Lepidoptera are valuable as pollinators and prey while parasitoids are important natural enemies of pests. Relatively little is known about the overwintering ecology of these taxa. However, conservation of the overwintering stage in their life cycle may be vital to protecting threatened populations. One potentially harmful disturbance to overwintering eggs, larvae, and pupa is the autumn removal or shredding of leaf litter in managed environments. Litter disturbance may cause direct mortality or alter habitat processes such as winter temperature buffering. However, a high diversity of canopy trees may mitigate the negative impacts of seasonal litter removal by increasing initial densities of source communities. Here, we use a large-scale tree biodiversity experiment (BiodiversiTREE) at the Smithsonian Environmental Research Center to examine the impacts of leaf litter addition, removal, and shredding on spring-emerging lepidopteran and parasitoid communities while assessing if a diverse tree canopy can mitigate the negative effects of litter disturbance. We manipulated leaf litter (control, removal, shredded, addition) in 1x1m quadrats using a randomized block design (48 blocks) within experimental monoculture or polyculture plots of trees. We tracked soil temperature buffering (defined as the difference between the soil and air temperature) and collected spring-emerging insects from each quadrat.
Results/Conclusions
When air temperatures are cold (low < 0°C), litter manipulation altered soil temperature buffering (F3,47=18.27, p=<0.0001). Specifically, removal of leaf litter decreased soil temperature buffering by 0.95°C compared to the control (Posthoc Tukey Test, p=<0.0001), but addition and shredding treatments were not different from the control. When air temperatures are warmer (low > 0°C), litter manipulation also altered soil temperature buffering (F3,47=44.08, p=<0.0001). Compared to the control, leaf litter removal decreased soil temperature buffering by 1.85°C (Posthoc Tukey Test, p=<0.0001) and shredding decreased soil temperature buffering by 0.70°C (Posthoc Tukey Test, p=0.024). Leaf litter addition had no effect. On both cold and warm days, we found that removing leaf litter decreased soil temperature buffering but adding leaf litter had no effect. The buffering mechanism of shredded leaves differed depending on whether the soil was warming or cooling. These findings lead us to predict that spring emergence data will show decreased abundance and a simplified community in litter removal plots due to winter mortality. This study provides landowners with data on the efficacy of litter management as a conservation strategy to increase the ecological value of their properties.