Screening carbon‑efficient urban woody plants in high‑altitude Xining: seasonal photosynthetic dynamics and canopy carbon‑sequestration potential

Urban green spaces are increasingly expected to support municipal carbon mitigation, yet species-level evidence remains scarce for high-altitude cold cities. We quantified seasonal net photosynthetic rate (Pₙ) and estimated growing-season canopy CO₂ assimilation potential for 106 commonly planted urban woody species (58 trees and 48 shrubs) across sixteen urban green-space sites in Xining, China (2,261 m a.s.l.), during the 2025 growing season (May–October). Monthly diurnal Pₙ measurements were integrated with total leaf area estimates derived from field structural measurements, i-Tree Eco leaf-area parameters and shrub-specific allometric models. Across species, mean Pₙ generally followed a unimodal seasonal pattern, peaking in July at 9.49 ± 0.21 μmol m⁻² s⁻¹, with lower rates in spring and autumn. Estimated growing-season canopy CO₂ assimilation potential varied strongly among taxa and life forms. Ulmus pumila reached the highest value among trees (146.48 ± 33.71 CO₂ yr⁻¹), followed by Populus alba and Populus cathayana , whereas Hippophae rhamnoides ranked highest among shrubs (0.852 ± 0.152 kg CO₂ yr⁻¹), followed by Elaeagnus angustifolia and Ligustrum sinense . Deciduous broad-leaved species generally outperformed evergreen conifers, suggesting that high peak-season assimilation and large leaf area can outweigh longer leaf retention under a short growing season. Clustering grouped trees and shrubs into five performance classes, supporting site-specific planting templates for parks, roads and residential spaces that couple carbon performance with air-quality, thermal-comfort and amenity co-benefits. These results provide a one-growing-season screening baseline for carbon-efficient urban woody plant selection in Xining, pending multi-year monitoring, leaf-area validation and life-cycle carbon accounting.