Analysis of Ecohydrological Processes in Two Subalpine Conifer Site–Species Combinations on the Eastern Margin of the Qinghai–Tibet Plateau Based on Stable Isotopes

ABSTRACT Characterising isotopic compositions within the soil–plant‐atmosphere continuum (SPAC) is crucial for revealing ecohydrological processes, yet studies linking multiple water pools in subalpine forest critical zones remain insufficient. We sampled eight water pools, including snow, rain, stream, groundwater, soil water, root water, stem water and leaf water from Qinghai spruce ( Picea crassifolia ) and Qilian juniper ( Juniperus przewalskii ) site–species combinations on the eastern margin of the Qinghai–Tibet Plateau during April–October 2024. Hydrogen and oxygen stable isotopes, linear mixed‐effects models, Bayesian mixing models (MixSIAR), paired leaf‐stem isotope enrichment and the Craig–Gordon model were used to characterise seasonal isotopic patterns among water pools and to estimate water sources of root and stem water. The results showed that: (1) Isotopic values among different water pools varied significantly with month, plant organ and soil depth. Leaf water was enriched relative to paired stem water, with growing‐season mean Δ 18 O L values of 13.85‰ ± 9.97‰ and 12.65‰ ± 7.07‰ and corresponding Δ 2 H L values of 37.96‰ ± 26.81‰ and 28.43‰ ± 17.53‰ for the Picea crassifolia and Juniperus przewalskii site–species combinations, respectively. (2) Root and stem water showed significant seasonal shifts in soil‐water use, and both combinations relied on shallow (0–10/20 cm) soil water during wetter periods, whereas both used deeper (10/20–40 cm) soil water during drier periods. (3) Isotopic relationships among various water pools revealed coordinated but temporally variable ecohydrological connections within the SPAC. These findings support an isotope‐based conceptual model of water transport and isotopic transformation in subalpine conifer forest.