Currently, the limited urban space for carrying out ecological restoration and landscaping construction projects is mainly found in challenging urban sites. Quantifying the ecological impacts of urban greening engineering on difficult urban terrains and evaluating the ecological service value of foundational tree species are crucial for promoting green, low-carbon, and sustainable urban development. This study took Shanghai Expo Culture Park as an example, laid out 149 micro-sample plots and investigated 11 trunk species totaling 743 trees, which included ×Taxodiomera peizhongii, Magnolia grandiflora, Aesculus chinensis, Acer buergerianu, Metasequoia glyptostroboides, Triadica sebifera, Sapindus saponaria, Camphora officinarum, Ginkgo biloba, Prunus × yedoensis, and Acer coriaceifolium. The carbon sequestration, oxygen release, and cooling capacity of trees were calculated using the i-Tree Eco model, which reflected the potential evapotranspiration and transpiration in terms of cooling capacity. Results showed that carbon storage and annual carbon sequestration followed similar trends, both proportional to tree diameter at breast height (DBH). Camphora officinarum and Sapindus saponaria exhibited the highest performance, followed by Triadica sebifera, Prunus × yedoensis, and Aesculus chinensis, while Ginkgo biloba, Metasequoia glyptostroboides, and ×Taxodiomera peizhongii showed relatively lower values. Cooling and humidity-enhancing capacities varied significantly across diameter classes: small- to medium-sized Acer buergerianu and Metasequoia glyptostroboides demonstrated notable advantages, medium-sized Sapindus saponaria and Ginkgo biloba showed increasing benefits, and large-sized Triadica sebifera performed optimally. In conclusion, urban greening projects should integrate ecological benefit requirements, tree size specifications, and species selection to achieve site-appropriate afforestation and sustainable landscape development.