Michigan's Forest | Beech Bark Disease

The discovery of beech bark disease in 2000 forever changed the dynamic of Michigan’s hardwood forests, both in the Upper and Lower Peninsulas.  It’s first outbreak in North America occurred in Nova Scotia (a Canadian province) around 1890 by accident from an infected ornamental, from there it spread south and west, reaching the United States by 1929 in Massachusetts.  By 2000 it had made its way as far west as Michigan. 

But what is beech bark disease? BBD is caused by not only one aspect but two.  It’s caused by a sap-feeding scale insect and a fungus, and develops in three stages identified as the Advancing Front, the Killing Front, and the Aftermath Forest.  The Advancing Front refers to the stands where trees are infested with beech scale but not yet infected by the Nectria fungus. The next stage is the Killing Front, which refers to stands of trees where beech scale populations are high and the Nectria fungus infection is abundant.  And finally the last stage is the Aftermath Forests which refers to the stands that have experienced the first wave of beech mortality.

The American beech trees are first infested by the beech scale. Scale feeding allows infection by the Neonectria fungus, the fungus kills the wood, by blocking the flow of sap. The tiny scale insects can be found on the trunk and branches, feeding on the inner bark. White wax covers the bodies of the scales, when a tree is heavily infested, it’ll appear to be covered in white wool.  Small wounds on the tree allow for the fungus to enter the tree, killing woody tissue, sometimes creating cankers on the tree stems and large branches.  The trees affected gradually decline in health and eventually die, sometimes it can take years for a tree that is infested to die, leaving plenty of time for high wind storms to break off infested branches referred to as “beech snap” which can further spread the disease. 

What does this mean for Michigan forests? Michigan has over 32 million American beech trees, that's more than 7 million acres of maple-beech-birch type forests that could be potentially affected by this disease in Michigan.  To date, about 2.5 million beech have been killed by BBD.  Most of this loss is occurring in the eastern portion of the Upper Peninsula.  However, there are new infestations reported every year, with more and more occurring in the Lower Peninsula. 

In an attempt to salvage the high timber-valued trees, the DNR has stepped up hazard tree removals due to BBD and emerald ash borer.  Only about 3% of American beech trees are resistant to to beech bark disease, the resistant trees are being identified and used to produce resistant beech trees for the future.  The best way we Michiganders can help is by not transporting beech firewood or logs from infested areas to un-infested areas, and reporting sights with infested scale.  However controlling natural spread is nearly impossible since the scale and fungus are spread by wind and animals.

This article is part of the ongoing series on invasive species funded in part with funds from the Michigan Invasive Species Grant Program through the Department of Natural Resources, Environmental Quality, and Agriculture and Rural Development.

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