Effects of nitrogen addition and leaf age on needle traits and the relationships among traits in Pinus koraiensis

Leaf functional traits are essential for the growth and adaptation of trees to environmental changes. However, how traits and trait–trait relationships respond to different nitrogen (N) addition rates and leaf ages remain unclear for evergreen coniferous species. Our study aimed to investigate the effects of N addition and leaf age on needle traits and their bivariate correlations. Understanding resource utilization and distribution strategies during tree growth under the background of increased N deposition is important. We conducted a six-year manipulative field experiment in a Pinus koraiensis plantation in northeastern China with variable N addition levels: control (no N addition, CK); 20kgN ha-1yr-1 (low-N, LN); 40kgN ha-1yr-1 (medium-N, MN); and 80kgN ha-1yr-1 (high-N, HN). The needle trait data were collected in September. The morphological traits, i.e., specific leaf area (SLA) and leaf dry matter content (LDMC); and chemical traits, i.e., leaf nitrogen concentration (LNC), leaf phosphorus concentration (LPC), and leaf nonstructural carbohydrates (NSC) concentrations, were measured to investigate the changes in needle traits and their bivariate correlations. Fifteen sampled trees (each with four leaf ages) from each of the four N addition treatments were selected for measuring needle traits. We found that N addition and needle age significantly affected needle traits, and the effect of needle age was much stronger than that of N addition. N addition and needle age had no significant interaction effects on any of the traits except for the LNC. In addition to the SLA, N addition significantly affected other traits in current-year needles, while the needle age had a significant effect on all traits. Due to the dispersal trait values of current-year needles, there were almost no significant bivariate correlations between traits under different N addition rates. Only SLA–LDMC was found to differ among the various N addition treatments, and other traittrait relationships may be relatively stable because of the weak plasticity of older needles. Additionally, needle age significantly affected the slope of the SLA–LDMC relationship and the elevation of the LDMC–LPC and NSC–LNC relationships, indicating differences in the ecological strategies of needles at different ages. In summary, our findings emphasize that trait–trait relationships in current-year and older needles were inconsistent in response to N addition and that the optimal allocation of resources in needles was based on their functional needs during leaf aging.