Effects of Combined Nitrogen–Phosphorus on Biomass Accumulation, Allocation, and Allometric Growth Relationships in Pinus yunnanensis Seedlings after Top Pruning

<i>Pinus yunnanensis</i> (Franch), a species endemic to southwestern China, provides significant ecological and economic benefits. The quality of afforestation can be enhanced by promoting high-quality sprout growth. This study analyzed the effects of six fertilization treatments following top pruning: T1 (N: 0 g/plant^-1; P: 0 g/plant^-1), T2 (N: 0 g/plant^-1; P: 2 g/plant^-1), T3 (N: 0 g/plant^-1; P: 4 g/plant^-1), T4 (N: 0.6 g/plant^-1; P: 0 g/plant^-1), T5 (N: 0.6 g/plant^-1; P: 2 g/plant^-1), and T6 (N: 0.6 g/plant^-1; P: 4 g/plant^-1). The accumulation and allocation of aboveground biomass in roots, stems, and leaves of <i>P. yunnanensis</i> were measured, as well as changes in biomass per plant at 90 days (early stage), 180 days (middle stage), and 270 days (late stage) post-fertilization. At 90 days, root biomass accumulation in T3 was significantly higher, by 13.31%, compared to that in T1 (<i>p</i> < 0.05). The growth rates of stem and plant biomass followed the order T6 > T1 > T3 > T5 > T4 > T2. The biomass of sprouts and individual plants exhibited allometric growth under T1, T5, and T6 treatments. At 180 days, needle biomass allocation in T1 and T4 increased by 7.47% and 8.6%, respectively, compared to 90 days. Combined nitrogen-phosphorus fertilization significantly influenced aboveground biomass allocation, promoting growth more effectively than other treatments. By 270 days, the stem and individual biomass in T2 and T3 treatments showed significant differences (<i>p</i> < 0.01) compared to other treatments. Overall, root, stem, and sprouts were primarily influenced by phosphorus fertilization, while nitrogen fertilization notably promoted needle and leaf growth in later stages. The aboveground components were more affected by phosphorus fertilization. The combination of nitrogen and phosphorus fertilizers enhanced early-stage stem and sprouts, as well as late-stage root development.