Climate and Soil Constraints Shape Vegetation-Zone-Dependent Leaf C:N:P Stoichiometry of Quercus variabilis

Leaf C:N:P stoichiometry provides a functional link between plant nutrient composition and environmental adaptation. However, whether widely distributed tree species maintain consistent leaf stoichiometric patterns across contrasting vegetation zones remains unclear. Here, we investigated the leaf C:N:P stoichiometry of Quercus variabilis, a widely distributed tree species in China, across the subtropical evergreen broad-leaved forest zone (SEF) and the warm-temperate deciduous broad-leaved forest zone (WDF). We aimed to clarify between-zone differences in leaf stoichiometric traits and the role of climate and soil constraints in shaping vegetation-zone-dependent stoichiometric variation. Our results showed that leaf C:N:P stoichiometry of Q. variabilis differed between the SEF and WDF, with significant differences detected only for leaf N and leaf C:N. Overall, leaf C varied little across zones (CV = 3.13–3.42%), leaf N and leaf C:N showed moderate variation (CV = 10.07–13.26%), whereas leaf P, leaf C:P, and leaf N:P showed greater variability (CV = 33.28–39.93%). Environmental analyses further indicated contrasting patterns between zones: leaf stoichiometry in the SEF was jointly associated with climate and soil factors and showed a positive leaf C–N relationship, whereas that in the WDF showed a stronger soil association and a negative leaf N–P relationship. These findings suggest that Q. variabilis may adjust its leaf stoichiometry in a zone-dependent manner in response to contrasting climate and soil conditions. This study provides a theoretical basis for understanding nutrient-use strategies and environmental adaptation of widely distributed tree species under heterogeneous climate and soil conditions.