Increased nitrogen deposition may benefit male poplars over female conspecifics through enhanced resistance to both specialist and generalist herbivores

The deposition of atmospheric nitrogen has globally increased and has interfered with plant growth and resistance to herbivores. Previous studies have shown that numerous dioecious plant species exhibit sex-specific responses in growth and tolerance to increased atmospheric nitrogen deposition. However, whether these changes lead to sexual differences in herbivore resistance and subsequent intersexual competition between male and female conspecifics remains unknown. Here, we used female and male siblings of Populus deltoides to investigate the effect of simulated nitrogen deposition on herbivore resistance and competitive ability of male and female plant conspecifics under controlled and field conditions. We showed that simulated nitrogen deposition significantly increased the growth of both plant sexes, with females outperforming males. The herbivore feeding bioassays demonstrated that simulated nitrogen deposition decreased the resistance of both plant sexes to generalist and specialist herbivores, with female plants exhibiting lower resistance than males. This could be attributed to decreased levels of leaf structural defense in females in response to simulated nitrogen deposition. A short-term competition test in the climate chamber revealed that simulated nitrogen deposition increased the competitive ability of females over males, whereas herbivore feeding reversed this advantage. A long-term competition experiment in the field further confirmed that male plants may benefit from increased atmospheric nitrogen deposition through enhanced herbivore resistance and intersexual competition compared to their female conspecifics. The findings provide potential implications for the selection of suitable sex of dioecious plants during forestation in natural habitats experiencing high levels of atmospheric nitrogen deposition.