Response of Growth and Non-Structural Carbohydrates' Allocation in Pinus yunnanensis Seedlings to Simulated Sunflecks

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.