Street Tree Diversity in Eastern North America and Its Potential for Tree Loss to Exotic Borers

In light of catastrophic tree losses caused by Dutch elm disease, foresters recommended that the urban forest be diversified. The intent was to create a more sustainable urban forest that would not be decimated by a single pathogen or insect pest. However, recent introductions of deadly borers such as Asian longhorned beetle and emerald ash borer reveal that new introductions can have disastrous consequences for urban forests. An analysis of tree inventories from 12 cities in eastern North America reveals an overabundance of certain genera of trees such as Acer and Fraxinus. Introduced pests with broad host ranges such as the Asian longhorned beetle may be capable of killing or forcing intervention on large numbers of trees. Even cities that have diversified at a low taxonomic level (cultivar or species) may suffer greatly from the depredations of insect pests such as the emerald ash borer that specialize on plants at the generic level. Generalists capable of feeding on plants in several families will create problems for urban forests even when attempts have been made to diversify at higher taxonomic levels. Urban foresters should avoid planting susceptible taxa of trees, especially in cities that are overstocked in these taxa, and consider diversifying greatly the types of trees in cities.