Sex-specific associations between photosynthesis and nitrogen-assimilation traits under short-term foliar nitrogen deposition in dioecious Fraxinus velutina Torr.

Abstract Key Message Foliar nitrogen addition increased leaf growth and nitrogen-assimilation enzyme activities in both sexes of Fraxinus velutina Torr. Trait associations centered on photosynthesis in females and linked photosynthesis more broadly with nitrogen status and water use in males. Context Atmospheric nitrogen deposition is a key environmental change in urban ecosystems, where tree canopies intercept substantial reactive nitrogen inputs. Whether foliar nitrogen input alters associations among photosynthetic, structural, and nitrogen-related leaf traits, and whether these differ between the sexes of dioecious trees, is poorly understood. Aims We examined leaf trait responses to simulated short-term foliar nitrogen addition and compared trait associations between female and male F. velutina . Methods In a 14-year-old pure plantation, female and male trees were exposed to three canopy-applied nitrogen addition treatments (control, 25, and 50 kg N ha −1 year −1 ). We measured leaf gas exchange, morphological and anatomical traits, leaf nitrogen concentration, inorganic nitrogen pools, and the activities of four nitrogen-assimilation enzymes. Treatment and sex effects were assessed using two-way ANOVA, and trait associations were examined using correlations, association networks, and principal component analysis. Results Foliar nitrogen addition increased leaf area, thickness and dry mass, leaf nitrogen and nitrate concentrations, photosynthetic nitrogen-use efficiency, and key nitrogen-assimilation enzyme activities in both sexes. In females, trait associations centered on photosynthesis, leaf structural traits, and glutamine synthetase activity. In males, photosynthesis was associated with a broader set of traits related to leaf nitrogen status, inorganic nitrogen pools, and water use. Conclusion Under short-term foliar nitrogen addition, female and male trees showed different patterns of association among photosynthetic, structural, and nitrogen-related traits, highlighting the importance of considering tree sex when assessing nitrogen responses in urban forests.