Size-asymmetric and spatially structured competition shapes Populus euphratica stands in the lower Tarim river riparian zone

• Assessed intraspecific competition in P. euphratica stands of the lower Tarim River. • Quantified crowding using Hegyi’s competition index across size and spatial gradients. • CI declined sharply with increasing DBH and height, confirming size-asymmetric competition. • CI showed a bimodal pattern across crown area and peaked at ∼ 300 m from the river. • CI was positively related to local stem density, identifying dense clusters as competition hotspots. Hydrological alteration and groundwater decline have reshaped the structure of Populus euphratica riparian forests along the lower Tarim River, underscoring the need for structural indicators that diagnose neighborhood crowding and guide restoration. We quantified intraspecific competition using Hegyi’s competition index (CI) across six 50 × 50 m plots arrayed along a river-distance (hydrological-edaphic gradient), measuring DBH, height, crown area, and spatial position for 229 target trees. We treat river distance as a hydrological-edaphic gradient proxy rather than a single causal factor and evaluate CI jointly with tree size and local stem density to avoid over-attribution to distance alone. We tested three hypotheses: (i) competition intensity decreases with tree size (DBH, height, crown area); (ii) competition peaks at intermediate distance from the river; and (iii) competition intensity increases with local stem density. Because CI integrates neighbor size and spacing, it serves as a management-relevant structural indicator of local crowding and stand condition. CI declined strongly with DBH and height (size-asymmetric competition), showed a bimodal pattern across crown-area classes (peaks at 0–10 m 2 and 30–40 m 2 ), and reached a spatial maximum at ∼ 300 m from the river where stem density was highest. Class-level models corroborated a negative DBH effect on CI (p < 0.05) and weak quadratic terms for crown area; CI was positively associated with local stem density. These results identify structural thresholds (≤20 cm DBH; 0–2 m height; <10 m 2 crown area) and a mid-gradient crowding peak (∼300 m) that together diagnose where regulation should be prioritized. Management implications include selective release thinning of dense sapling clusters and conditional, small-pulse ecological water supplementation where site diagnostics indicate hydrologic limitation, coupled with long-term monitoring of density, size trajectories, and mortality.