Objective To elucidate the resource allocation patterns of leaves from different age classes of Pinus tabulaeformis and the mechanisms underlying their functional traits’ responses to nutrient addition, with the aim of providing a theoretical foundation for understanding the adaptive strategies of plants to nutrient fluctuations in forest ecosystems and their ecological significance.

Method In this study, we established a controlled nitrogen (N) and phosphorus (P) addition experiment in the lingkongshan Forest of Taiyue Mountain, Shanxi Province. We systematically measured eight functional traits across three major categories—structural, pigment, and chemical characteristics—in leaves of different ages from Pinus tabulaeformis. Combining this with dynamic monitoring of soil nutrients, we employed Pearson correlation tests and regression analysis to explore the synergistic and trade-off relationships among multiple traits in leaves of different ages. Additionally, we utilized redundancy analysis to identify key soil environmental factors significantly influencing the functional traits of Pinus tabulaeformis leaves and to elucidate the response patterns of these traits to nutrient inputs.

Result (1) Regarding structural and pigment characteristics, low N treatment significantly increased the specific leaf area (SLA) of current-year needles by 18.5%, while simultaneously reducing their leaf dry matter content (LDMC) by 7.96%; indicating that nitrogen application enhanced the resource acquisition capacity of current-year leaves; Chlorophyll (CHL) content increased significantly only in current-year leaves with N addition, while second-year CHL decreased significantly by 16.0% under low N treatment. In contrast, P addition and the combined N + P effect had limited impact. (2) Regarding chemical properties, low N and low P addition significantly increased P content in second-year needles. High P and N + P additions significantly suppressed P content in both current-year and one-year-old needles. All three nutrient addition treatments significantly increased carbon (C) content in current-year leaves, but none significantly altered leaf N content. This reflects distinct differences in the response of leaves of different ages to exogenous nutrient inputs, with the adaptation process primarily achieved through adjustments in P allocation and changes in carbon accumulation. (3) Leaves at different developmental stages showed marked functional differentiation. Current-year leaves had lower leaf thickness (LT) and LDMC, but higher SLA, leaf N content, and leaf P content, indicating an acquisitive “fast investment-return” strategy. In contrast, older leaves had higher LT and LDMC, but lower SLA, leaf N content, and leaf P content, reflecting a conservative “slow investment-return” strategy. These findings indicate that leaves of different ages in Pinus tabulaeformis adapt to changes in the nutrient environment through strategic differentiation. (4) Leaf traits were generally significantly correlated, and the trade-off relationships among traits varied with leaf age, indicating that functional coordination and resource allocation patterns shift across developmental stages. (5) The dominant soil factors driving changes in leaf functional traits differed among leaf ages: soil pH was the main factor for current-year leaves, soil total phosphorus (TP) for 1-year-old leaves, and soil water content (SWC) for 2-year-old leaves. These results suggest that leaves at different developmental stages differ in their sensitivity to soil environmental conditions.

Conclusion Nitrogen and phosphorus additions significantly affected the functional traits of Pinus tabuliformis needles, and leaves of different ages showed distinct responses to nutrient inputs. These findings provide new insights into the mechanisms by which evergreen species with leaves of different ages adapt to nitrogen and phosphorus deposition, and offer a theoretical basis for further understanding how plants optimize resource allocation through trait coordination under changing nutrient conditions.