150 Survey Context Exploring how managers make decisions on tree spe- cies selection could improve education and poten- tially lead to an increase in benefits provided by those trees. Professional organizations such as the ISA could help to bridge this gap and increase awareness of tree species selection issues. Different tree species provide different benefits and are adapted for different sites; understanding how managers—those responsi- ble for tree species selection—make these selections can inform regional education to improve tree selec- tion by managers. Selecting species in urban areas that are well-suited to the existing conditions on the site will help mitigate loss of tree species to a chang- ing climate. Forest ecosystems that are more diverse and are managed sustainably will be more resilient to climate change (Spittlehouse and Stewart 2004). There has been a limited body of research examin- ing how professional certification influences urban forest management. There have been a number of studies that have used empirical evidence to justify ISA certification (Carlson 1995; Green 2002; Elmen- dorf et al. 2005); however, little research investigates how this training is operationalized in arboriculture practices. Other studies have examined how different professional organizations play a role in the urban forest, such as the Society of Municipal Arborists (e.g., Young 2010). Ries (2017) found that participa- tion in the Municipal Forestry Institute (MFI) leader- ship training increased the likelihood of various levels of leadership outcomes. While there are an increasing number of certifications, there has not been any explicit exploration of how ISA Certified Arborists® differ from those that are not. The use of quantitative statistical analysis can help to elucidate trends and explore relationships between variables that could lead to more informed tree species selection. Tree species selection has been examined through a variety of lenses: climate adaptability (Roloff et al. 2009), residential attitudes (Kirkpatrick et al. 2012), and those who plant on public land (Conway and Vander Vecht 2015). This study primarily builds on a study conducted in Toronto, Canada, by Conway and Vander Vecht (2015) exploring tree species selection on pub- lic land. Conway and Vander Vecht (2015) found the factors most influential in tree species selection for urban foresters were increasing canopy cover, native species, and increasing local and overall tree species diversity. They collected data on four groups: garden ©2020 International Society of Arboriculture Petter et al: Municipal Tree Species Selection centers, the Toronto Urban Forestry Department, non- profit organizations, and nurseries. They found some differences in selection criteria between organiza- tions, but there was also a lot of overlap in species planted. The Toronto Urban Forestry Department had a general policy to diversify their canopy, plant native species, and increase canopy cover. By conducting a partial replication of the methods in Conway and Vander Vecht (2015) over a broader geographical scale, the goal of this study was to explore how municipali- ties may differ in tree species selection, rather than how stakeholders differ within a municipality. This survey was designed to explore how managers are using existing research, as well as how they are using social, ecological, and economic criteria, to influence tree species selection. As in Conway and Vander Vecht (2015), we surveyed managers regarding their tree species selection criteria. However, the focus of this study was on how these selection criteria differed based on ISA certification and municipality size. METHODS Study Area The study population consisted of representatives from Tree City USA designated cities across the PNW states of Oregon and Washington. There are 61 Tree Cities in Oregon, where roughly half the popula- tion of the state resides within a Tree City. The largest Tree City in the state is Portland, with a population around 609,000, and the smallest is Rivergrove, with a population around 480 (Arbor Day Foundation 2016a). There are 90 Tree Cities in Washington, where about 47% of the population lives in a Tree City. The largest Tree City is Seattle, with a popula- tion around 653,000, and the smallest is Farmington, with a population around 140 (Arbor Day Foundation 2016b). Targeting Tree Cities helped to eliminate those cities that are not actively managing their urban forests. Regional Context The climate of the PNW is influenced by the Pacific Ocean, the Cascades, and the Olympic Mountains. These mountain ranges cast a rain shadow over the eastern high desert portions of Washington and Ore- gon. The substantial decline in precipitation is a dom- inant factor in influencing the composition of flora (Albright and Peterson 2013). The more moderate climate allows for a greater diversity of tree species in the western portions of the states (McPherson et al.
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