28 imperative for diversity is an aesthetic preference for planting monocultures of the same tree species along streets, avenues, and boulevards (Trowbridge and Bassuk 2004). This practice, which dates back to 16th century Europe (Couch 1992), contributed to the overplanting of Ulmus americana (American elm) as a street tree in the United States. Despite the lessons ostensibly learned from the devastation wrought by Dutch elm disease (DED, Ophiostoma spp.), the overplanting of urban tree species continues to be operative today. For example, Vander Vecht and Con- way (2015) found Acer spp. to comprise 33.1% of all street trees in Toronto, Canada; and an analysis of managed municipal tree populations in the state of Montana, United States found the percentage of Fraxinus spp. (ash) to be at least 40% in eighteen municipalities east of the Continental Divide and to be 70% in two of those municipalities (Montana Department of Natural Resources and Conservation 2017). The overplanting of urban tree species is not simply a North American phenomenon; it also occurs worldwide. For example, Thaiutsa et al. (2008) found 42% of street trees in Bangkok, Thailand to be Ptero- carpus indicus (Amboyna wood); and Tang et al. (2016) randomly sampled six districts in the center of Beijing, China and found most roads to have only one tree species and Styphnolobium japonicum (Japanese pagoda tree) to account for more than 50% of sam- pled trees. When the ALB was found in Worcester, Acer spp. was the most prevalent street tree genus, comprising 79.7% of all city street trees, and Acer platanoides (Norway maple) was the most prevalent street tree species, comprising 60.8% of all city street trees (Freilicher et al. 2008). Recognizing the threat posed by the ALB to both urban and nonurban forests, the Massachusetts Department of Conservation and Rec- reation (DCR), the United States Department of Agri- culture’s Animal and Plant Health Inspection Service (APHIS), and the United States Forest Service (USFS) partnered in an intensive ALB eradication effort which resulted in the removal of more than 35,000 publicly and privately managed Worcester trees (Kotsopoulos 2017). Although the ALB is a polyphagous pest that attacks other tree genera besides Acer spp., including Aesculus spp. (hor- sechestnut), Populus spp. (poplar), Betula spp. (birch), and Ulmus spp. (elm)(Hu et al. 2009), it is reasonable to assume that, if Worcester’s street tree ©2020 International Society of Arboriculture Cowett and Bassuk: Street Tree Diversity in Massachusetts, USA population had been more diverse, the number of tree removals would have been fewer and the impact of the ALB infestation less severe. Street trees are typically located in the public street right-of-way. Although comprising a minority of all urban trees (Dwyer et al. 2000), they often receive special attention due to their public function, and together with park trees are the components of the urban forest which municipalities are most able to manage directly. To sustainably manage street trees, assessing the diversity of the street tree population is essential (Raupp et al. 2006; Sjöman et al. 2012). This assessment is usually facilitated through a street tree inventory, whether a complete, partial, or sample inventory. Because the Worcester Department of Public Works and Parks had conducted a citywide street tree inventory between 2005 and 2006, the city was able to select tree species and genera that would not only be resistant to the ALB, but would also increase the diversity of the street tree population when it replaced ALB tree removals (Freilicher et al. 2008; Freilicher 2011). Street tree management occurs at numerous geo- graphic scales (e.g., parcel, block, neighborhood, municipality, state, country, etc.) by many different actors (e.g., property owner, arborist, tree board, non- governmental citizen group, public utility, govern- ment official, etc.)(Clark et al. 1997; Mincey et al. 2013). This multiplicity of scales and actors makes altering the structure of urban tree populations, such as increasing street tree diversity, a challenging endeavor. In the United States, the USFS partners with individ- ual states in developing statewide plans that delineate management goals and strategies for each state’s urban forest, including its street tree component, and municipal management plans are encouraged to be consistent with the statewide plan (Hauer et al. 2008). In Massachusetts, the DCR is charged with adminis- tering the state’s Urban and Community Forestry Program and advocates greater street tree diversity for municipal streetscape plantings (Massachusetts DCR 2017). However, what is true at the municipal level is also true at the statewide level: implementing increased diversity depends on identifying species and genera that may be overplanted and species and genera that are less prevalent. To this end, some states in the United States have conducted statewide street tree assessments employing a wide range of tech- niques to obtain the information needed to make
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