Unpacking the effects of tree and stand age as drivers of leaf damage in eastern deciduous forests

Background/Question/Methods

: Insect populations contribute a huge proportion to biodiversity, however, several studies suggest that their populations are declining globally. Declines in insect herbivore populations can affect tree growth and forest demography over time, either directly through alterations in herbivory or indirectly through their effects on leaf microbial damage. The current work on herbivore-tree interactions comes largely from tropical forests or studies conducted in temperate forests during insect population outbreaks. To investigate insect communities and leaf damage in non-outbreak years, we surveyed temperate forest stands ranging from 27 to 286 years old at the Smithsonian Environmental Research Center (SERC) in Maryland, USA across two sampling periods (summer and early fall) in 2021. We deployed flight-intercept traps to assess insect communities across stand ages. Leaves from seedlings, saplings, and mature trees were collected from three tree species common to the mid-Atlantic region: American beech (Fagus grandifolia), sweet gum (Liquidambar styraciflua), and tulip poplar (Liriodendron tulipifera). The percent and type of herbivore damage (chewing, mining, skeletonizing, galls) and percent of microbial damage were visually estimated for each leaf.

Results/Conclusions

: We found that SERC forests support a diverse insect community (104 families) that spans a wide variety of feeding guilds (15 guilds). Insect family richness (R2=0.28, p=0.02) increased with stand age, while evenness tended to decrease (R2=0.17, p=0.07). Total insect abundance increased with stand age (R2=0.19, p=0.059), however, herbivore abundance was not related to stand age. Leaf damage by insect herbivores differed among tree species and life stages (X2(4)=16.5, p=0.0024). Herbivore damage was lowest on seedlings. Additionally, herbivore damage was highest on beech in the summer and tulip poplar in the fall. Sweet gum herbivory increased with stand age, regardless of tree life stage, but we found no relationship for beech or tulip poplar. Microbial damage differed by species and life stage (X2(4)=123.9, p< 0.001), with the lowest damage on beech. Specifically, beech seedlings had higher damage than saplings and mature trees, while sweet gum and tulip poplar seedlings had lower damage compared to other life-stages. Overall, our work provides an important baseline of tree leaf damage in temperate forests during a non-outbreak year, which is critical for tracking future consequences of expected insect declines in this widespread and functionally important ecosystem.