Aims <p>While hydrogen isotope offsets in plant water are well-documented, the spatial variability of oxygen isotopes (δ¹⁸O) within plants, particularly in relation to soil–plant water dynamics, remains poorly understood. This study aims to investigate the vertical and developmental variations of xylem δ¹⁸O in <i>Abies georgei var. smithii</i> and assess its potential impact on misinterpretations of soil water utilization.</p> Methods <p>We analyzed δ¹⁸O distributions in xylem tissues of <i>Abies georgei var. smithii</i> representing three growth stages (seedlings, saplings, juvenile trees). Sampling was conducted at multiple heights (e.g., stem base, middle stem, and branches) to assess vertical isotopic variability within individual plants.</p> Results <p>Results showed significant spatial variations in xylem δ¹⁸O, with enrichment levels increasing with sampling height. Stem water δ¹⁸O values were -11.88 ± 0.59‰ (seedlings), -12.84 ± 0.64‰ (saplings), and -12.82 ± 0.57‰ (juvenile trees), consistently depleted compared to branch water. The degree of δ¹⁸O fractionation in branches relative to stem water was 15.81%, 12.20%, and 20.60% for seedlings, saplings, and juvenile trees, respectively.</p> Conclusions <p>The MixSIAR model analysis revealed that the calculated proportions of water absorbed from different soil layers varied significantly when using δ¹⁸O data from different plant sampling heights. This suggests that inferences about soil water utilization by <i>A. georgei var. smithii</i> are sensitive to sampling location, potentially leading to overestimation of water uptake from shallower soil layers with higher sampling height. These findings underscore the importance of considering within-plant isotopic variation and fractionation effects for accurately quantifying plant water uptake patterns and understanding soil–plant water relations.</p>