If you have lived in Minnesota during the last fifty years, you will probably remember when Dutch Elm Disease took its toll during the “great depression” of trees in the community. Trees all over the cities had to destroyed and removed because of this disease that devastated their elm population. There are lessons to be learned from this information. The Emerald Ash Borer history is very closely compared to Dutch elm disease. There are good reasons why this correlation has been brought up. Minnesotans remember how the elm trees were virtually wiped out in our cities.
St.Paul, MN, saw the discovery of Dutch Elm disease, but the widespread destruction and removal did not happen until sometime in the 1970s. Many cities that lost all their elm planted ash trees in their place. So, because of that, there is often a comparison made between the Dutch Elm disease and the emerald ash borer, especially in the cities. To see if this is a valid comparison, let us go back further in our history. Here is a different example of deforestation: the loss of American chestnut and chestnut blight. Because of this chestnut blight, there was the first mass destruction of a forest. American chestnut is a big tree that is native from Maine to Mississippi. In the years between 1900, when the blight was first discovered in NYC, and 1940 when most of the trees had already been infected and destroyed, many of these great trees were destroyed. What impact does this pest have on our forests? All ash trees bigger than 1″ in diameter are usually destroyed by the emerald ash borer. Chestnut blight, however, did not kill all the trees, but instead caused them to become merely sprouts from the roots and trunks of these formerly large trees. The American chestnut trees are pretty much lost as far as the makeup of our forests go. Dutch Elm disease usually kills about 80% of the trees that it infects. Elm is a smaller percentage of our city and forests than before this Dutch elm disease invaded, but they do exist in cities because of fungicides.
In forests, the elm remains as a small tree. The American chestnut in eastern forested areas has become pretty much nonexistent. You have to take a look at the rate at which these diseases actually spread as well. The spread and death of chestnut trees averaged about 32-40 km/year according to Baker and Tainter in 1996. Dutch elm disease spread numbers were more difficult to come by, but it was first named in 1930 in Ohio and showed up in St. Paul, MN, in 1961. There were only 30 cases of it reported during the first seven years. Detroit, Michigan in 2002 saw the first incidence of the emerald ash borer insect. According to McCullough, Siegert, Telewski, and Liebhold in 2008, it was reportedly in the state after dendrochronological analysis shows it was there as soon as around 1995. Dendrochronology is the scientific method for dating trees and assessing environmental impacts on such, by using the growth rings of the trees. Scientists in Michigan went to various locations and took ash trees, dated them, and took note of the year that emerald ash borers attacked. They took that data and referred to weather patterns to try to figure out when the ash borer insects first came to Michigan.
We do not know the exact date when the emerald ash borer was discovered in the United States, but we do know that these insects were here long before they were reported and identified and before the actual efforts to manage and quarantine these areas began. In the last 15 years, emerald ash borers have spread to 14 different states, including from Minnesota to Virginia, and from Missouri to New York. Chestnut blight is not spread the same way as Dutch Elm disease and Emerald ash borers: through people transporting wood. Chestnut blight mainly spread through a natural process. The future for ash does not bode well. Michigan data shows that there is very little left-over ash in areas where the insects have attacked, and the ash seeds only survive for about 8 years. Both scientists and volunteers have been working tirelessly in MN to try to collect and preserve some of the ash genes for years so that scientists can take those genes and have something to work with after the borers have gone through an area. Some stands in the northern area of MN are made up of about half ash trees. Elm, on the other hand, was never really a majority tree in stands even though it is a vital tree in forests. Some Appalachian forests have about 25% of American chestnuts. The entire impact of Dutch Elm disease was therefore less than the greater impact of Emerald ash borers in some parts of northernmost Minnesota. In an interesting note, however, ash is seen in a higher percentage in some MN forests, probably due to the demise of the elm trees. Ash took over the space left by the elms.
An ironic fact is that this guide recommends that disease-resistant American elm be used in seven out of nice timber areas to replace ash. You might take this to mean that the forest has come around full circle; one disease removed the elms to help the ash become more prominent, and now the ash are being destroyed and elms are taking over as the dominant trees. Dutch elm disease was a huge problem, but just looking at the Emerald ash borer history, it would seem that this is an even greater problem. However, with the advent of greater scientific discoveries, and funding to support these advances, it seems the forest is growing once again.