Nitrogen Fertilization Reshapes Nutrient Resorption, Leaf Economic Traits, and Biomass Allocation in Female and Male Populus cathayana

Nitrogen (N) enrichment is increasingly prevalent in terrestrial ecosystems and can profoundly alter plant nutrient-use strategies. However, how dioecious tree species integrate nutrient resorption, growth allocation, and leaf economic traits across nitrogen fertilization gradients remains poorly understood. Here, we examined sex-specific responses of nutrient resorption efficiency, biomass accumulation, root-to-shoot (R/S) ratios, and leaf economic traits, including leaf thickness, leaf mass per area (LMA), and leaf vein density (LVD), in female and male Populus cathayana subjected to four N fertilization levels (0-200 mg kg⁻¹). Nitrogen fertilization markedly increased biomass and net photosynthetic rate while reducing the R/S ratio in both sexes, reflecting a shift toward aboveground investment. Leaf economic traits declined with increasing N supply, indicating a transition toward a fast-return resource-use strategy. Nitrogen resorption efficiency (NRE) decreased significantly under nitrogen enrichment, whereas phosphorus resorption efficiency (PRE) increased and resulted in NRE:PRE ratios < 1 across fertilization levels, revealing a nitrogen-induced phosphorus limitation. Correlation analyses further showed that NRE was positively associated with R/S ratio, leaf thickness, LMA, and LVD but negatively related to total biomass, whereas PRE exhibited opposite trends. These coordinated responses highlight that P. cathayana integrates growth allocation, leaf morphological adjustments, and nutrient resorption to maintain stoichiometric balance under nitrogen fertilization. The findings offer new insights into nutrient-use strategies in dioecious tree species and contribute to predicting plant adaptation under anthropogenic nitrogen deposition.