Invasives and Forests

“Forests and forestry are part of our nation’s lifeblood
socially, economically and ecologically”

Chief Tom Tidwell U.S. Forest Service

Find out why ash trees are lingering among dead ash. Understand why scientists and land managers have spent over a decade studying populations of a nonnative, invasive pest known as emerald ash borer.  This current research using lingering ash may be the key to success.

Subheads: Control EABStrength in Diversity

White ash in fall color. Image by Kelly van Frankenhuyzen


Over the last century, nonnative plants and insects have been invading forests and urban areas in the United States. Invasive plant and insect outbreaks have become more common in forests and urban areas in recent decades. An average of 2.5 nonnative forest insect species invades North America each year [1].

Ash trees have severely declined on private and public lands in Michigan and Ohio in the last decade. According to U.S. Forest Service Northern Research Station, 8 billion ash were on U.S. timberlands prior to the beetle attacking and killing ash. The  number of trees have declined, since the pest has established populations in 31 states.

According to Deb McCullough [2] and Dan Herms [3] hundreds of millions of ash have been killed by emerald ash borer ( EAB, Agrilus planipennis).

Click here for T. Poland transcript

In 2002, the emerald ash borer was discovered in Canton, Michigan and Windsor, Ontario, Canada. Emerald ash borer began to spread into surrounding states like Ohio in 2003 and Indiana in 2004 [4].

Invasive species have impacted other trees like the American chestnut and the American elm. In the early 1900s, an air carried fungus began affecting the American chestnut. It was later identified as chestnut blight. Once discovered, the fungus spread through most of the Mid-Atlantic states. According to Scott Tepke, a forester on the Allegheny National Forest in Marionville, Pennsylvania, by the 1950s, the American chestnut had become endangered throughout it’s entire range. Fewer than 3.5 million American chestnut trees were eliminated by the blight between 1900 and 1940 [5]. Approximately 200 million mature elm were killed by the Dutch elm disease (also a fungus) in the northeastern United States between the late 1920s and the 1970s [6].

American Elm tree in East Lansing, Michigan. Image by Kelly van Frankenhuyzen


Tree diversity is an important concept, especially when invasive pests or pathogens become established and spread, killing healthy trees by the millions. Tree species that co-evolved with insects or  pathogens are more likely to develop a mechanism of resistance [7]. In Asia, where emerald ash borer is native, ash trees are not facing serious declines. While some environmental stressors impact ash trees in Asia, the damages to the ash population in Asia is no where near like it is in North America. Emerald ash borer has evolved with the ash trees in Asia, unlike the ash in America.

A resistance breeding program using lingering ash from the forests is being modeled after the American Chestnut Foundation program. Propogation programs have been around for centuries especially those geared toward agriculture and crops. According to Jennifer Koch, a research biologist with the U.S. Forest Service Northern Research Station in Delaware, Ohio similar techniques are being used to propagate ash for resistance to emerald ash borer, like scientists are doing with the chestnut program.

The long term goal of Koch’s research is to identify resistance to emerald ash borer in exotic species and/or select North American lingering ash to establish a breeding program. The use of exotic ash may have value to a nursery because ash are important street trees. Ash were the replacement trees after the Dutch elm disease and popular for their aesthetic and environmental value in cities. Scientists have created offspring by breeding a parent generation with a donor parent, containing a resistance gene. Then the offspring, from that pairing, are crossed with the parent generation. This is known as backcross. These newer generations are created through careful selection of North American species. In the near future, ash trees may be able to establish resistance to emerald ash borer and re-establish populations in forests [8].

Trees play an important role economically and environmentally. Research conducted by the U.S. Forest Service Northern Research Station in 2017 estimated that before the arrival of EAB, ash trees on U.S. timberlands (forest suitable or managed for timber harvests) were valued at $282.25 billion. Trees offer many health benefits such as lower stress, increase physical activity, and improve air quality [8a]. This research has been important in densely populated cities, where ash trees were frequently planted in landscaping. Several studies have shown a link between the environment and improved health in areas with more green space and trees.

Ash trees also serve as wind breaks for farmers and help to filter and clean water. Northern Research Station scientist Stephen Handler said the “most important product we get from forests is not wood, it’s water.”

Watch a short video introducing the cast of people involved in the story and research related to lingering ash and propogation.