372 Petter et al: Tree Selection of Tree City USA Cities in the Pacific Northwest impact. The benefits provided by trees can be increased by utilizing current science and sound tree management, but may be hindered by neglect or lack of management (Pincetl et al. 2013). There have been a number of studies that have examined, and in some cases demonstrated, the potential downsides of trees, including hazards, pollen, maintenance costs, and infrastructure damage (Roy et al. 2012). So, while large municipalities’ pledges to plant a million trees seem noble, they must choose the proper trees to realize the full potential from planting and not exacerbate issues through poor tree selection. This could lead to an even age distribution, which could lead to large removal costs when the forest matures. Additionally, this would result in a large loss of benefits and can- opy cover all at the same time. It is important to have both tree species diversity and age diversity. Pests and Pathogens Urban forests are subjected to harsh growing condi- tions and face a wide variety of pests and pathogens. Historically, pathogens such as chestnut blight (Cryphonectria parasitica) and Dutch elm disease (Ophiostoma ulmi) have caused large scale decline in North American urban forests. Pests and pathogens burden cities with enormous tree removal costs and loss of benefits, often in a relatively short time period (Raupp et al. 2006). One of the pests that is decimat- ing the urban forests in the Midwest is the emerald ash borer (EAB)(Agrilus planipennis). Since its dis- covery in 2002, this insect has caused substantial destruction of the ash genus (Fraxinus spp.) in urban areas. Cities are often more susceptible to pests due to planting practices (e.g., inadequate soil volume) and the large influx of goods (e.g., wood products). An increase in pests complicates tree species selection (Poland and McCullough 2006). Within a 15-year period in the USA, insects that are commonly associ- ated with wood were found during routine inspec- tions of goods originating from over 100 different countries; insect species that were found more fre- quently were more likely to be established in the USA (Haack 2006). Even without pest infestations, urban trees often have shorter life spans and higher mortality rates. Factors such as tree species, size, health, main- tenance, and land use are all significant factors in deter- mining survival and growth rate (Nowak et al. 2004). Selecting the right tree for the right place can help to increase the survival rate and minimize conflicts. ©2020 International Society of Arboriculture Urban Forest Diversification In order to avoid large scale loss of the urban forest resource, Santamour (1990) suggested that no more than 10% of a tree species, 20% of a genus, or 30% of a family should be planted in the confines of a munic- ipality. This rule was based on empirical evidence and professional advice to foster diversity, not as a result of controlled scientific research. As research progresses, it is clear that the 10-20-30 rule is not the only rule to consider when selecting tree species. A survey of the Nursery and Landscape Association showed that there is a heavy reliance on cultivars, which are genetically identical to each other. For many tree species there are only a small number of cultivars that comprise the majority of the plantings (Iles and Vold 2003). Reliance on a few cultivars results in less diversity within a species. Raupp et al. (2006) point out that this formula does not take into account susceptibility of multiple species to a singular pest. Another example is that if 2 species of ash were planted, and each ash species represented 10% of the urban forest, then 20% could be wiped out by EAB. When attempting to create a more diverse forest, spe- cies should be selected that minimize risk of overlap- ping pest problems; this could manifest itself as further consideration of tree diversity within families and orders (Raupp et al. 2006). Conversely, past research has argued for the perpetuation of the oldest tree spe- cies in the urban forest. These species are thought to be well-suited to the harsh urban conditions and pro- vide ideal form (Richards 1983). While there is good reason to plant species tolerant of the tough urban environment, it does not adequately account for the introduction of foreign pests, changing climates, and other unanticipated changes. Most municipalities have the potential to increase tree species diversity as well as the potential to create more diverse habitat types to preserve biodiversity and create more resilient urban forests. For example, many parks provide valuable tree habitat and can be used to increase connectivity and serve as biodiverse hot spots in cities. Unsurprisingly, larger parks tend to have higher levels of tree diversity (Cornelis and Hermy 2004). A survey of personnel responsible for tree inventories at their nurseries in Washington State suggested that support for tree species diversity is high, but knowledge of what diversity means is varied; less than half of the respondents thought planting more than 10% of the same species increased susceptibility
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