Determinants of herbivory on seedlings in a subtropical forest: Insights from plant apparency, Janzen-Connell effects, and functional traits

Herbivory is widely known to play a critical role in structuring plant communities. However, much less is known about what factors determine herbivory in forest ecosystems. Here, we monitored herbivory damage on more than 2000 seedlings in a subtropical forest (2022 and 2023), to identify the key biotic drivers of interspecific variation in seedling herbivory. We found a positive correlation between seedling height and herbivory, evidence for the “plant apparency” hypothesis. Both increasing proportion of herbivory on local conspecifics and conspecific adult density were associated with higher herbivory on focal seedlings, evidence for the Janzen-Connell hypothesis. Interestingly, interactions with ants, as an example of interspecific mutualisms, were found to increase herbivory damage, while decrease seedling mortality in the same research system, highlighting the pivotal role of biotic mutualisms in mediating plant-herbivore interactions. Furthermore, leaf functional traits accounted for the greatest proportion of variance in seedling herbivory, indicating that they served as the primary determinant regulating herbivory pressure at the seedling stage. Overall, our study provides insight into the intricate interplay of seedling height, Janzen-Connell effect and functional traits in determining seedling herbivory and consequently assembly of forest ecosystems. These findings advance our understanding of subtropical forest community dynamics and provide empirical support for evidence-based conservation strategies in such ecosystems. • Seedling height exhibited a positive effect on seedling herbivory, evidence for the “plant apparency” hypothesis. • Plant chemical content and the synthesis of secondary metabolites could strongly influence standing herbivory. • Higher density of heterospecific ant-associated seedlings generally reduced standing herbivory on focal seedlings, indicating that ants may indirectly protect plants located near ant-associated seedlings.