups from Norway spruce (Picea abies) across three Swiss forest sites. We show that tree-ring cellulose δ18O preserves water source information more effectively than δ2H, making it better suited for ecohydrological reconstructions. We propose δ2H of tree-ring lignin methoxy groups as an alternative proxy for soil water sources, supported by strong correlations where cellulose failed to track soil water isotopes. Significant linear isotopic relationships within and across sites enable the development of transfer functions that link tree-ring to water sources, particularly precipitation and xylem water. We exemplify how these transfer functions can be used to estimate the seasonal origin of water sourced by trees during the growth period. Our findings enhance the interpretation of environmental water isotope signals in tree rings and promote the use of tree-ring isotope-based tools for retrospective retrieval of forest water dynamics.