Riparian forest soils fairly resistant to Emerald Ash Borer infestations

What do you need to know?

The Emerald Ash Borer is an invasive insect pest that once it attacks an ash tree, the ash tree has a very low chance of survival. Ash trees growing in riparian forests, which are forests growing close to a body of water, play an important role in regulating nutrient and particulate flow to the nearby aquatic ecosystems. Emerald Ash Borer infestations can dramatically change the structure of a forest. In this study, researchers found that despite the structural changes to forests, forest soils are fairly resistant to the devastating effects of Emerald Ash Borer infestations. This soil buffering helps to mitigate the effect of the Emerald Ash Borer on adjacent streams. 

Why is this research important?

Emerald Ash Borer, Agrilus planipennis, is a small metallic green beetle. The Emerald Ash Borer is originally from Asia, but invaded North America in 2002. Since then, it spread steadily across southwestern Ontario and other parts of Canada. It is the main reason for many dead ash trees that are lining city and suburban streets. The main damage to ash trees is done by the larvae of the Emerald Ash Borer. The larvae feed between the bark and the sapwood. The sapwood is important for the transportation of water and nutrients within a tree. This damage to the sapwood is what ultimately kills ash trees when they are infected by Emerald Ash Borers.

Ash trees don’t only grow in cities but occur throughout forests and they are especially abundant in riparian forests. Riparian forests are defined as forests that grow adjacent to a body of water. As such, riparian forests live in close connection to the water, and play an important role in regulating the flow of nutrients and of particulate material into the aquatic ecosystem.

Emerald Ash Borer infestations result in very high ash tree mortality. These dead ash trees can lead to significant changes in the forest ecosystem. Dead trees do not produce leaves and eventually will topple over. The standing and fallen dead trees create large light gaps inside the forest canopy. These canopy gaps enable much more light to reach the forest floor, and this, in turn, affects many other important ecosystem processes within the riparian forest but also potentially within the adjacent waterways.

Imagine, for example, the reduction of litter that results from having dead trees in a forest. Leaf litter brings nutrients back into the soil. Therefore, less leaf litter could result in fewer nutrients, and because of the intimate connection between riparian forests and streams, this could also lead to fewer nutrients to fuel productivity of a stream, and ultimately translate into less fish abundance.

What did the researchers do?

Researchers established plots of 5m x 5m dimension in two riparian forests in Southwestern Ontario. One site was located close to Essex, Ontario. At this site, Emerald Ash Borer killed a significant amount of ash trees eight to ten years ago. The second forest was located near Glencoe, Ontario, where ash trees experienced severe mortality due to Emerald Ash Borer infestation one to three years ago. For both forests, they chose plots that were not affected by ash mortality and compared them to plots that had significant ash mortality. This allowed them to compare short and long-term effects on soil nutrient dynamics.

For all the plots, they measured several important parameters:

• Open canopy percentage to indicate how much tree canopy was “missing” in a given plot
• Amount of litterfall to estimate how much litter is produced in the different plots
• Amount of herbaceous vegetation growing on the forest floor
• Amount of carbon and nitrogen found in the soil as an indicator of how nutritious the forest soil was and to measure the potential of nutrients leaching to adjacent streams
• Soil nitrogen mineralization which measures how successfully microbes are converting nitrogen into a format that can be taken up by plants

What did the researchers find?

Forest plots that experienced Emerald Ash Borer mortality had a more open forest canopy, resulting in more herbaceous ground vegetation. Part of that observed herbaceous vegetation was regenerating ash trees. However, it is predicted that ash trees will only be able to persist in shrub form ten years or so after the pest invasion before becoming infected again.

In the plots that experienced high ash tree mortality, there was also generally a reduction in leaf litter. That reduction in litterfall did result in less flux of nutrients to the forest soil. But interestingly, despite that reduction in nutrient fluxes, there was no difference measured in the soil nitrogen and carbon content. The fast and prolific growth of the ground vegetation might be able to take up nitrogen from the soil and prevent leaching into the ground and surface water. That will also help to maintain nitrogen in the riparian forest system and ultimately provide a buffer for soil nutrient content. They also didn’t find any difference in soil nitrogen mineralization rates.

Forest plots that experienced Emerald Ash Borer mortality had a more open forest canopy, resulting in more herbaceous ground vegetation. Part of that observed herbaceous vegetation was regenerating ash trees. However, it is predicted that ash trees will only be able to persist in shrub form ten years or so after the pest invasion before becoming infected again.

In the plots that experienced high ash tree mortality, there was also generally a reduction in leaf litter. That reduction in litterfall did result in less flux of nutrients to the forest soil. But interestingly, despite that reduction in nutrient fluxes, there was no difference measured in the soil nitrogen and carbon content. The fast and prolific growth of the ground vegetation might be able to take up nitrogen from the soil and prevent leaching into the ground and surface water. That will also help to maintain nitrogen in the riparian forest system and ultimately provide a buffer for soil nutrient content. They also didn’t find any difference in soil nitrogen mineralization rates.

Overall, the researchers concluded that even though the Emerald Ash Borer causes significant damage to forests and leads to structural changes, the riparian forest soil seems surprisingly resistant to the effects of that pest.

About the researchers

Dr. Paul Sibley is a faculty member at the School of Environmental Sciences (SES), University of Guelph, ON, Canada. David Dutkiewicz is a former SES M.Sc. Student and now works as an Entomologist at the Invasive Species Center, Great Lakes Forestry Centre, Natural Resources Canada, Canadian Forest Service in Sault Ste. Marie, ON, Canada. David Kreutzweiser and Paul Hazlett work for the Great Lakes Forestry Centre, Natural Resources Canada, Canadian Forest Service, Sault Ste. Marie, ON, Canada.

Keywords

Emerald Ash Borer; Riparian forest; Agrilus planipennis; Fraxinus; invasive species, forest pest; soil nutrients\

Citation

Sibley, P. K., Dutkiewicz, D., Kreutzweiser, D. P., & Hazlett, P. (2020). Soil and Nutrient Cycling Responses in Riparian Forests to the Loss of Ash (Fraxinus spp. L) from Emerald Ash Borer (Agrilus planipennis, Fairmaire). Forests, 11(5), 489. https://www.mdpi.com/1999-4907/11/5/489