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Mixed cultivation with legumes may alleviate the nitrogen (N) limitation of monoculture <i>Eucalyptus</i>. However, how leaf functional traits respond to N in mixed cultivation with legumes and how they affect tree growth are unclear. Thus, this study investigated the response of leaf functional traits of <i>Eucalyptus urophylla</i> × <i>Eucalyptus grandis</i> (<i>E. urophylla</i> × <i>E. grandis</i>) and <i>Dalbergia odorifera</i> (<i>D. odorifera</i>) to mixed culture and N application, as well as the regulatory pathways of key traits on seedling growth. In this study, a pot-controlled experiment was set up, and seedling growth indicators, leaf physiology, morphological parameters, and N content were collected and analyzed after 180 days of N application treatment. The results indicated that mixed culture improved the N absorption and photosynthetic rate of <i>E. urophylla</i> × <i>E. grandis</i>, further promoting seedling growth but inhibiting the photosynthetic process of <i>D. odorifera</i>, reducing its growth and biomass. Redundancy analysis and path analysis revealed that leaf nitrogen content, pigment content, and photosynthesis-related physiological indicators were the traits most directly related to seedling growth and biomass accumulation, with the net photosynthetic rate explaining 50.9% and 55.8% of the variation in growth indicators for <i>E. urophylla</i> × <i>E. grandis</i> and <i>D. odorifera</i>, respectively. Additionally, leaf morphological traits are related to the trade-off strategy exhibited by <i>E. urophylla</i> × <i>E. grandis</i> and <i>D. odorifera</i> based on N competition. This study demonstrated that physiological traits related to photosynthesis are reliable predictors of N nutrition and tree growth in mixed stands, while leaf morphological traits reflect the resource trade-off strategies of different tree species.