Seasonal differences in insect herbivory on woody plant seedlings along a subtropical elevational gradient

<b>Premise: </b>Theories of plant-herbivore interactions hold that seedlings are more vulnerable to herbivory in warmer and more stable climates at lower elevations. Hypotheses of plant apparency, resource concentration, and resource availability have been proposed to explain the variability in leaf herbivory. However, the seasonal differences of leaf herbivory on seedlings remain unclear.<b>Methods: </b>To address this issue, we measured the percent and frequency of leaf herbivory in understory seedling communities within a subtropical forest during two seasons (May and October), covering an elevational gradient from 290 m to 1370 m. In total, 2,890 leaves across 696 seedling individuals belonging to 95 species were measured. Beta regression analyses were used to assess the effects of biotic and abiotic factors (leaf area, seedling height, community height, plant diversity, light, and soil nutrients) on leaf herbivory.<b>Key results: </b>Plant seedlings exhibited single-peaked elevational patterns of percent and frequency leaf herbivory. The effects of drivers on leaf herbivory varied in two seasons. Leaf herbivory was influenced by leaf area, seedling height, plant diversity, canopy openness, and soil nutrients in May, while mediated mainly by seedling height, plant diversity, canopy openness, and soil nutrients in October. The findings are only consistent with the resource concentration hypothesis, with lower leaf herbivory on seedling leaves in high plant diversity, but not with the results predicted by the plant apparency and resource availability hypotheses.<b>Conclusions: </b>Overall, our study highlights the importance of seasonal herbivory differences toward a comprehensive understanding of leaf herbivory in subtropical forests.

**研究前提：** 植物-植食者互作理论认为，幼苗在低海拔地区更温暖且更稳定的气候条件下，更易遭受植食作用。目前已提出植物可见性假说（plant apparency）、资源浓度假说（resource concentration）与资源可用性假说（resource availability），用以解释叶片植食率的变异规律，但幼苗叶片植食作用的季节差异仍未明确。
**研究方法：** 为解决该科学问题，本研究于亚热带森林的林下幼苗群落中，在5月与10月两个季节开展叶片植食率百分比与发生频率的测定，样地覆盖了290米至1370米的海拔梯度。本次研究总计测定了隶属于95个物种的696株幼苗个体的2890片叶片。采用β回归分析（beta regression）评估生物与非生物因子（叶面积、幼苗株高、群落高度、植物多样性、光照、土壤养分）对叶片植食率的影响。
**主要结果：** 植物幼苗的叶片植食率百分比与发生频率均呈现单峰型海拔分布格局。各驱动因子对叶片植食率的影响在两个季节存在显著差异。5月的叶片植食率受叶面积、幼苗株高、植物多样性、冠层开度（canopy openness）及土壤养分的调控，而10月的叶片植食率则主要受幼苗株高、植物多样性、冠层开度及土壤养分影响。本研究结果仅符合资源浓度假说：在植物多样性较高的样地中，幼苗叶片的植食率更低，但并不符合植物可见性假说与资源可用性假说的预测结果。
**研究结论：** 总体而言，本研究强调了季节间植食作用的差异，对于全面理解亚热带森林叶片植食格局具有重要意义。