Variation of the water-use efficiency and nutrients of Pinus armandii under different ages and growth season

Seasonal changes in carbon (C), nitrogen (N), and phosphorus (P) stoichiometry and water-use efficiency (WUE) in the needles of Pinus armandii were investigated. The needles of P. armandii were sampled in January, May, August, and November 2021. The results showed that (1) the needle δ13C values of young P. armandii forests were higher than those of middle-aged P. armandii forests, and these differences were significant in winter. (2) Both young and middle-aged P. armandii forests were restricted by N and P, and P. armandii was more restricted by N in summer, autumn, and winter. (3) Needle δ13C values were significantly positively correlated with P and negatively correlated with N, C/P, and N/P in young P. armandii forests; P. armandii (especially young stands) can compensate for decreases in N-use efficiency by increasing WUE. The C, N, and P content and their stoichiometric ratios were affected by a combination of season, stand age, and WUE. This study contributes to an in-depth understanding of the adaptive mechanisms of nutrients and WUE in P. armandii in the subtropical region. Our findings also have implications for the management of P. armandii plantations in the study area.