By Colleen Matula and Robert Godfrey
Wisconsin Department of Natural Resources
WISCONSIN – Black ash wetland forests in the Western Great Lakes face some dramatic challenges. The arrival of Emerald Ash Borer (EAB), combined with various climate change stressors, presents some unique challenges for forest managers to maintain wetland forest systems.
While black ash trees historically have been a species of low economic value in Wisconsin, they have always provided many ecosystem benefits to rare species, hydrology and water quality. Significantly, black ash has always been of cultural importance for Native Americans. How should we adapt forest management practices to protect them?
Colleen Matula, DNR Forest Ecologist and Silviculturist in Ashland, directs a project funded by the Great Lake Restoration Initiative (GLRI) to develop successful silviculture strategies to maintain resilience in black ash wetland forests in the Lake Superior Basin.
The project is trying to figure out which species can be successfully propagated following the loss of black ash from the landscape due to Emerald Ash Borer, while at the same time determining which of those species are more likely to withstand the effects of climate change. Answering these questions about maintaining forest cover and water flow despite these threats is complicated.
“We’re worried about the loss of forests and impacts to the forested wetland systems,” Matula said. “We need to learn what species will tolerate these sites and whether we can keep these areas forested. Our goal is to stop them from converting to invasive shrubs or grasses.”
Black ash wetland forests are complex ecosystems. Several habitat types exist across the northern portion of Wisconsin. You can find them in large, isolated wetlands and drain-ways along streams. They cover over 700,000 acres in the state, mainly in the north. They are crucial for recycling water and nutrients, and their eventual replacements will require some careful study.
“We see these stressors coming down the pipe, and not all counties in the North have EAB that we know of, but we still have to prepare for the infestation of emerald ash borer in every county and the losses of ash throughout the forest,” Matula stated.
Emerald Ash Borer is a green jewel beetle native to northeastern Asia that feeds on ash species. Females lay eggs in bark crevices on ash trees, and larvae feed underneath the bark to emerge as adults after one to two years. As a flying insect, EAB can infect all ash in its path from one-half mile up to 25 miles a year, depending on winter temperatures. People can also spread them unintentionally by bringing contaminated firewood from an infected area to a non-infected area.
Many northern counties are trying to figure out how to manage lowland ash forests before EAB arrives. The impact to local sites includes flooding and soil erosion and an overall decrease in the forests’ health. One tool that can assess productivity and quality is the Wetland Forest Habitat Type Classification System (www.dnr.wisconsin.gov/topic/forestmanagement/wfhtguide). The classification can help foresters identify productivity by identifying specific types of plants present at the site.
Matula’s project is a five-year project that began in 2019. Black ash sites were selected in four counties: Iron, Ashland, Douglas and Bayfield. Direct seeding and planting various tree species took place on 100 acres of harvested and unharvested black ash wetland forests. The project addresses the effectiveness of planting, prepares the sites for potential change in water flow patterns and increases species diversity by planting a variety of tree species. Initial results and recommendations are expected to be released by 2024.
Replacing species is a complex problem involving multiple issues, from very wet soils to drier areas with good drainage, some with poor drainage, some right along streams and others in wetland areas. One device Matula has been using is a sounding probe that measures the depth of organic and mineral soil layers. Wetland soils are made up of many layers. Understanding more about the depth and composition of these layers can help shed some light on species tolerance and hydrology. This data can help them select the correct tree species for each planting environment.
Each site can have different layers throughout the soil. For example, lowland ash areas usually have a deep organic muck layer. With that probe, we can assess the depth of mineral soil. Some sites have a shallow organic to mineral layer, and their drainage is better. In other locations, deep organic layering can be over five feet deep. Sometimes, the only species that can tolerate that site is ash because it can handle more prolonged periods of flooding.
Not only is the depth of the organic soil significant, but water flow and other features around the wetlands, like landform features, can play an important role. Some of these wetlands were historically hemlock or hardwood stands, not black ash. During heavy logging in the late 1800s, the areas became swamped and would not regenerate back to hemlock, so black ash took over.
Some species are seeded by hand in the winter because they need a cold period before germinating. In Jan. 2021, small two-acre strips were directly seeded in the project area on snowshoes. This technique could be utilized on a larger scale with aerial seeding. Decisions on seeding techniques are based on the soil conditions and how a seed tree species germinates. Certain species were planted as seedlings in the spring.
“We want to see which technique is more efficient, and we’re evaluating the costs of each method, rather than just the success of the species itself,” Matula said. “Sometimes it’s better for a seed to germinate in its natural environment rather than the nursery. We’re testing all kinds of things to see what works and doesn’t.”
Replacement species for this study included tamarack, white birch, red maple, eastern white pine, northern white cedar, black spruce, swamp white oak, American elm, hackberry, yellow birch and balsam poplar. Eastern hemlock is not on the list right now because of a lack of seed or seedling availability and difficulties with germination.
Throughout the project area, deer browsing is also a concern. Foresters set up exclosures (areas where animals are kept away) and non-exclosed areas around different tree species to look at the preferences of wildlife to feed on the leaves, soft shoots or fruits of the various species. They also use bud caps, a piece of paper that protects the tree bud, to deter deer.
“A lot of folks that work in the forested wetlands and waters program are concerned about the loss of ash because we have miles and miles of trout streams that need forested cover,” Matula explained. “When Emerald Ash Borer comes through, it impacts that habitat.”
Hydrology, or water flow, is another critical element with the impending demise of the black ash. When EAB comes through and wipes out the stands, the water table will rise, creating swamp-like conditions. This can cause issues with local water flow, such as the flooding of roads.
Last but not least, black ash provides critical cover for many wildlife species.
“There are important rare species that exist in these areas,” Matula stated. “When we were gathering data this spring, we found several rare species of orchids. We must keep these areas forested with appropriate species that can handle the effects of climate change. In southern Wisconsin, near the Kettle Moraine, many terrestrial invasive species seem to take over these post-EAB invasion areas. These species lower the quality of forested areas and contribute to the loss of available habitats for wildlife and fish.”