Effects of Top-Pruning Intensity Gradient on Root System Architecture and Allometric Patterns in Pinus yunnanensis Franch. Seedlings

<i>Pinus yunnanensis</i>, is an ecologically and economically important tree species in southwestern China. However, its natural renewal is relatively lagging behind, and it is difficult to achieve sustainable development. Apical removal (top-pruning) can eliminate apical dominance, stimulate sprouting, and provide high-quality scions for clonal propagation. Root systems are a critical foundation for sprouting capacity. In this study, one-year-old <i>P. yunnanensis</i> seedlings were subjected to four treatments: removal of 3/4 (H1), 2/4 (H2), or 1/4 (H3) of the seedling height, and a non-topped control group (CK). The objective was to investigate the seedlings' responses in terms of root morphology, biomass allocation, and allometric growth. The results showed that by May, biomass allocation in the topped treatments increased by 13.37%, 11.01%, and 7.86%, respectively, compared with the control, and also exhibited higher coefficients of variation. Under the H2 treatment, both fine and coarse roots accounted for a higher proportion of total root biomass and displayed stronger water-retention stability. With increased top-pruning intensity and time, root volume, specific root length, root tissue density, and root tip number were the first to respond, indicating the onset of allometric growth. Notably, in May, the growth rate of specific root surface area followed the order: H3 > H1 > CK > H2. These findings suggest that the root system adapts to environmental changes by modulating growth patterns among various indicators to optimize resource allocation and enhance adaptability.