Objectives The seasonal changes of carbon (C), nitrogen (N), phosphorus (P) contents and their ecological stoichiometric characteristics in different organs of Pinus armandi young forest were studied, and the allometric growth relationship was analyzed to predict the possibility of nutrient limitation of young pine trees and improve the management.

Methods In January, May, August, and November of 2021, young P. armandii tree samples were collected from Yiliang County State-Owned Garden Forest Farm, Kunming City, Yunnan Province, China. The plants were divided into needles, branches, stems, and roots for the analysis of C, N and P contents, and the ecological stoichiometric ratios were then calculated in different seasons.

Results 1) Sampling site, growing season and their interactions had significant (P<0.05) effects on C, N and P contents and their stoichiometric ratios. C and P contents and C∶P ratios were mainly affected by the growing season, while N, C∶N ratios and N∶P ratios were mainly affected by the sampling site. 2) The C contents in all the organs of P. armandii in winter were significantly higher than in the other seasons. The N contents in leaves and branches in winter were significantly higher than in spring and summer, and the P contents in leaves and stems in summer were significantly higher than in autumn and winter. The C contents in the organs of young pine trees were in order of stem > branch > leaf > root, the N contents were in order of leaf > branch > root > stem, and the P contents were in order of root > leaf > branch > stem. When N : P < 14, the growth and development of plants was thought more limited by N element. In this study, the N : P ratios of all the organs were less than 14, indicating that the main limiting nutrient element for the growth of young P. armandii forest was N. 3) Needle N content had the largest plasticity index (0.79), followed by leaf P content (0.64), indicating that the nitrogen and phosphorus in the needles of young P. armandii forests were susceptible to external environmental influences. 4) The C-N ratios of leaves, branches and stems showed a significant allometric relationship, reflecting the different distribution and regulation modes of C and N content in P. armandii tree organs.

Conclusions  N and P nutrition in young P. armandii needles is easily affected by the growth seasons. Appropriate N and P fertilizers, especially the restrictive element N, should be applied for promotion of P. armandii growth.