Long-Term Nitrogen Addition Drives Contrasting Nutrient Allocation Strategies in Overstory Poplar Trees and Understory Herbs

Long-term nitrogen (N) addition induces N saturation in forests, but its impacts on nutrient allocation across plant organs and life forms remain unclear. Utilizing a 12-year N addition experiment in a coastal poplar plantation in southeastern China, which included five treatments (0 CK, 50 N1, 100 N2, 150 N3, and 300 N4 kg N ha−1 yr−1), we examined N, phosphorus (P), and potassium (K) concentrations in leaves and roots of overstory poplar trees and understory herbs in 2024. Results showed understory leaf N increased by 14.9–31.4% in vigorous growth, while overstory leaf N rose only 7.0% under the highest N treatment. By late growth, N3 treatment understory herbs had 17.2% and 21.9% higher leaf and root N, respectively. In contrast, overstory root N decreased by 15.7% under N1, but root P increased by 55.4% under N2. These findings reveal that understory herbs preferentially allocate nutrients to leaves, whereas overstory poplar trees shift allocation to roots under elevated N. Such divergent strategies may reshape interlayer nutrient competition, thereby affecting forest structure and function and providing theoretical and practical guidance for predicting N addition responses and plantation management.