Abstract: Aims Insect herbivory leads to leaf damage in subtropical forests, but it remains unclear whether nitrogen (N) and phosphorus (P) additions alter the insect resistance of saplings. Methods In this study, we conducted an eight-year nitrogen and phosphorus addition experiment in an evergreen broadleaf forest to determine effects of N and P on leaf herbivory damage, leaf stoichiometry, mechanical resistance, and chemical resistance of saplings. Important findings Saplings exhibited strong resistance to herbivory, with an average leaf damage rate of 5.4%, which was unaffected by N addition, P addition, or combined N and P addition. However, significant interspecific differences were observed, with light-demanding species showing significantly higher damage rates than shade-tolerant species. N addition and combined N and P addition significantly increased leaf nitrogen concentration per unit mass but did not significantly alter leaf stoichiometric ratios. P addition significantly increased leaf phosphorus concentration and decreased the carbon-to-phosphorus ratio. Leaf mechanical resistance such as force to tear and force to punch remained unchanged across all treatments. In contrast, N addition significantly increased amino acid content, and P addition elevated total phenolics, while tannin levels remained stable. Notably, light-demanding species exhibited higher damage due to higher leaf N concentration, whereas shade-tolerant species exhibited lower damage due to stronger mechanical resistance and chemical defense. These findings suggest that saplings in subtropical forests maintain high resistance to background insect herbivory pressure through stable physical defenses and flexible chemical defenses, potentially enhancing the ecological stability of community natural regeneration under global change.

Key words: long-term nutrient additions, in-situ experiment, leaf chewing, secondary metabolites.