22 Steenberg et al: A Social-Ecological Analysis of Urban Tree Vulnerability Research has shown that adaptive capacity is driven by an array of social processes not necessarily affixed to wealth, including place attachment, common concern for neighborhood improvement (e.g., crime reduc- tion), and the presence of community leaders (West- phal 1993; Manzo and Perkins 2006; Tidball and Krasny 2007). Household-scale, qualitative research will provide valuable insight into these social pro- cesses in future work. Given their longevity and stationary nature, trees and forests are generally vulnerable to environmental change, where manifestations of change in urban for- est structure and function may lag considerably in their response to drivers of change (e.g., changes in management practices). Current urban forest struc- ture is often a function of decades-old management decisions (Boone et al. 2010). The disparity between commonly-planted tree species and overstory species composition in the neighborhood, coupled with ongo- ing decline of green ash and its removal from tree planting schedules, points toward the likelihood of considerable future change in ecosystem conditions. Moreover, Norway maple, which was the dominant overstory species, was an extremely popular urban tree in previous decades but is now no longer favored in Toronto’s urban forestry plan and planting sched- ules because of its potential to become invasive (City of Toronto 2013). In addition to these potential lag effects in species composition, the observed species- specific mortality and shifts towards smaller, orna- mental species may also correspond to declines in future ecosystem service supply irrespective of urban stressors and disturbances. Many ecosystem services are strongly associated with larger, longer-lived tree species with large leaf areas (Nowak and Dwyer 2007), which may indicate future declines in ecosys- tem service supply due to changing planting prefer- ences in tree species. Moreover, populations of mature urban trees, especially with low species and age diversity, may provide high levels of ecosystem services but also be highly vulnerable due to their sensitivity to pests, storms, and age-related decline (Steenberg et al. 2017a). These issues reinforce the temporal nature of vulnerability and associated impacts (Adger 2006; Steenberg et al. 2017a). Urban forest vulnerability assessments require both hind- sight in the form of monitoring (Roman et al. 2013), but also foresight in the form of ecological modeling to explore future scenarios of management and dis- turbance (Steenberg et al. 2017b). ©2019 International Society of Arboriculture Vulnerability science offers an integrative lens through which to explore risk and loss of function in highly complex, social-ecological systems like the urban forest (Turner et al. 2003; Adger 2006; Grove 2009; Steenberg et al. 2017a). Vulnerability also has many synergies with the concept of resilience that is of increasing importance in urban planning, though Steenberg et al. (2017a) argue that a vulnerability lens addresses drivers of change that are often external to resilience frameworks. Much of the research investi- gating mortality and decline in urban forests focuses primarily on stressors and disturbances. Moreover, vulnerability assessment might also be a useful sup- plement to existing assessments of tree safety and risk (Ellison 2005). This study affirms that there is a need to investigate how these stressors interact with urban forest structure and surrounding human popu- lations to reduce or inflate vulnerability in order to reliably predict the likelihood of potential loss of eco- system services. Moreover, many of the established relationships between urban forests and socioeco- nomic variability are based on two-dimensional tree canopy cover data at broader spatial scales. There are far fewer studies (e.g., Shakeel and Conway 2013) investigating urban forest ecological processes at finer scales using empirical field data from multiple time periods. However, further research is needed that tests both the reliability and validity of indicator design in different neighborhoods, cities, and scales. With increasing attention paid to urban forests by municipalities (Ordόñez and Duinker 2013) and com- munity groups (Conway et al. 2011), the demand for management information that goes beyond quantify- ing ecosystem structure and function to assessing urban forest vulnerability is of increasing interest. Acknowledgements. We are indebted to Ryerson University research assistants Amber Grant and Claire Stevenson-Blythe, who assisted with field data collection. Thank you to the residents of the Harbord Village neighborhood and the Harbord Village Residents Association for permitting field data collection and sharing existing data. Funding for this project was provided by the Natural Sciences and Engineering Research Council of Can- ada (NSERC) and Ryerson University. Some statistical consult- ing support was provided by David Kremelberg. This research was, in part, conducted and funded during the lead author’s Ful- bright exchange at the USDA Forest Service’s Northern Research Station in Syracuse, New York. Fulbright Canada is a joint, bi -national, treaty-based organization created to encourage mutual understanding between Canada and the United States of America through academic and cultural exchange.
January 2019
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