Effects of Simulated Nitrogen and Phosphorus Deposition on Dioecious Populus cathayana Growth and Defense Traits

Human activities have increased the imbalance in atmospheric N and P deposition, which changes soil nutrient availability and subsequently affects the structure and function of terrestrial ecosystems. Dioecious plants are important parts of terrestrial ecosystems and are characterized by sex-related differences in their response to the external environment and always exhibit a skewed sex ratio, which makes them more vulnerable to climate change and increases their risk of extinction. However, little attention has been paid to the effects of unbalanced N and P deposition on these plants, especially on their defense traits. In this study, we used dioecious <i>Populus cathayana</i> to investigate the influence of gradient N and P deposition on the correlation between growth and defense traits. The results showed that although the different rates of N and P deposition enhanced biomass accumulation in both sexes to varying degrees, the most substantial biomass increment was noted under a lower-nitrogen and higher-phosphorus (LNHP) treatment regimen, with females showing an approximately 112% increase and males a 47% increase in total biomass. In response to varying levels of simulated N and P deposition, males and females adopt distinct strategies for biomass allocation. Although declines in root biomass were observed in both sexes as nutrient availability increased, the decrement was more marked in males; under the LNHP treatment, it dropped by about 11%, while under a high-nitrogen and high-phosphorus (HNHP) treatment, the decrease was about 35%. Conversely, females demonstrated a heightened propensity to allocate biomass towards leaf development. Furthermore, with increasing N and P deposition, there was a general reduction in the concentrations of physical and chemical defense substances within the leaves of both sexes. Nonetheless, the correlations between defense substances, nutrient element content, non-structural carbohydrate (NSC) content, and dry biomass were more pronounced in males, suggesting a greater sensitivity to defense substance responses in males than in females. Overall, these results indicate that there is sexual dimorphism in the accumulation of chemical substances in male and female <i>P. cathayana</i> under unbalanced N and P deposition and they provide a technical and theoretical basis for predicting the population dynamics of dioecious plants, maintaining the stability of poplar populations, and constructing high-productivity poplar plantations globally in the future.