Arboriculture & Urban Forestry 45(1): January 2019 vulnerability (Steenberg et al. 2017a) was used to assess 2014 data describing 806 public trees in a res- idential neighborhood in Toronto, Ontario, Canada. The framework consists of a series of quantitative indicators of exposure, sensitivity, and adaptive capacity that describe the built environment and asso- ciated stressors, urban forest structure, and the neigh- borhood’s human population, respectively. Tree mortality, condition, and diameter growth rates were then assessed using an existing tree inventory from 2007/2008. A bivariate analysis was first conducted to test for significant relationships of vulnerability indicators with mortality, condition, and growth. A multivariate analysis was then conducted using mul- tiple linear regression for the continuous condition and growth variables and a multilayer perceptron neural network for the binary mortality variable. With much of the global population increasingly living in cities and urbanization rates on the rise, ongoing research and science-based tools for understanding the causes of urban forest change and decline are essential for developing planning strategies to reduce long-term system vulnerability. METHODS Study Area The study was conducted in a centrally located, down- town residential neighborhood, Harbord Village, in Toronto, Ontario, Canada. The neighborhood was selected because of its existing, spatially-referenced tree inventory. As of 2011, Harbord Village had 8,583 residents, a population density of 13,484 persons/ km2 , and total area of 0.6 km2 , and was predominately comprised of semi-detached residential dwellings, with approximately 1,600 households (Keller 2007; Statistics Canada 2012). There are commercial land uses along main street sections, with several larger multi-unit and institutional parcels, and three small public parks. Urban forest researchers and Harbord Village residents conducted a tree inventory in 2007 and 2008 to inform their strategic urban forest man- agement plan (Keller 2007). Dominant tree species in the neighborhood include Norway maple (Acer plat- anoides), green ash (Fraxinus pennsylvanica), hon- eylocust (Gleditsia triacanthos), white cedar (Thuja occidentalis), silver maple (Acer saccharinum), and horsechestnut (Aesculus hippocastanum). Natural- ized species that have grown from seed (in-grown) that are common include white mulberry (Morus 13 alba), tree-of-heaven (Ailanthus altissima), and Man- itoba maple (Acer negundo). Toronto has a continental climate with hot, humid summers and cold winters, with a mean annual precipitation is 834 mm and a mean annual temperature of 9.2°C (Environment Can- ada 2015). The city is within the Deciduous Forest Region and Mixedwood Plains Ecozone (Ontario Min- istry of Natural Resources 2012). Data Collection and Processing Data collection took place during the growing season of 2014. A total of 806 publicly-owned trees (i.e., street trees and trees in front-yard rights-of-way, parks, and schoolyards) were re-inventoried and matched with data from the existing 2007/2008 tree inventory. Of the 806 trees inventoried in 2007/2008, 672 were still living in 2014 during field data collec- tion. Residential backyard trees were omitted from the study due to access constraints. The 806 trees rep- resent a full survey of 24 city blocks covered in the original inventory. In addition to the standard tree inventory metrics of species, diameter at breast height (DBH), and location, a series of indicators of urban forest vulnerability were assessed for each tree (Table 1). Newly planted trees were also measured for descriptive purposes but were not used in subsequent statistical analysis. The design of the urban forest vulnerability assess- ment framework and selection of indicators are described in Steenberg et al. (2017a). Specific indica- tor selection and design were further refined accord- ing to the study’s scale of assessment (i.e., individual trees), data availability, and feasibility. Indicators in the framework are assigned to the vulnerability sub- categories of exposure, sensitivity, or adaptive capac- ity. Exposure indicators (Table 1) represent external stressors and disturbances that cause tree decline and mortality, and subsequently a decline in ecosystem service supply. While some of the exposure indica- tors represent direct stressors (e.g., vandalism), most characterize indirect relationships between stress and the surrounding environment, all of which have been previously identified as important causes and cor- relates of tree decline and/or mortality (Randrup et al. 2001; Nowak et al. 2004; Trowbridge and Bassuk 2004; Jutras et al. 2010; Lu et al. 2010; Lawrence et al. 2012; Koeser et al. 2013; Steenberg et al. 2018). The main data source for exposure indicators was field data collected during this study. Additionally, 2011 census data were used to measure population ©2019 International Society of Arboriculture
January 2019
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