In North American forests infested by the emerald ash borer (Agrilus planipennis), downed ash trees lead to significant changes in the forest ecosystem, for years to come. At first, the effects of gaps in the forest canopy are greatest; later, coarse woody debris on the forest floor becomes the greater influence, driving changes in the presence and activity of ground-dwelling invertebrates.

By Kayla I. Perry, Ph.D.

Investigating the ecological impacts of invasive species is an important area of research as the frequency of biological invasions continues to increase globally. In the case of invasive insects, such research contributes to understanding the cascading ecological impacts on native forest ecosystems.

For instance, formation of canopy gaps is a natural process in forests, but the widespread, nearly simultaneous creation of gaps from the invasive emerald ash borer (Agrilus planipennis) is an unprecedented phenomenon. As part of my Ph.D. research, I focused on understanding the effects of this disturbance on ground-dwelling invertebrates and their community structure, diversity, and function in forest ecosystems. With my doctoral advisor, Daniel Herms, Ph.D., at Ohio State University, we proposed a conceptual model that predicts ash tree mortality caused by emerald ash borer (EAB) would result in an inverse relationship over time between the effects of canopy gaps and coarse woody debris from downed ash trees on ground-dwelling invertebrates. In other words, the effects of canopy gaps diminish over time with canopy closure, while the effects of ash coarse woody debris increase and change over time as trees fall and decompose.

We outlined this model in a study published in 2016, and we have followed it up in a new study, published last week in the open-access Journal of Insect Science, in which we quantified the effects of canopy gaps and ash coarse woody debris on ground-dwelling invertebrate communities during late stages of EAB-induced ash mortality.

In short, we found that, as our model predicts, canopy gaps had minimal impacts during the late stages of EAB infestation, but the accumulation and decomposition of ash debris had a more significant influence on invertebrate community structure and composition. Ground-dwelling invertebrate activity-abundance, evenness, and diversity were highest near minimally decayed ash logs, more so than near more decayed ash logs. Moreover, soil moisture emerged as an important factor, often mediating the strength and direction of invertebrate responses to ash debris and stages of decomposition. For example, the activity-abundance of slugs was higher near minimally decayed ash logs in forest plots with drier sandy soil but higher near more decayed logs in plots with more moist soil. Activity-abundance of springtails was lower near more decayed ash logs in dry plots and near minimally decayed logs in moist plots. These taxon-specific responses resulted in distinctly different invertebrate communities near woody debris of the same decay stage in forest plots with dry versus moist soil.

A variety of springtail species were found to be active on the forest floor in a study on the effects of downed ash debris on forest ecosystems after an invasion of emerald ash borers. (Photo credit: Kayla I. Perry, Ph.D.)

This study was conducted in forest stands that experienced nearly 100 percent ash mortality in southeastern Michigan, near where EAB first established in North America. These long-term EAB monitoring plots were established in 2004, two years after EAB was first detected here, and have been used in other studies to investigate the ecological impacts of EAB. As  EAB continues to spread across the continent essentially unabated, forest ecosystems in other areas will progress through the various stages of ash mortality and the ensuing effects of the invasive pest’s destruction.

Our findings suggest that ash mortality caused by EAB can indeed have long-term impacts on forests, beyond the obvious direct effects of ash tree mortality. Moreover, our results highlight the importance of downed coarse woody debris in forest ecosystems. Management strategies that consider the volume and diversity of woody debris decay stages can aid in the maintenance of ground-dwelling invertebrate biodiversity in forest ecosystems. Ground-dwelling invertebrate communities are taxonomically and functionally diverse and contribute to essential ecosystem services, including decomposition, nutrient cycling, and maintenance of soil structure. Because they are sensitive to changes in the environment, these ground-dwelling invertebrates are useful for detecting and characterizing forest responses to disturbance and are often used as indicators of broader ecological changes or trends.

Kayla I. Perry, Ph.D.

Kayla I. Perry, Ph.D., is a postdoctoral researcher at Ohio State University’s Ohio Agricultural Research and Development Center in Wooster, Ohio. Email: perry.1864@osu.edu