Water use complementarity in old-growth mixed forests versus competition in younger successional mixed forests

• Coexisting species in old-growth forests partition water use temporally. • Younger successional forests display overlapping water use and competition. • Physiology explains 92.1–99.8 % of interspecific tree-ring δ¹⁸O offsets. • Tree water-use strategies shift flexibly across successional stages. Water use strategies among coexisting species across successional gradients critically influence forest climate resilience and biodiversity conservation, but remain poorly understood for high-elevation regions. We investigated water use patterns of co-occurring species at three sites on the southeastern Tibetan Plateau and the eastern Himalayas using tree-ring oxygen isotopes (δ 18 O TR ), isotope-enabled model outputs (LMDZ4iso), and Roden-model-based mechanistic simulations. Statistical comparisons using Steiger’s Z-tests and bootstrap resampling reveal clear temporal partitioning among species at the old-growth sites (Wache and Baima) and substantial overlap at the younger successional site (Xincuo). Specifically, at Wache, Larix griffithii predominantly used August-October soil moisture (September peak), whereas Picea spinulosa accessed July-October soil water (August peak). At Baima, Larix potaninii utilized July-October soil moisture with September peak uptake, while Abies georgei accessed June-October soil water peaking in July. Conversely, at the Xincuo site, species in the younger successional forest, including Abies georgei and Hippophae tibetana , exhibited significant water use overlap from June to September (July peak). Simulations based on the Roden framework indicate that physiological processes, including stomatal and biochemical fractionation, explain 92.1 %–99.8 % of the observed interspecific δ¹⁸O TR offset (∼3 ‰). Nevertheless, δ¹⁸O TR chronologies still reliably capture interspecific differences in the seasonal timing of source-water uptake. Moreover, A. georgei acts as a flexible “bridge species,” shifting from temporally partitioned water use in old-growth forests to overlapping resource use at the younger site, where shallow soils likely constrain niche differentiation. These results suggest that forest succession may enhance niche complementarity by increasing effective soil-water niche space. Such shifts in resource partitioning may play an important role in maintaining forest resilience under projected regional warming and precipitation change. Integrating δ¹⁸O TR observations with isotope-enabled model outputs and mechanistic simulations provides a useful framework for investigating species-level ecohydrological strategies in alpine forests.