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.

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.