From sharing to partitioning: Stable isotopic evidence for flexible water use strategies mediating plant coexistence on the Tibetan Plateau

Study region The southeastern Tibetan Plateau, specifically subalpine riparian zones and closed forests dominated by Abies georgei var. smithii . Study focus Alpine plant coexistence shifts from resource sharing to vertical niche partitioning across moisture gradients, yet how these strategies operate in riparian zones versus closed forests remains unclear. We employed δ¹ ⁸O analysis and the MixSIAR model to quantify water source contributions, and assessed niche breadth (WNB) and overlap (Proportional Similarity, PS) to elucidate coexistence mechanisms. New hydrological insights for the region In riparian zones, shrubs showed a distance-dependent decrease in river water use (23.4–18.1%), and coexisting species exhibited high niche overlap (PS > 0.91) and broad niches (WNB: 3.8–3.9), indicating a coexistence strategy based on resource sharing. In fir forests, water use was strongly age-dependent: juvenile and young trees primarily utilized deep soil water (40–100 cm, WNB: ∼3.0), while fir seedlings and understory herbs competed for shallow water (0–20 cm, WNB: ∼2.5, PS up to 0.94), likely constraining fir regeneration. As trees matured, niche overlap decreased (PS to 0.73), demonstrating a shift toward vertical niche partitioning under moisture limitation. Our findings represent a mid‑summer snapshot and require seasonal validation. Nonetheless, they reveal a moisture‑mediated continuum of coexistence mechanisms—from resource sharing to vertical niche partitioning—that is vital for predicting community dynamics and guiding conservation of this vulnerable fir under climate change-induced hydrological shifts.