In 1980, 27%, and in 2003/2005, 42% of the tree taxa were considered overplanted. This indicates that we are relying on fewer tree species today than we were in 1980. In 1980, silver maple (Acer saccharinum), blue spruce (Picea pungens), crabapple (Malus sp.), and ash (Fraxinus sp.) made up more than 5% of the total tree composition in the urban forest. In 2003/2005, Arborvitae, silver maple, Norway maple, blue spruce, ash and Norway spruce each made up more than 5% of the total urban forest. When considering the public trees, there were eight different species on public property that comprised more than 5% of the tree composition in both 1980 and in 2003/2005. On private property, in 1980, four species comprised more than 5% of the total private tree composition, and in 2003/2005, six species made up more than 5% of the private tree composition.
When comparing the genera found in the urban forest, it is apparent that Acer is overrepresented. In 1980, Acer made up over 22% of the total urban forest and in 2003/2005 it was 24% of the total. Of the public trees, the genus Acer is even more overrepresented. In both years, 1980 and 2003/2005 Acer represents nearly 40% of the public trees. The amount of Acer on private property is similar to the amounts in the total trees. In 1980, Acer was almost 20% and in 2003/2005, Acer was just over 21% of the private trees. There are very good reasons for avoiding mass plantings of the same species and genera; e.g. American elm (Ulmus americana L.) with Dutch elm disease and ash with emerald ash borer (Agrilus planipennis Fairemaire) are two examples. It seems that the genus Acer has replaced the American elm as being overplanted and may now be waiting for a calamity to happen, e.g. Asian longhorned beetle (Anoplophora glabripennis Motschulsky). If it becomes established in these Midwest cities, it would dramatically change the urban forest by decimating about 24% of the trees.
There is a question, or concern, about native versus exotic tree species. Some advocate that only native species of trees should be planted in urban forests. However, many native tree species simply do not do well in urban situations. Clinging to the few proven native species has already led to certain species and genera being overplanted. This can lead to devastation by uncontrolled pests or diseases, not necessarily of native origin. American elm and ash are both native species that were overplanted in many cities and today both are being or have been destroyed by exotics. Interestingly, in 1980, 89% of all tree species considered to be over planted were native, with the exception of the Norway maple. In 2003/2005, 75% of the all tree species that are considered overplanted in this study was native, with the exceptions of the Norway maple (Acer platanoides), Norway spruce (Picea abies), and linden (Tilia sp.).