298 Raupp et al.: Street Tree Diversity process. More recently, the importance of increasing the di- versity of tree species in urban forests has found its way into the plant selection process as a buffer against catastrophic losses resulting from pests (Grey and Deneke 1986; Miller 1997; Gerhold and Porter 2000). One approach to help guide the planning process has been to set diversification goals or guidelines for urban forests. After DED, scientists began creating formulas to help guide the planning process. Barker (1975) was among the first to suggest that street tree plantings needed to be diversified. He recommended that a particular kind of tree should not exceed 5% of the density of the entire tree population. This concept was later extended to include genera as well as species by Moll (1989) who suggested that no more than 10% of trees in a community be of the same genus and 5% be of the same species. Miller and Miller (1991) recognized that one impor- tant limitation in attempting to meet a goal of no more than 5% of any one tree would be the availability of enough proven plant material in a region to implement this goal. They recommended no more than 10% of a single species as a goal for commonly planted trees. Other authors have set this di- versification goal at levels both higher and lower. Grey and Deneke (1986) took a more liberal view and suggested that no tree species should make up more than 10% to 15% of the total population. Gerhold and Porter (2000) stressed the im- portance of diversifying the urban tree population but sug- gested that a single species or clone along a street should not be prohibited. Santamour (1990) adopted a more comprehen- sive strategy and suggested that urban forests could be pro- tected from pest outbreaks if no more than 10% of a single species, 20% of a single genus, or 30% of a single family of plants were used. An important consideration that may be overlooked in the rush to diversify forests is consideration of the adaptability and potential longevity of replacement trees. Richards (1982/ 1983) persuasively argued that trees selected to diversify the urban forest must be well suited to the rigors of the complexly stressful conditions found in urban ecosystems. Using inven- tory data from Syracuse, New York, Richards (1982/1983) demonstrated that species varied greatly in their ability to adapt to stressful urban conditions. Adaptability and longev- ity of species in stressful urban habitats must weigh heavily in the tree selection process. Efforts to diversify the urban forest that fail to account for inherent differences of cultivars or species to survive may result in the selection of trees that are ill-suited for the rigors of cities (Richards 1982/1983). The result of poor choices may be increased mortality, re- duced lifespan, and ultimately greater costs when failed or failing trees must be removed and replaced. CURRENT AND FUTURE RISKS TO STREET TREES In the last decade, two important and devastating insect pests have been introduced to North America, the Asian long- ©2006 International Society of Arboriculture horned beetle, Anoplophora glabripennis, and the emerald ash borer, Agrillus planipennis. Both species are believed to be Asian in origin and were likely transported to this country as larvae in low-quality lumber used for crating and packing material. In recent years, Asian longhorned beetle popula- tions have become established in several states, including New York (New York City, several cities in Long Island), Illinois (Chicago), New Jersey (Jersey City, Rahway, Car- teret), and in Canada (Toronto-Vaughn). Emerald ash borer was initially detected in the summer of 2002 in six counties in Michigan. Since then, detections have been made in more than 30 counties throughout the state. Several counties in Ohio, Illinois, and Indiana have reported infestations and nearby Ontario, Canada, has an infested area. In the spring of 2003, the emerald ash borer arrived in Maryland with ship- ments of infested nursery stock from Michigan. These two species of wood-boring beetles share an ability to attack and kill seemingly healthy trees of most age classes in both urban and native forest stands. Despite the enormous destructive potential of these two insects, much is still to be learned about their ability to colonize, use, and kill different species and cultivars of trees commonly planted in cities in North America. However, Sawyer (2003, 2005) compiled lists of trees commonly found in urban forests and catego- rized them according to their suitability as hosts for Asian longhorned beetle. Host suitability was based on records of relative infestation rates in North America, rearing results, laboratory studies, and Chinese literature. Although some variation in preference and performance has been noted at the specific level for Asian longhorned beetle (Sawyer et al. 2006), intervention tactics, including tree removal and insec- ticide applications, are still made at the generic level in eradi- cation programs (Sawyer 2003, 2005; USDA 2005). Genera considered to be very good hosts included Acer, Salix, and Ulmus. Good hosts included Aesculus, Betula, and Platanus and occasional hosts included Albizzia, Celtis, Fraxinus, Populus, and Sorbus. Questionable hosts included Hibiscus, Malus, Morus, Prunus, Pyrus, Quercus, Robinia, and Tilia. These are genera in which egg laying has been noted but development has not been generally confirmed in the field. Genera that have no record of use in the United States in- cluded Alnus, Eleagnus, Melia, and Ailanthus. With respect to the emerald ash borer, ash species known to support infestations in North America include F. pennsyl- vanica, F. americana, F. nigra, and F. quadrangulata, and horticultural cultivars of these species (Herms et al. 2004a). In Asia, several ash species are hosts with F. mandshurica and F. chinensis being primary hosts (Herms et al. 2004a). At the present time, the entire native and cultivated ash resource in North America is considered at risk. This includes at least 16 endemic ash species as well as naturalized species and cultivars used in landscapes (Cappaert et al. 2005).
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