Nutrient resorption rather than root acquisition offsets increased N limitation induced by the stand development of a secondary Pinus tabuliformis forest in North China.

Plants allocate nitrogen (N) and phosphorus (P) to green leaves through a cost-optimal trade-off between root acquisition and leaf resorption, which affects plant nutrient limitation and cycling. However, our understanding of the nature of this positive trade-off under nutrient limitation in forests with stand development under N deposition is limited. We carried out experiments along age sequences of 42, 55, and 65 in a P. tabuliformis forest under five N treatment levels (0, 5, 10, 15 and 20 g N m− 2·yr− 1). We multiplied a multivariate functional variable, INPS (the integral N: P stoichiometry of plants), which reflects the overall nutrient limitation in plants, to explore its response strategies under N treatment during stand development. We found that the P. tabuliformis forest experienced progressive N limitation with stand development under N deposition, as evidenced by the decreased N: P ratios across needles, twigs, and fine roots, and the whole plant (INPS). Thus, to cope with this progressive N limitation and maintain stoichiometric homeostasis, P. tabuliformis forest adopted divergent nutrient utilization strategies for N and P. Regarding the N, the NRE-N (nutrient resorption efficiency of N) did not decrease significantly as a coordinated response to the declining NAC-N (nutrient acquisition capacity of N) along the chronosequence. Conversely, for P, the relative P limitation weakened relative to N along the age sequence in P. tabuliformis forest led to a compensatory trade-off between NRE-P (nutrient resorption efficiency of P) and NAC-P (nutrient acquisition capacity of P), consistent with nutrient-economy and adaptive strategies. N deposition generally alleviated N limitations in P. tabuliformis forest yet significantly decreased the NRE-N and NRE-P across all age classes. However, the ratio of NRE-N: NAC-N exhibited a substantial increase along the chronosequence and negatively linearly correlated with the INPS. These findings indicate that NRE-N acts as a compensatory mechanism to offset age-related progressive stand-level N limitation. Our results suggest that resorption relieves nutrient limitations, a strategy used broadly by woody plants, providing a comprehensive framework for understanding forest adaptation strategies in a changing environment.