Quick facts
- Emerald ash borer is an invasive insect that threatens Minnesota’s ash trees.
- Managing ash woodlands should focus on maintaining forest cover.
- Planting non-ash trees early can help maintain forested cover, even if you lose your ash.
- A well-planned harvest can help maintain a healthy future forest.
Ash is a common and important tree in Minnesota. In addition to being widely planted in urban areas and communities, our forests contain over a billion ash trees. The most common ash species in Minnesota woodlands are black ash (Fraxinus nigra) and green ash (Fraxinus pennsylvanica) up north. White ash (Fraxinus americana) is more common in southern Minnesota.
Emerald ash borer (EAB) is an invasive, non-native beetle that kills ash trees. It is moving across Minnesota, threatening the state’s vast woodland ash resource.
In northern Minnesota, black ash is the most common ash species growing in wet forest ecosystems. Typically, these woodlands have many ash trees and few other species. There is concern that if ash is wiped out, a rising water table might lead to long-term replacement of forest cover with swamp or brushland habitat.
Large-diameter ash trees are more attractive to EAB. Woodland owners with many large-diameter ash trees may focus management on maintaining tree cover when dealing with EAB, even if the ash component of these stands is lost.
Management should focus on keeping all forested sites forested. Planting a variety of trees and increasing the number of non-ash trees before EAB arrives can help maintain forest cover and possibly slow the spread of EAB.
Management options
It is important to fully understand the characteristics of your woodland before selecting a management technique. When replacing ash, consider the quantity and quality of the existing ash trees.
Woodland owners need to understand their own goals and specific site conditions before making decisions about timber harvesting and regeneration. Foresters and other natural resource professionals can conduct a woodland inventory and help prepare a management plan.
Measurements for a woodland inventory:
- Tree measurements such as diameter, height, species, age and volume.
- Stand measurements such as timber volume and stocking (size and spacing of trees across the site).
- Site measurements such as depth to the water table and site quality.
- The proportion of mature trees that are ash versus non-ash species.
- The number of existing seedlings of non-ash species on the site.
These variables help in understanding the potential of the forest. The same measurements can also help to determine wildlife habitat and placement of recreational trails.
Once the inventory is done, management methods should be applied on time to lessen or eliminate the effects of EAB.
- Try to harvest healthy trees before an attack from EAB.
- Create space for non-ash seedlings to put down strong roots and grow more quickly.
- Limit the number of ash trees in the woodland to control the spread of EAB.
Choosing a specific timber harvesting and regeneration method depends on the quality and quantity of timber produced by the forest. Higher-quality woodlands on well-drained soils with faster-growing ash trees have more management options compared to lower-quality sites. A forester can help you understand your options.
- The focus of timber harvesting is to help transition to a woodland more resilient to EAB.
- The type of timber harvest and regeneration method chosen is collectively called a silvicultural system.
- The way trees are removed affects the future of the woodland.
Ash is often found in lowland areas in Minnesota, especially up north. In woodlands with more ash, harvest activities should be carried out only when the ground is frozen or dry.
- Consider how the water table will be affected before choosing an appropriate harvesting method.
- Wetter sites have a greater chance of the water table rising after harvesting and have a higher risk of “swamping.”
Clearcut
- Harvest most trees in a stand, except reserve trees as recommended by your forester and Minnesota’s forest management guidelines. Clearcutting opens up a lot of growing space at once.
- Avoid clearcutting on wet sites due to the risk of increased water table rise.
- Clearcutting in strips that are harvested over two or three cuts may reduce water table rise and deter browse from white-tailed deer on non-ash species. Retaining some mature trees will likely reduce water table rise.
Seed tree
Retain mature trees at low densities as seed sources to populate a future forest.
- Seed tree harvesting opens up a lot of growing space at once, helping established seedlings grow more quickly.
- Seed tree harvesting is generally not recommended on wet sites due to the risk of swamping.
Shelterwood
Harvest all trees over two or three cuts to gradually open up growing space and encourage advanced regeneration before a final clearcut.
Consider for wet sites if:
- Other non-ash species are present in the canopy.
- Existing seedlings are abundant and healthy.
- Implementing a shelterwood in strips, and keeping a large number of mature trees to help maintain water balance.
Single-tree selection
Individual trees of all sizes and ages are periodically removed to form an uneven-aged stand.
- Opens up relatively little growing space, maintaining mature forest cover throughout the woodland.
- Consider using in floodplains or mesic hardwood forests on dry or frozen ground.
Group selection
Trees are removed in small groups (almost like mini-clearcuts) up to 1/2 acre in size.
- Local research suggests that group selection can be effective on wet sites to minimize water table rise and transition to non-ash species.
- Group selection can also be used in floodplains or mesic hardwood forests on dry or frozen ground.
Because timber harvesting makes more growing space available, harvesting and regeneration are interconnected. Because native ash currently has no defense against EAB, regenerating non-ash species is recommended to maintain resilient woodlands for the future.
Regeneration is the process of replacing trees in a woodland with new trees. A woodland can be regenerated in three ways:
- Natural seeding
- Stump sprouting
- Planting new seeds or seedlings
Natural seeding
Natural seeding from nearby trees is an example of natural regeneration. Talk with a forester and carefully observe what non-ash species are thriving in or near your ash woodland. Consider helping them spread by creating growing space near those trees and collecting and dispersing seed throughout your woodland.
Stump sprouting
Stump sprouting is also an example of natural regeneration. Ash, oak, maple and birch trees are some tree species that can reproduce naturally through stump sprouting. Stump sprouts grow from exposed stumps and are genetically identical to the parent tree. They tend to grow fast because they already have a large root system and stored energy.
It is important to note that black ash trees generally produce several stump sprouts after harvest.
Planting new seeds or seedlings
Planting new seeds or seedlings is an example of artificial regeneration.
Artificial regeneration is more expensive than regeneration from natural seeding but allows you to select the appropriate tree species, genetic characteristics and tree spacing by planting.
Replacement trees can be selected depending on site characteristics and the native plant community of the woodland.
Finding help
The first step is to contact a local forester to help you understand your options. Because ash prices are relatively low and harvesting can be a challenge, it may be hard to find a logger, particularly for small harvests far from a road or on particularly wet sites.
Cost-sharing may be available to help with ash diversification. A good place to start is contacting a local Minnesota Department of Natural Resources Cooperative Forest Management forester or another local landowner service provider.
D’Amato, A.W., Palik, B.J., Slesak, R.A., Edge, G., Matula, C. & Bronson, D.R. 2018. Evaluating adaptive management options for black ash forests in the face of emerald ash borer invasion. Forests 9: 348.
Great Lakes Silviculture Library. Case studies from ash forest cover types.
Slesak, R.A., Lenhart, C.F., Brooks, K.N., D’Amato, A.W. & Palik, B.J. 2014. Water table response to harvesting and simulated emerald ash borer mortality in black ash wetlands in Minnesota, USA. Canadian Journal of Forest Research 44: 961-968.
University of Minnesota Extension. 2019. Ash Management Guidelines for Woodland Owners. University of Minnesota. St. Paul, MN, 68 p.
Reviewed in 2024