The Advantage of Tree–Shrub–Grass Vegetation Structures in Urban Green Spaces for Mitigating Atmospheric Pollutant NO2

Urban green spaces, as crucial components, can effectively mitigate atmospheric pollutants such as NO2. However, the heterogeneous driving forces and the underlying quantitative mechanisms of different vegetation community structures in response to NO2 exposure remain insufficiently explored. This study utilized a laboratory-based artificial fumigation method to examine the NO2 mitigation benefits and response mechanisms of three representative vegetation structures (tree–shrub–grass, tree-shrub, and tree-grass) as well as their monoculture communities under NO2 stress. The objective was to elucidate the variations in NO2 reduction capacity and the adaptive mechanisms associated with different vegetation structures. The results demonstrated that, under NO2 exposure, the NO2 reduction rate of the tree–shrub–grass mixed community was 56.15 mg·h−1. NO2 stress inhibited leaf morphogenesis and plant growth. However, the tree–shrub–herbaceous community enhanced its NO2 tolerance by reallocating photosynthetic products and increasing epidermal thickness, stomatal density, and the compactness of tissue structure, thereby strengthening its mechanical barrier function. The NO2 reduction rate was positively correlated with parameters such as net photosynthetic rate, stomatal density, leaf width, transpiration rate, and stomatal conductance, but negatively correlated with fluorescence and the leaf length-to-width ratio. A comprehensive evaluation based on fuzzy membership functions ranked the NO2 mitigation and tolerance capacities of plant communities as follows: the tree–shrub–herbaceous community exhibited the strongest capacity for NO2 reduction and stress tolerance. Thus, in NO2-polluted areas, priority should be given to establishing tree–shrub–grass composite vegetation structures, whose multidimensional resistance mechanisms provide both a theoretical foundation and a technical pathway for the ecological restoration of urban green spaces.