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

Do vertical gradients in temperate forest structure insect herbivore
communities?  We tested the hypothesis that the increase in light
intensity from understory to forest canopy level drives differences in
leaf physical traits and budburst phenology that impact insect herbivores
and thus play a role in structuring both herbivore communities and the
leaf damages they cause. Twelve sugar maple (Acer saccharum) sites were
monitored in southern Quebec, examining insect herbivore patterns from
understory to the shaded and sun canopy over the summers of 2020, 2021,
and 2022. Additionally, we recorded leaf physical traits, temperature,
humidity, and sun exposure. Our findings revealed that leaf thickness
increased along the vertical gradient in 2021, making leaves less
favorable to herbivores in the canopy level. Accordingly, we recorded a
consistent decrease in insect herbivory damage rates from the understory
to the shaded canopy and sun canopy in 2020 and 2021, driven by leaf
cutters, skeletonizers, stipplers, and leaf miners. These results support
our hypothesis that variation in plant physical traits due to sun exposure
contributes to the vertical stratification of insect damage. In 2022, the
gradient of insect herbivore abundance corroborated the observed damage
trends from the previous years. Moreover, we calculated an average annual
herbivory rate of 9.1% of the leaf surface in our study site, suggesting
limited evidence supporting a significant contribution of background
herbivory to the decline of sugar maple forests. 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.