Dynamic responses of growth rhythms and allometric relationships in Pinus yunnanensis seedlings under combined nitrogen and phosphorus application

This study investigated the effects of combined nitrogen (N) and phosphorus (P) application on the growth rhythm, optimal fertilization, and allometric growth of Pinus yunnanensis seedlings. To find out, we set up a two-factor, three-level factorial experiment and tracked growth using logistic models, response surface analysis, and allometric equations. Combined N and P application significantly affected the growth rhythm and growth amount of P. yunnanensis seedlings. Under P-deficient treatment, seedling growth was suppressed, and ground diameter grew faster than seedling height, suggesting that insufficient P supply may alter the growth relationship between height and ground diameter. The high-N and high-P treatment (T9) significantly extended the rapid growth period and exhibited better growth performance. Response surface analysis revealed that the effects of combined N and P application on seedling growth varied across growth stages, with the optimal fertilization combination being N: 0.8 g·plant⁻¹ and P: 6 g·plant⁻¹ . Allometric analysis showed that the seedling height-ground diameter growth relationship dynamically shifted between isometric and allometric growth under different treatments. In conclusion, combined N and P application can regulate the growth rhythm, growth amount, and the growth relationship between seedling height and ground diameter. The T9 treatment performed better during the observation period, whereas phosphorus deficiency may limit growth. Moreover, the seedling response to the initial N and P supply exhibited dynamic changes over the growth process. Future studies could further establish treatments with different fertilization timings and application methods to clarify the N and P response characteristics at various growth stages and to identify appropriate nutrient management practices.