As a fast-growing tree species, Ochroma lagopus Swartz has a greater demand for nutrients, and urea and slow-release fertilizer addition can promote the growth and development of O. lagopus. This experiment explored the effects of urea and slow-release fertilizer on the growth and nutrient uptake of O. lagopus plantation forests. The results of the study can provide a theoretical basis for the cultivation and management of O. lagopus plantation forests. At the end of June 2023, a two-year-old O. lagopus was used as the study’s object at Mengwing Farm, Mengla County, Xishuangbanna Dai Autonomous Prefecture, Yunnan Province, China. Fertilizer is applied through root outer ring application. Seven treatments of no N control (CK) and urea and slow-release fertilizers 300 g/plant (N1, H1), 450 g/plant (N2, H2), and 600 g/plant (N3, H3) were set up to determine the breast height, diameter at breast height, as well as the nutrient contents of leaves, branches, roots, and soils of the trees. The DBH and tree height were higher under N addition than in the CK, and both were highest in H3 at 90 d after fertilization, with 16.92% and 14.64% higher than CK, respectively. The overall change in C content of each organ was not obvious, N content increased with the increase in fertilizer application, and the change pattern of P content was not consistent. Soil N content showed a significant increase with the increase in N application, while P and K content showed a trend of increasing and then decreasing, and soil N content of slow-release fertilizer treatments was higher than that of urea treatments under the same amount of N application. The soil N content was higher in the slow-release fertilizer treatment than in the urea treatment regardless of the amount of N applied. The soil N content was highly significantly positively correlated with the N content of leaves, branches, and roots. There was a highly significant allometric growth relationship among the C, N, and P content of each organ. The N addition had a significant effect on the growth and the nutrient content of each organ and soil. O. lagopus mainly adapted to changes in the soil N content by adjusting the N and P content of each organ, and the optimal effect was observed in H3. In the future, an appropriate amount of P fertilizer should be applied to complement N fertilizer, urea and slow-release fertilizer can also be applied.

To elucidate shade adaptation mechanisms in Pinus yunnanensis seedlings across different ages, this study established five light gradients (100% full sunlight as control or CK, 80% as L1, 45% as L2, 30% as L3, and 5% as L4) for experimental treatments on one- and three-year-old seedlings. By analyzing dynamic changes in non-structural carbohydrates (NSCs) and their components within needles, stems, and roots—combined with a phenotypic plasticity assessment, a correlation analysis, and a principal component analysis—we explored the carbon metabolic adaptations under shade stress. The key results demonstrate the following: (1) Increasing shade intensity significantly reduced the NSCs in the needles and roots of both age groups. The stem NSCs markedly decreased under L1 and L2, indicating “carbon limitation.” However, under severe shade (L3 and L4), the stem NSCs stabilized while the stem soluble sugars gradually increased. In three-year-old Pinus yunnanensis seedlings under the L3 treatment, the ratio of soluble sugars to starch in the stems reached as high as 5.772 g·kg⁻¹, yet the stem NSC content showed no significant change. This pattern exhibited “growth stagnation-carbon enrichment” characteristics. This reveals a physiological strategy for maintaining stem carbon homeostasis through a “structure–metabolism” trade-off under carbon limitation. (2) Shade adaptations diverged by age: one-year-old seedlings employed a short-term “needle–root source–sink reallocation” strategy, whereas three-year-old seedlings developed a “root–stem–needle closed-loop homeostasis regulation” mechanism. (3) Age-specific shade thresholds were identified: one-year-old seedlings required >80% full light to maintain a carbon balance, while three-year-old seedlings exhibited enhanced root carbon storage under moderate shade (45–80% full light). This study clarifies the physiological mechanisms by which P. yunnanensis seedlings of varying ages optimize shade adaptation through organ-specific carbon allocation, providing a theoretical foundation for shade management in artificial forests and understory seedling conservation.

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.