Scale-Dependent Effects of Three-Dimensional Urban Vegetation on Land Surface Temperature and Air Pollutants: Synergy and Trade-Off Zones in Guangzhou, China

Urban greening programs in Chinese megacities are premised on the assumption that vegetation cools the surface and improves air quality simultaneously, yet greenness is usually quantified in two dimensions and the thermal and air-quality services are evaluated separately, so the places where they reinforce or conflict remain unmapped. We measured effective leaf area index (LAIe) in 570 quadrats across 30 vegetated plots in Guangzhou, China, and found that, under the spatial and temporal conditions tested, planar spectral data represented it poorly: Moderate Resolution Imaging Spectroradiometer (MODIS) LAI was only weakly correlated with field values (r = 0.15, n = 395), 87.9% of LAIe variance occurred within plots, and all 16 locally calibrated vegetation-index models showed no skill under plot-grouped cross-validation (CV-R2 ≤ 0). We therefore represented three-dimensional greenness with a structural proxy, layered vegetation volume (LVV, m3 m−2), defined as the product of cell-mean 10 m canopy height and tree-cover fraction (a cover-weighted canopy-height index), and coupled it with warm-season Landsat land surface temperature (LST) and 1 km estimates of PM2.5 (2016–2022), warm-season daily maximum 8 h average (MDA8) O3 (2016–2022) and NO2 (2019–2022) on 1, 2, 3 and 5 km grids, analyzed with ordinary least squares (OLS), geographically weighted regression (GWR), multiscale geographically weighted regression (MGWR) and extreme gradient boosting with Shapley additive explanations (XGBoost–SHAP). The results show the following: (1) MGWR achieved lower corrected Akaike information criterion (AICc) than OLS and GWR at every scale where it was fitted (2, 3 and 5 km), and the LVV term operated at a bandwidth of 43 neighbors (median radius ≈ 12 km) versus ≥863 neighbors (≈108 km, effectively citywide) for meteorological covariates; (2) one standard deviation of LVV (7.4 m3 m−2) was associated with 1.3 °C lower warm-season LST (significantly negative in 75% of cells at |t| > 1.96 and 37% under MGWR-adjusted critical values), 0.30 μg m−3 lower PM2.5 (69% and 50%) and 1.3 μg m−3 lower NO2; (3) the citywide O3 effect was null, but MGWR revealed offsetting local responses (31% of cells significantly negative, 17% positive), and within-cell greening was associated with rising O3 (+0.19 SD, p < 0.001), and annual coefficients turned non-negative after 2020 as nitrogen oxides (NOx) declined; (4) the PM2.5 benefit leveled off near 13.6 m3 m−2 (bootstrap 95% CI 8.4–15.2), while the cooling response flattened gradually with a poorly constrained breakpoint, and cells classified as full synergy, partial synergy, O3 trade-off and insensitive covered 44.6%, 36.3%, 15.0% and 4.1% of the municipality, with trade-off cells concentrated in the hottest, most built-up southern lowland belt; under stricter significance criteria the classified shares contracted, while the trade-off cells remained in this belt. These results suggest that structure-based, zone-specific greening targets may help secure co-benefits while anticipating a possible ozone penalty in NOx-rich areas.