PS 28-92 - What drives insect herbivory patterns in a sugar maple forest? Vertical stratification of bottom-up pressures on insect herbivores

Background/Question/Methods

The decline of sugar maple (Acer saccharum) forests in Quebec accentuates the importance of studying herbivorous insects as a driving force in forest ecosystems. In this regard, we aim first to evaluate the extent to which herbivores may contribute to this decline and second to document insect biodiversity supported by maples that could be threatened by this decline. For these purposes, we monitored a total of twelve sugar maples within the sugar maple dominated forest in southern Quebec, to investigate the patterns of herbivores in a vertical stratification from understory to canopy. Three sampling sessions occurred in the summers of 2020 and 2021 to consider temporal changes as well. Temperature, relative humidity, and sun exposure were recorded and leaf quality was assessed in 3 strata (sun canopy, shade canopy, and understory) of each tree. The rate of herbivory was quantified through leaf damage analysis, specifically focusing on affected surface percentage and damage type.

Results/Conclusions

Overall herbivory damage on sugar maple decreased moving up from the understory to the shade canopy and sun canopy in 2020 but showed no vertical pattern in the following year. Among the different insect guilds studied, the damage inflicted by leaf cutters and leaf skeletonisers followed the same pattern of decrease with rising height in the canopy in both years of the study, as did damage by leaf stipplers and leaf blotch miners in 2020. This trend is likely explained by the increase in light intensity and leaf thickness moving from the understory to the sun canopy level, making leaves harder for herbivores to feed on. This shows variation in leaf trait (bottom-up force on herbivory) in a vertical gradient of the sugar maple tree. Although sampling periods had no significant effect on the total herbivore damage of leaves in 2020, herbivory damage was significantly increased toward the third sampling in 2021. This seasonal pattern was followed by some damage types in the first year of sampling but was not consistent in the following year. These findings show that vertical stratification of herbivory communities varies in time both within and between years and that different feeding guilds respond differently.