Arboriculture & Urban Forestry 32(6): November 2006 Arboriculture & Urban Forestry 2006. 32(6):297–304. 297 Street Tree Diversity in Eastern North America and Its Potential for Tree Loss to Exotic Borers Michael J. Raupp, Anne Buckelew Cumming, and Erin C. Raupp Abstract. 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. Key Words. Asian longhorned beetle; diversity; emerald ash borer; risk; street trees; sustainability; urban forestry. At one time, the American chestnut, Castanea dentata, was a dominant species of eastern deciduous forests. In the early 1900s, nursery stock from Asia was imported to New York with the chestnut blight fungus, Cryphonectria parasitica. Never having been exposed to this fungus, populations of the American chestnut had little or no resistance to the pathogen. The blight swept through forest stands killing 3.5 billion chestnuts by 1940 (Roane et al. 1986). At the present time, it appears unlikely that the American chestnut will ever assume its dominant role in eastern forests despite some progress to find resistant cultivars and inoculate our forests with less virulent strains of fungi to reduce the lethal impact of this pathogen. The exotic pest that captured the attention of urban forest- ers and changed our thinking about the design of urban for- ests was Dutch elm disease (DED) caused by Ophiostoma ulmi. American elm was one of the most commonly used street trees in the United States before the introduction of DED. American elm has many attributes such as a magnifi- cent habit, fast growth rate, tolerance to a wide range of soils and moisture conditions, and hardiness that made it a favored street tree. Elms were often planted in pure stands that lined the streets of many cities in the United States. DED was first reported in Europe in the 1920s as the agent killing millions of elm trees. It was introduced into the United States from Europe around 1930 in logs to be used for veneer. Since its introduction, it has killed millions of elm trees in the United States and throughout the world. The heavy use of elms in cities and huge costs associated with their removal stretched municipal budgets and left many city streets barren (Sinclair and Campana 1978). DIVERSITY IN STREET TREE POPULATIONS One of the most important lessons learned from these cata- strophic events was the dire consequence that low levels of street tree diversity had for urban forests when exotic pests were introduced. Diversity is an important buffer against catastrophic tree loss in natural and managed forest systems alike. The aftermath of the gypsy moth, chestnut blight, and the hemlock woolly adelgid has been a change in the com- position of the eastern hardwood forest. Lands once domi- nated by chestnut (Aesculus), oak (Quercus), and hemlock (Tsuga) now contain a greater diversity of trees such as maple (Acer), birch (Betula), sassafras (Sassafras), black locust (Ro- binia), and tulip poplar (Liriodendron) (USFS 2003). What have urban foresters learned about preventing or avoiding losses resulting from exotic pests? The DED disas- ter forced urban foresters to broaden the scope of factors considered when selecting trees for cities. Before this event, architectural features such as form, habit, color, and environ- mental tolerances to temperature, soil composition, and mois- ture regimes were the primary drivers in the plant selection ©2006 International Society of Arboriculture
November 2006
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
