The stoichiometry of carbon, nitrogen and phosphorous in plants can reflect the interactions between plants and their environment. The interplay between plant nutrients, climatic factors, and soil properties and the underlying regulatory mechanisms are pillars of ecology but remain underexplored. In this study of plant C–N–P stoichiometry and nutrient resorption in Castanopsis hystrix groves in three cities (Guangzhou, Zhongshan, and Lechang) that represent an urban–rural gradient in Guangdong Province, South China, we explored potential relationships among NO₂ concentrations, diameter at breast height (DBH), and resident human population. Mean annual temperature, mean annual precipitation, insolation duration per year, and the human resident population differed significantly among the three cities. Soil C-N-P was always highest in suburban Lechang, and the concentration of NO₂ was highest in urban Guanghzou (55.33 ± 0.67 μg m^–3) and positively correlated with the resident population and leaf N:P. Our findings suggest that C–N–P stoichiometry of C. hystrix was better explained by NO₂ than by soil C–N–P stoichiometry and that nutrient resorption was better explained by leaf nutrients and DBH than by NO₂ and soil stoichiometry. Our study supports the hypothesis that rapid urbanization influences NO₂ concentrations and microclimate, which may jointly change the stoichiometry of plant nutrients in the forest ecosystems.