10 Steenberg et al: A Social-Ecological Analysis of Urban Tree Vulnerability Arboriculture & Urban Forestry 2019. 45(1):10–25 A Social-Ecological Analysis of Urban Tree Vulnerability for Publicly Owned Trees in a Residential Neighborhood James W. N. Steenberg, Andrew A. Millward, David J. Nowak, Pamela J. Robinson, and Sandy M. Smith Abstract. The urban forest is a valuable ecosystem service provider, yet cities are frequently degraded environments with a myriad of stressors and disturbances affecting trees. Vulnerability science is increasingly used to explore issues of sustainability in complex social-ecological systems, and can be a useful approach for assessing urban forests. The purpose of this study was to identify and explore drivers of urban forest vulnera- bility in a residential neighborhood. Based on a recently published framework of urban forest vulnerability, a series of indicators of exposure, sensitivity, and adaptive capacity that describe the built environment, urban forest structure, and human population, respectively, were assessed for 806 trees in Toronto, Ontario, Canada. Tree mortality, condition, and diameter growth rates were then assessed using an existing 2007/2008 inventory. 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 multiple linear regression for the continuous condition and growth variables and a multilayer perceptron neural network for the binary mortality variable. Commercial land uses and commercial buildings adjacent to trees con- sistently explained higher mortality rates and poor tree conditions. However, at finer spatial scales it is important to differentiate between dif- ferent causes and correlates of urban forest decline within commercial land uses. Tree species, size, and condition were also important indicators of vulnerability. Understanding the causes of urban forest change and decline are essential for developing planning strategies to reduce long- term system vulnerability. Key Words. Condition; Growth; Mortality; Neighbourwoods; Urban Forest; Vulnerability Assessment. The urban forest is a valuable ecosystem service pro- vider and represents essential green infrastructure for many cities. However, cities are highly altered, densely settled, and frequently degraded environ- ments with a myriad of stressors and disturbances that create difficult conditions for tree establishment and growth (Nowak et al. 2004; Trowbridge and Bassuk 2004; Steenberg et al. 2017a). Consequently, urban trees are often in poor condition and frequently have reduced longevity (Roman and Scatena 2011; Koeser et al. 2013), both of which translate to a reduction in ecosystem services (Nowak and Dwyer 2007). Cases and causes of decline in urban forest structure and function need to be identified, assessed, and modeled. Such research can inform the processes of urban design and policy development, as well as urban for- est management, so that unnecessary tree decline and ©2019 International Society of Arboriculture mortality are avoided and the benefits that urban inhabitants receive from trees are maximized. The built environment is a source of stress for urban trees, especially in higher-density neighborhoods. Build- ing density, height, and type affect irradiation (i.e., sunlight available for photosynthesis and plant growth), the physical growing space for trees, and the microclimate of urban areas (Jutras et al. 2010). Moreover, construction activities and conflicts with above- and belowground utilities and other gray infrastructure are common sources of urban tree decline and mortality (Randrup et al. 2001; Koeser et al. 2013; Steenberg et al. 2018). Land use is highly influential on urban forest ecosystems (Nitoslawski et al. 2017), and is indeed indicative of the presence of many of these stressors. Land uses with higher human populations and building densities, as well as abundant
January 2019
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