Many seedlings and a few young trees have recently been observed in Pinus yunnanensis forests, reducing the natural regeneration ability and succession. Shade treatments were applied to potted 1-year-old P. yunnanensis seedlings, and the shade net was opened at noon to simulate light patches. We used four treatments, i.e., 80% shade with 1 h light at noon (T80-1), 80% shade all the time (T80), 95% shade with 1 h light at noon (T95-1), and 95% shade all the time (T95), and a control (natural light). We analyzed the effects of light patches on the growth and C:N:P stoichiometry of P. yunnanensis seedlings. (1) Shading significantly inhibited seedling growth, with height increments reduced by 29.59% and 47.40% under T80 and T95, respectively, and basal diameter increments decreased by 10.97% and 14.41%. (2) Shading reduced biomass across organs, with total biomass under T95 being only 39.02% of CK, but midday light patches alleviated this inhibition (T80-1 total biomass increased by 137.90% compared to T80). (3) Under high shading (T95), seedlings prioritized photosynthetic product allocation to aboveground parts (needle biomass proportion reached 58.01%), while light patches (T80-1) enhanced coarse root biomass (137.90% higher than T80). (4) Shading significantly increased needle C:N and C:P ratios (T95 increased by 69.01% and 129.93% compared to CK, respectively), with N:P > 16 indicating phosphorus limitation; light patches (T80-1) reduced N:P to 14–16, mitigating co-limitation by N and P. The study demonstrates that P. yunnanensis seedlings adopt conservative strategies under shading by adjusting biomass allocation and stoichiometry to adapt to low-light conditions, while midday light patches enhance photosynthetic efficiency and nutrient utilization. We recommend forest thinning to increase understory light patches, thereby improving natural regeneration and promoting sustainable forest management of P. yunnanensis forests. These findings highlight the importance of light management in P. yunnanensis forests to enhance regeneration by regulating understory light patches.

Ochroma lagopus, a fast-growing tropical tree species, faces fertilization challenges due to slope heterogeneity in plantations. This study examined 3-year-old Ochroma lagopus at upper and lower slope positions under five treatments: CK (no fertilizer), F1 (600 g/plant), F2 (800 g/plant), F3 (1000 g/plant), and F4 (1200 g/plant) of secondary macronutrient water-soluble fertilizer. Growth parameters and N-P-K stoichiometry were analyzed. Key results: (1) Height increased continuously with fertilizer dosage at both slopes, while DBH peaked and then declined. (2) At upper slopes (nutrient-poor soil), fertilization elevated leaf P but reduced branch N/K and increased root P/K. At lower slopes (nutrient-rich soil), late-stage leaf N increased significantly, with roots accumulating P/K via a “storage strategy”. Stoichiometric thresholds indicated N-K co-limitation (early-mid stage) shifting to P limitation (late stage) on upper slopes and persistent N-K co-limitation on lower slopes. (3) PCA identified F4 (1200 g/plant) and F1 (600 g/plant) as optimal for upper and lower slopes, respectively. This research provides a theoretical basis for precision fertilization in Ochroma lagopus plantations, emphasizing slope-specific nutrient status and element interactions for dosage optimization.

The ongoing global climate change has led to an increase in the frequency and complexity of drought events. Pinus yunnanensis, a native tree species in southwest China that possesses significant ecological and economic value, exhibits a high sensitivity to drought stress, particularly in its seedlings. This study investigates the response mechanisms of non-structural carbohydrates (NSCs, defined as the sum of soluble sugars and starch) and the stoichiometric characteristics of carbon (C), nitrogen (N), and phosphorus (P) to repeated drought conditions in Pinus yunnanensis seedlings. We established three treatment groups in a potting water control experiment involving 2-year-old Pinus yunnanensis seedlings: normal water supply (CK), a single drought (D1), and three drought–rewatering cycles (D3). The findings indicated that the frequency of drought occurrences, organ responses, and their interactions significantly influenced the non-structural carbohydrate (NSC) content and its fractions, as well as the C/N/P content and its stoichiometric ratios. Under D3 treatment, stem NSC content increased by 24.97% and 29.08% compared to CK and D1 groups (p < 0.05), respectively, while root NSC content increased by 41.35% and 49.46% versus CK and D1 (p < 0.05). The pronounced accumulation of soluble sugars and starch in stems and roots under D3 suggests a potential stress memory effect. Additionally, NSC content in the stems increased significantly by 77.88%, while the roots enhanced their resource acquisition by dynamically regulating the C/P ratio, which increased by 23.26% (p < 0.05). Needle leaf C content decreased (18.77%) but P uptake increased (8%) to maintain basal metabolism (p < 0.05). Seedling growth was N-limited (needle N/P < 14) and the degree of N limitation was exacerbated by repeated droughts. Phenotypic plasticity indices and principal component analysis revealed that needle nitrogen and phosphorus, soluble sugars in needles, stem C/N ratio (0.61), root C/N ratio (0.53), and stem C/P ratio were crucial for drought adaptation. This study elucidates the physiological mechanisms underlying the resilience of Pinus yunnanensis seedlings to recurrent droughts, as evidenced by their organ-specific strategies for allocating carbon, nitrogen, and phosphorus, alongside the dynamic regulation of nitrogen storage compounds (NSCs). These findings provide a robust theoretical foundation for implementing drought-resistant afforestation and ecological restoration initiatives targeting Pinus yunnanensis in southwestern China.

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.

In recent years, it has been found that the phenomenon of ‘only seedlings but no young trees’ is very serious in P. yunnanensis forest, which is very unfavourable to the natural regeneration and succession of seedlings in P. yunnanensis forest. Through research on the growth and non-structural carbohydrates (NSCs) content of various organs under different shading treatments, this study provides a theoretical basis for understanding the regeneration difficulties of P. yunnanensis and strengthening the scientific conservation of P. yunnanensis forests. In this study, we set up shade treatments for potted P. yunnanensis seedlings by constructing shade shelters and simulated sunflecks by opening the shade net at noon; we set up five treatments, namely the control (natural light), 80% shade with the net open at noon for 1 h (T80-1), 80% shade all the time (T80), 95% shade with the net open at noon for 1 h (T95-1), and 95% shade all the time (T95). The changes in seedling height and diameter and the NSCs content of various organs of P. yunnanensis seedlings were determined after shading. The results showed that 80% and 90% shading significantly inhibited the growth of P. yunnanensis seedlings and reduced the biomass of each organ. While the needle–biomass ratio of P. yunnanensis increased, the fine root–biomass ratio and root–crown ratio tended to decrease. The starch content and NSCs content of each organ decreased, and the soluble sugar–starch ratio of each organ tended to increase. Under the simulated sunfleckssunfleckstreatment, P. yunnanensis seedlings had increased aboveground biomass investment and also decreased storage of thick root starch, which was decomposed and invested into the aboveground part. This indicates that the transient high intensity of Sunfleckssunflecksmitigation alleviated the adverse effects of shading on seedling carbon reserves and increased the adaptability of P. yunnanensis seedlings to prolonged shading.