The oxygen (δ18O) and hydrogen (δ2H) isotope compositions of leaf and xylem water shape tree-ring isotope baselines, while the fraction of sugars undergoing isotopic modification downstream of leaves (fO, fH) determines the dominant hydrologic signal. However, limited information on the seasonal dynamics of these isotope sources and on the drivers of f variation constrains tree-ring isotope interpretation. We measured intra-annual δ18O and δ2H in stem water, sugar, starch, and tree-ring α-cellulose of beech and spruce over two growing seasons. Using modelled leaf water δ18O and δ2H, we estimated seasonal f values and examined their relationships with nonstructural carbohydrate concentrations and climate. Tree rings primarily recorded δ18O and δ2H signatures of leaf water, despite seasonal changes in fO and fH. We found no clear transfer of intra-annual xylem water isotopic signals into sugars, starch, or cellulose. Seasonal fO and fH can be negatively correlated. Both were related to climate variables, but only fO was correlated with nonstructural carbohydrate concentrations. Thus, isotopic fractionation downstream of leaves does not always override the seasonal imprint of leaf water in tree rings. These findings provide insight into the controls on the fO-fH covariation, supporting more robust interpretations of climate variability from tree-ring isotope records.
Diao et al. (Wed,) studied this question.