Functional-Unit Differentiation, DBH Size Structure, and Species-Level Contributions of Urban-Tree Standing Carbon Stocks in Qingdao, China: Implications for Sustainable Green-Space Management

Urban trees are important biological carbon pools in cities, yet their standing carbon stocks and the associated methodological uncertainty regarding their evaluation often vary among functional spaces. This study assessed 2442 individual trees representing 148 species in 116 standard 20 m × 20 m plots in the core urbanized area of Qingdao, China. Individual-tree carbon stocks were estimated from diameter at breast height, tree height, wood basic density, and allometric equations and were analyzed across urban functional units (UFUs), DBH classes, and species. Under the baseline Chave model, the sampled trees stored 153.337 Mg C, equivalent to 562.237 Mg CO2, with a mean carbon density of 33.047 Mg C ha−1. Residential districts contributed the highest total carbon stock, whereas public affairs service districts had the highest carbon density. Trees with DBH ≥ 40 cm accounted for only 3.93% of individuals but contributed 29.50% of the total carbon stock, and the top 20 species contributed approximately 79.0%. Alternative China-specific equations increased total estimates by 9.38–11.24% but covered only 10 of 148 species and 21.38% of trees at most; the UFU ranking and the main size- and species-contribution patterns remained unchanged. A root-to-shoot parameter sensitivity analysis likewise affected absolute estimates but not relative patterns. These findings support conserving mature trees, maintaining medium-sized trees as future carbon reserves, and targeting greening improvements to low-carbon-density functional spaces while recognizing the uncertainty inherent in absolute carbon-stock estimates.