Forest Insect and Disease Newsletter

Tackling the black ash management challenge

by Harvey Tjader, DNR

Wet back ash forest

Emeral ash borer Photo by Marianne Prue, courtesy of Forestry images

In the early 1990's, the emerald ash borer was inadvertently introduced in Michigan, probably hitchhiking on imported wood products. By the time it was discovered in 2002, it was well established. Few ash have survived in the areas where emerald ash borer has established, and no native ash trees have shown any resistance.

American entomologists have been trying to find ways to stop the spread of the insect and control its population outbreak. The primary method has been a campaign to stop the transportation of firewood. On its own, the beetle doesn't travel far and its spread would be quite slow if left up to them. It can, however, travel 70 miles per hour down the freeway when firewood is moved by people from an area of infestation to a new spot.

Systemic pesticides can be injected into valuable ash shade trees to provide them protection, but the treatment must be repeated and is not practical in the forest. Introducing Asian insects that parasitize the beetle is another option. Such biological controls cannot be pursued frivolously. Before any introduction, an in-depth study is necessary to ensure that we aren't importing another non-native species that could become a pest.? Biological controls typically do not eliminate the targeted pest, but bring its population into balance with its new ecosystem, establishing a new homeostasis or steady state.

Developing hybrids between the beetle-resistant Manchurian ash and our native black ash is another tactic being examined. This is also a slow process. Such breeding programs have been pursued for decades with eastern white pine, American elm and American chestnut. Climatic factors may help. In a 2010 study conducted by the U.S. Forest Service and the Minnesota Department of Agriculture, in St. Paul and Grand Rapids, Minnesota, 90 percent of emerald ash borer larvae were killed when exposed to -35°F.

Of particular concern are the wet forests where the canopy is overwhelmingly comprised of black ash. Foresters are attempting to address the problem through diversification. They are encouraging other tree species to adopt a larger role, which often involves some harvesting of mature ash trees and planting other species.?

It's a daunting task. You can't just put a paint line around an ash stand and allow a logger to clear-cut it like an aspen stand. In the absence of mature trees that pump water out of the ground and release it to the atmosphere through transpiration, the water table may rise to the point of preventing anything from growing except grasses, sedges and alder shrubs. This is the same result we expect if we do nothing and allow the emerald ash borer to kill the ashes — a transformation of forest to lowland grass or brush. Such a change would not be a mere loss of trees and timber. These wet ash forests are host to an amazing array of flora and fauna, many of which could not survive in a grass or brush environment.

Wet back ash forest

Wet black ash forest. Photo by Harvey Tjader, DNR

Black ash forest management requires a little finesse. A significant number of trees must be preserved on the site to prevent "swamping." Swamping is a hydrologic response that makes the site inhospitable to trees.? Preparing a black ash timber sale requires a forester to mark the individual trees to be cut or provide detailed instructions to the logger regarding which trees to select and reserve.

Harvest must be on frozen ground. Even in Northern Minnesota, the ground under black ash forests does not always freeze in ?winter due to lateral groundwater movement. The soils in these forests are weak and comprised of several inches to several feet of peat or muck lying above the mineral subsoil. Entering the forest with heavy equipment during unfrozen conditions can result in rutting, disturbed groundwater flows and stuck machinery.?

Wet back ash forest

Black ash veneer logs on a Kelliher MN log landing, destined for Korea. Photo by Mike Bater, DNR

Marketing, a task often left to the logger, is also challenging. Black ash timber is not in high demand. Scattered locally-owned sawmills throughout Minnesota buy some black ash, along with other species. They may produce pallets with the lower quality logs and export any veneer quality logs, often to Asia. Exporting logs requires a site visit from a skilled log grader to evaluate and tag each log. An inspector from the U.S. Department of Agriculture must inspect each log to ensure a potential insect pest isn't sent to another country.?

The oldest use of black ash is in basket-making. Developed by American Indians, basket making involves beating on the outside of a log with a mallet or a club. The concussion breaks the cell walls of the porous spring wood, allowing the denser summer wood to be peeled off in long strips. The strips are then woven into baskets. Black ash is the only native species that can be used for this type of basket making, although trials with Manchurian ash have proven successful. This is a small industry, to be sure, but one that is culturally important.

Logging and marketing is only half the job. Planting trees is the next phase of diversifying the wet black ash forest.? Numerous species are indigenous to this habitat, but only black ash tends to dominate. To survive in this habitat, a plant has to have strategies for dealing with a fluctuating water table that alternates between saturated and dry conditions. A shallow fibrous root system and specialized breathing cells make black ash ideally suited.

Wet back ash forest

An exposed black ash root, showing rows of specialized breathing cells. Photo by Harvey Tjader, DNR

Whether any other species will assume a dominant role in the absence of black ash is not known. Some of the species to consider planting include tamarack, quaking aspen, white cedar, yellow birch, red maple, paper birch, American elm, balsam fir, red elm, balsam poplar, basswood, white pine, white spruce, bur oak, cottonwood and black spruce. If any of these species are already present in the stand, they may reproduce by seed if they are reserved from harvest. Quaking aspen and balsam poplar can be expected to reproduce by root sprouting if they are harvested during the logging operation. They, along with cottonwood may also be established by rooted cuttings. Seedlings of the remaining species must be planted. Broadcasting seed is another alternative.

Planting these forests could be difficult. They may be too wet in the spring, so containerized seedlings could be planted later in the growing season. Planting such a wide variety of species, each with its own preferred microsite and shade condition, is more technical than a single-species planting job. The need to communicate detailed instructions to migrant tree planters who speak little or no English could slow production rates and may rule out the usual tree-planting contractors. Finally, there is the challenge of tending the young trees. Many wet black ash forests are located near deer yards. The seedlings may be too attractive as a food source for deer that are sequestered in adjacent cedar stands during a harsh winter. Caging, fencing or repellents will likely be required. Reducing competitive vegetation is another concern. Some stands are lush with grasses, shrubs and other plants that will respond vigorously to an increase in sunlight. Thus, there is the likelihood that mechanical or chemical treatments will be required to give tree seedlings a chance.

It's obvious that this is a formidable task that cannot be taken lightly, but one that we can't turn our backs on. If we do nothing, we could lose 406,000 acres of ash-dominated forest in Minnesota.