Loss of Black Ash from Emerald Ash Borer Infestation: Operational Studies in Minnesota and Michigan

There are 2.1 billion ash trees across Minnesota, Michigan, and Wisconsin, which are susceptible to emerald ash borer infestation. Black ash (Fraxinus nigra), which dominates wetland areas in Minnesota and Michigan, are equal if not more susceptible to emerald ash borer compared to other ash species. The loss of black ash in black ash wetlands, where 40 to 100 percent of the forest canopy is composed of black ash trees, is leading to substantial changes in forest structure, water levels (including an increased risk of flooding), nutrient cycling, carbon sequestration, and wildlife habitat.

Following the discovery of emerald ash borer in Minnesota in 2009, scientists at the Northern Research Station (NRS) installed an experiment at the Chippewa National Forest in northern Minnesota that simulated the short-term effects of emerald ash borer infestation by girdling black ash trees and testing adaptive management strategies to increase the resilience of these wetlands. NRS scientists also established a companion study at the Ottawa National Forest in Michigan in 2010 that used a similar girdling treatment to assess short-term effects but also included a harvest with logs left in place treatment to simulate long-term effects. Both studies have since offered insights into the effects of black ash loss due to emerald ash borer and management strategies that might be effective.

Key Findings

• The loss of black ash due to emerald ash borer is expected to increase the water table in black ash wetlands, which increases the potential for flooding and creates a challenging environment for tree regeneration.
• Black ash leaf litter is high in nitrogen; without it, nutrient cycling is expected to slow down and the quality of the habitat provided by litter to wildlife is expected to decline.
• Black ash loss leads to increases in emissions of carbon dioxide and methane, two greenhouse gases that contribute to climate change.
• Plantings of American elm, swamp white oak, red maple, and silver maple were found to survive better following black ash loss in these experiments than other tested tree species, especially if planted on high spots (hummocks). 

Photo Credit

USDA Forest Service photo by Brian Palik

Black ash trees girdled to simulate impacts of emerald ash borer 

Approach

Black ash wetlands in Minnesota and Michigan tend to differ in size and location on the landscape. For example, black ash wetlands in Minnesota tend to be large expanses, 10 to 150 hectares (25 to 370 acres), whereas black ash wetlands in Michigan tend to be smaller (1 to 2 hectares, or 2.5 to 5 acres) and located within depressions. Installing operational studies in both states allows for a comparison of black ash wetlands across the different conditions in which they can exist.

The operational study at the Chippewa National Forest in Minnesota, installed in 2009, examines several approaches to managing in anticipation of EAB invasion. Treatments implemented in this study, which were each replicated eight times, include:

1. Clearcut with removal of all trees
2. Group selection (e.g., harvesting 0.04-hectare gaps constituting 20% of the plot)
3. Short-term emerald ash borer mortality (i.e., girdling all black ash trees greater than 6 centimeters in diameter)
4. Control (i.e., unharvested forest)

In addition to simulating short-term effects of emerald ash borer infestations, the operational study at the Ottawa National Forest in Michigan, installed in 2010, also simulates long-term effects of infestations. In this long-term treatment, they cut the black ash trees but left them on site. Treatments implemented in this study, which were each replicated nine times, include:

1. “Girdle”, i.e., short-term emerald ash borer mortality (girding all black ash trees greater than 2.5 centimeters in diameter)
2. “Ash Cut”, i.e., long-term emerald ash borer mortality (felling all black ash trees greater than 2.5 centimeters in diameter with a chainsaw and leaving them on site)
3. Control (i.e., unharvested forest)

Outcomes

These experiments provide insights into the ecological effects of emerald ash borer infestation on black ash wetlands.

Current results suggest that the loss of black ash:

• will lead to an increase in herbaceous and woody shrub cover, which competes with regenerating seedlings potentially leading to non-forested wetlands.
• will lead to an increase in water table, which could lead to increased flooding.
• increases in greenhouse gas emissions.is expected to change nitrogen cycling rates due to the loss of nitrogen-rich black ash litter, though so far, no significant effect of black ash mortality on nitrogen cycling has been detected.

These experiments are also testing management strategies in light of expected ash mortality. Results from Minnesota suggest that a combination of forest harvesting and understory planting may help black ash wetlands maintain their function and ecosystem services. Suitable replacement species identified in these studies include American elm, silver maple, and swamp white oak. Similar studies in Michigan and northern Wisconsin testing the effect of microsites on seedling survival indicate similar results for American elm and silver maple, but also suggest that red maple and northern wide cedar are viable black ash replacement species, and that seedlings planted on high spots or hummocks have twice the survival than those in hollows.