Allometric Growth Patterns and Phenotypic Plasticity Indices of Different Grades of Annual Pinus yunnanensis Franch. Seedlings at Different Growth Stages

Pinus yunnanensis Franch. is a native pioneer and economically important tree in Yunnan Province in China. In this study, over 1400 annual seedlings were used. Following national or regional official seedling quality standards, seedlings were classified into three grades, namely Grade I, Grade II, and Grade III by using mean ± 1/2 standard deviation method according to the height of seedlings (H ± 1/2σ). Morphological traits including seedling height, ground-line diameter, root length, and root average diameter were measured from September 2022 to December 2023 for each grade. A power-law allometric growth model was constructed, and the standardized major axis method was used to analyze the allometric relationships between plant height and ground-line diameter as well as between root length and root average diameter. The results showed that higher grade seedlings exhibited stronger synergistic plasticity, accelerating allometric growth and enhancing phenotypic plasticity. A significant positive correlation was found between plant height and ground-line diameter growth rates, with ground-line diameter showing greater plasticity. Grade I seedlings demonstrated clear advantages, with mean allometric rates of 0.5860 for plant height versus ground-line diameter and 1.6315 for root length versus root system. The phenotypic plasticity index for ground-line diameter was high across all three grades, but actual thickening varied by grade due to different initial diameters, with Grade I and II seedlings growing much more than Grade III. For plant height, the index ranged from 0.3 to 0.8, with values of 0.6–0.7 for Grade I, 0.3–0.7 for Grade II, and 0.6–0.8 for Grade III. These findings provide a scientific basis for evaluating seedling quality, breeding, reproduction, and improving survival and growth in later-stage afforestation.