Sex-specific divergence in leaf phosphorus economy in Fraxinus velutina Torr. under short-term simulated nitrogen deposition

Atmospheric nitrogen (N) deposition can substantially alter nutrient balance and internal nutrient recycling in trees, yet how dioecious species regulate phosphorus (P) economy in a sex-specific manner under N enrichment remains poorly understood. We investigated how short-term N enrichment differentially modifies leaf functional traits, leaf stoichiometry, and N and P resorption efficiencies (NRE and PRE) in female and male Fraxinus velutina Torr. using a field canopy-spray experiment with two N addition levels. Although N addition consistently promoted leaf structural traits, neither sex nor the N × S interaction significantly affected leaf morphological traits, suggesting that sexual dimorphism in this species manifests primarily in nutrient economy rather than leaf structure. Under N enrichment, P-related traits exhibited pronounced sex-specific responses, with significant N × S interactions: under high N, females increased leaf P concentration and exhibited lower N:P, whereas males increased leaf N concentration, resulting in higher N:P. Nitrogen addition reduced NRE in both sexes, but PRE responses differed markedly: PRE declined progressively in males under elevated N, while females maintained a moderate reduction in PRE. Standardized major axis and multivariate analyses further showed that nutrient resorption in males was more strongly associated with current-season stoichiometric variation, whereas in females, nutrient resorption showed a stronger association with P status. Together, these findings identify P balance as the primary axis of sex-specific nutrient regulation under N enrichment, with females maintaining more stable internal P homeostasis than males. This distinction underscores the need to account for plant sex in assessments of dioecious tree responses to increasing atmospheric N deposition.