Seasonal Variation Drives Leaf Anatomical Structure, NSC Allocation, and C:N:P Stoichiometric Characteristics in...

Ochroma lagopus is a fast-growing, low-density broadleaf tree species with increasing importance for tropical plantation forestry and lightweight bio-based materials. However, juvenile plantations may be sensitive to seasonal changes in water availability and soil nutrient status, particularly in tropical monsoon regions with strong drywet season transitions. In this study, two-year-old O. lagopus plantations in Xishuangbanna, Yunnan Province, China, were used to compare leaf anatomical structure, non-structural carbohydrate (NSC) components in leaves and roots, and C:N:P stoichiometric characteristics between the wet and dry seasons. Correlations among leaf structure, carbon allocation, and nutrient stoichiometry were also analyzed. The results showed significant seasonal differences in leaf anatomical structure. Leaves in the wet season had more developed vascular bundles and midrib tissues, whereas leaves in the dry season had deeper substomatal chambers and higher tissue looseness. NSC components were mainly affected by organ differentiation, whereas seasonal effects were weak. Roots maintained relatively high and stable carbohydrate levels. C:N:P stoichiometric characteristics were affected by both season and organ, and root P concentration and P-related ratios were more sensitive to seasonal transition. Leaf and root N/P ratios were generally below 14, indicating a nutritional status associated with relative N limitation. Correlation analysis showed that leaf anatomical traits were significantly correlated with leaf NSC, C, N, and P concentrations and stoichiometric ratios, revealing a coordinated structural carbon nutrient response of O. lagopus during the dry wet season transition. These findings suggest that the response of juvenile O. lagopus plantations to seasonal environmental variation is not driven by water availability alone, but is jointly regulated by leaf structural plasticity, root carbon pool stability, nutrient leaching during the wet season, and P availability in red soil. This study provides a theoretical basis for the introduction and cultivation, nutritional diagnosis, and seasonal management of juvenile O. lagopus plantations in tropical regions.