Sap Rising and the Borer Returns

Ferns and forest vegetation grow between black ash trees
Ferns and forest vegetation grow between black ash trees
Even in areas where the emerald ash borer hit the hardest, ash has persisted on the landscape.

The Emerald Ash Borer (EAB) is a widespread ecological pest. But in his guest blog, Joe Shannon highlights why ecologists focus on solutions rather than fixating on the problem.

Spring has finally sprung, and with it, EAB. No doubt ash is in trouble, but ongoing research and examination of historical records have yielded tools and actions for landowners, managers and city officials to respond to this invasive species.

Emerald Ash Borer

"Don't move firewood, it bugs me!" These signs and bumper stickers are a familiar sight around the Great Lakes region and beyond. There are a number of invasive species that can hitch a ride in firewood, but one of the best known is Emerald Ash Borer (EAB).

EAB was first identified near Detroit, Michigan in 2002. After a steady crawl and some big leaps, EAB is in 31 states and two Canadian provinces. All North American ash species are susceptible to EAB and that has people worried. It’s not hard to understand why; people see towns cut and chip all the ash trees along their streets, assuming they must be gone for good. The worry leads people to respond with panic, or with apathy. But maybe there are reasons not to panic.

"There is value in action, but now is not the time to panic."Rob Venette, US Forest Service, Northern Research Station

The key to avoiding panic and apathy is knowledge and action. Ash trees can grow in mixed-upland forests, ash-dominated wetlands, floodplains, streets, lawns and parks. Lessons learned from one species or in one setting may not translate directly to others. So, what can be done about EAB? It turns out there are a lot of answers.

Gathering the answers

Last summer, the School of Forest Resources and Environmental Science at Michigan Tech partnered with the US Forest Service Northern Research Station (USFS-NRS), University of Vermont, University of Minnesota, the Fond du Lac Band of Lake Superior Chippewa (FLBLSC), the Lake Superior National Estuarine Reserve (LSNERR) and the Minnesota Forest Resources Council to bring together more than 170 foresters, ecologists, wildlife and fisheries managers, entomologists and social scientists, from seven states, seven tribes and three countries for a workshop in Duluth, Minnesota. These researchers and managers walk through forests, wade in wetlands, survey the public and comb through historical timber harvest records, trying to determine the impacts of EAB and suggest useful actions. Many of the researchers contributed their work to an upcoming special issue of the journal Forests. This issue will be publicly available and provides a single reference for many of the questions around ash and EAB.

The attendees discussed a range of topics, from native ash resistance to silvicultural treatments to the best approaches in public spaces. All discussion led to actions that can slow the spread of EAB, prepare forests and trees before infestation and respond post-infestation.

Ash is here to stay

Even in areas where EAB hit the hardest, ash has persisted on the landscape. Sometimes the ash is fighting to survive, as stump sprouts and root suckers, like the green ash Daniel Kashain (Wayne State University) studies in southern Michigan. In other places, researchers including Kathleen Knight (USFS-NRS) and Charlie Flower of the University of Illinois, Chicago, are finding surviving native trees, which are already being incorporated into a plant breeding program for resistant stock (report a survivor). Directly treating individual trees with pesticides will also allow an arborist to protect a handful of significant trees along a street, in a lawn or on private forest land.

a group of people stand in semicircle in an open meadow

By taking a step back and looking beyond the threatened trees to the desired function and form of the forest, we can make better management decisions in the face of EAB and other invasives.

Slowing down EAB

After learning that ash is not going extinct in the next year, most people’s minds turn towards slowing the spread of EAB. Monitoring techniques for EAB can include insect traps, branch sampling and visual inspection. Mark Abrahamson from the Minnesota Department of Agriculture showed how efficient visual inspection is to detect infestation early by looking at woodpecker damage and dying crowns. Removing newly infested trees is one of the best direct actions to slow the spread of EAB.

Another direct action is the controlled release of non-native wasps that feed on eggs or larvae of EAB. Research by Jonathan Osthus from the Minnesota Department of Agriculture, has shown that some of these parasitoids can be effective at reducing EAB population growth, and Jian Duan from the USDA Agricultural Research Service, found that native parasitoids are responding to EAB and can play an important role.

So does temperature. Cold winters are often assumed to limit EAB, especially in areas where they’re proud of their harsh winters (ahem…Northern Minnesota). While extreme cold does slow infestation, it doesn’t eradicate EAB, so managers there need to stay vigilant; one warm winter can let them spread.

Mixed forests are built to respond

The response of trees, shrubs, and herbaceous plants to EAB-induced mortality is variable, and depends on how much ash is present, how wet or dry is the site, what other species are already present and a host of other factors. Trees in stands with fewer ash are more susceptible to EAB than trees in more ash-dominant stands, but mixed forests are best equipped to deal with change. EAB mortality takes one to four years for a given tree; during that time, surrounding trees expand their growth and saplings and sub-canopy trees begin to grow into the canopy. The composition of the forest shifts, but the function of the forest can remain the same.

Silviculture can prompt the response we want

Of particular interest to many at the workshop was black ash, which is often dominant in wet, seasonally inundated sites, where loss of the ash canopy could shift the site from forested wetland to marsh. Species diversification is needed to retain these often expansive forested wetlands. Resiliency can be added using commercial timber harvests and intermediate treatments to promote non-ash species. Projects led by Dustin Bronson of the Wisconsin Department of Natural Resources (WDNR), Tony D’Amato of the University of Vermont, Colleen Matula (WDNR) and Justin Pszwaro of The Nature Conservancy, are among researchers nationwide looking at various silvicultural practices for black ash through controlled experiments, and evaluation of the results of past and current timber sales. Understanding which techniques best prepare ash forests for EAB helps guide future management. People like Greg Edge of the Wisconsin DNR and John Kotar of Terra Silva Consulting, have developed worksheets and site classifications to evaluate actions for a given stand.

people sit at rows of tables looking at a screen with a forest projected on it.

Artificial regeneration can further strengthen sensitive locations

Some forested wetlands are considered vital, and to ensure they retain their current role, may require proactive action such as planting seedlings. The species can be present on the site, or, as Brian Palik of the USFS-NRS explained, climate change projections may dictate other suitable species.

Site visits during the workshop highlighted some of these sensitive forests. The black ash wetlands on the Fond du Lac Reservation, studied by Christian Nelson and Shannon Kesner with FLBLSC, filter and buffer water flowing into wild rice lakes, which are sensitive to water level fluctuations. The floodplain ash in the Superior, Wisconsin Municipal Forest, studied by Nick Bolton from Michigan Tech and Shon Schooler at LSNERR, slow spring flows and reduce sediment transport along the Pokegama River, a tributary to the St. Louis River estuary.

Looking forward

Perhaps the greatest tool in slowing the spread of EAB is public education. Transporting less wood and quickly responding to early infestations will slow EAB. Slower spread will mean more time for researchers and managers to improve recommendations. When action is required in urban settings, the work of researchers like Ingrid Schneider from the University of Minnesota and Chris Wynveen of Baylor University, can serve as a guide to retain the character of a place.

Tony D’Amato gave the final presentation of the workshop summarizing the current successful strategies and the remaining questions. In that presentation, he highlighted how combining controlled experiments with timber harvest implementation and monitoring, is the best way to keep advancing the science of EAB and ash forests.

By taking a step back and looking beyond the threatened trees to the desired function and form of the forest, we can make better management decisions in the face of EAB and other invasives.

"Keep using silviculture to manage forest ecosystems that are threatened by an insect versus just harvesting a threatened tree."Tony D’Amato

The researchers and managers mentioned by name in this article are just a sample of many who shared their knowledge as presenters and participants at this workshop.

Michigan Technological University is a public research university, home to more than 7,000 students from 54 countries. Founded in 1885, the University offers more than 120 undergraduate and graduate degree programs in science and technology, engineering, forestry, business and economics, health professions, humanities, mathematics, and social sciences. Our campus in Michigan’s Upper Peninsula overlooks the Keweenaw Waterway and is just a few miles from Lake Superior.