Vertical stratification of leaf physical traits exerts bottom–up pressures on insect herbivory in a sugar maple temperate forest

Do light vertical gradients in temperate forest structure insect herbivore communities? We tested the hypothesis that the increase in light intensity from understory to forest canopy drives differences in leaf physical traits and bud burst phenology that impact insect herbivores and thus play a role in structuring both herbivore communities and the leaf damage they cause. Understanding these interactions is essential for addressing knowledge gaps in the dynamics of temperate deciduous forest ecosystems. Twelve sugar maple (Acer saccharum) sites were monitored in southern Quebec, examining insect herbivore patterns from understory saplings to mature tree‐shaded and sun canopy (where intensity is highest and canopy cover lowest) over the summers of 2020, 2021 and 2022. Additionally, we recorded leaf physical traits and sun exposure. Our findings revealed that leaf thickness increased along the vertical gradient in 2021, making mature tree leaves in the canopy less favourable to herbivores than understory sapling leaves. Accordingly, we recorded a consistent decrease in insect herbivory damage rates from understory to shaded and to sun canopy in 2020 and 2021, driven by leaf cutters, skeletonizers, stipplers and leaf miners. These results support our hypothesis that variation in leaf physical traits contributes to the vertical stratification of insect damage. This variation in leaf traits can be linked to light levels or to tree ontogeny. In 2022, the gradient of insect herbivore abundance corroborated the observed damage trends from the previous years. We calculated an average annual herbivory rate of 9.1% of the leaf surface in our study site. Overall, our study highlights the importance of vertical gradients in structuring insect herbivore communities and emphasizes the role of leaf traits in mediating these interactions. In addition, the average annual herbivory rate suggests limited evidence supporting a significant contribution of background herbivory to the decline of sugar maple forests.