©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(6): November 2023 315 and applicability to policy decisions. We report the results of a case study in a neighbourhood in Vancou- ver, Canada with both low building density and low canopy cover in 2020, envisioning a future for the area in which both of these attributes would shift sub- stantially by 2050. In addition to these overall crite- ria, the future scenario sought to model, and thereby investigate, the implications of 3 additional planning decisions that reflect policy priorities in the region: (1) designing the urban forest in a manner that would achieve greater public-health benefits via the integra- tion of “blue-green streets” as a specific form of NBS; (2) prioritizing the inclusion of tree species that would be maximally resilient under a changing climate; and (3) integrating planting programs into the existing network of trees on both public and private property to ensure the highest possible diversity across the urban forest. This study was designed as a complement to Czekajlo et al. (in press), which offers a comprehen- sive description of the underlying scenario-development methodology, by elucidating the effects of embedding specific policy priorities. MATERIALS AND METHODS The City of Vancouver’s Urban Forestry and Sustainability Planning Located in southwestern British Columbia, Vancou- ver is Canada’s third largest municipality (following Toronto and Montreal) and has the highest population density of any Canadian municipality according to the 2021 Census, at 5,750 people per square kilome- ter (City of Vancouver 2022b). The city’s population increased by 4.9% between 2016 and 2021 to a total of 622,248 residents, comprising a quarter of the pop- ulation of the broader metropolitan region and account- ing for 17% of the region’s growth over this time period (City of Vancouver 2022b). Within the city, growth rates and population density vary widely, with some neighbourhoods reporting a population increase of nearly a quarter across the 5 years in between cen- suses and others remaining essentially flat. The down- town core reported the highest population density of any location in Canada, at 18,837 people per square kilometer compared to Toronto’s figure of 16,608 and Montreal’s of 8,367 (City of Vancouver 2022b). This pattern of urban densification is expected to continue over the coming decades, with the region projected to grow by nearly 1.5 million residents by 2050 (Metro Vancouver 2022). trees face additional pressures as their surrounding environments densify, including soil compaction due to land development, solid impediments to root pro- liferation (such as building foundations and roads), and toxic levels of heavy metals deposited in soil from high levels of vehicular traffic (Day et al. 2010). Finally, many of the direct, short-term effects of climate change pose perils to tree health as well as human health. One recognized way to help address future unknown disturbances is to carefully and stra- tegically increase the diversity of the tree population in terms of age, size, and species (Morgenroth et al. 2016). In British Columbia (BC), Canada, which includes the site of the current case study, recent years have been marked by a number of climate-related disasters, including devastating wildfires, a record- breaking heat dome, and severe flooding (Crawford 2022). Across coastal areas of the region, such floods are projected to become more common in the future, with simulation models indicating that the atmospheric rivers responsible for extreme precipitation will become 50% more frequent under projected warming scenar- ios (Espinoza et al. 2018). In addition to this increase in precipitation in the wet season, models indicate significantly less precipitation during the dry season, with the length of dry spells expected to increase by 22% over the historical average by the 2050s (Metro Vancouver 2016). Each of these climate-related disas- ters can negatively impact the urban forest. Wildfires can result in increased air pollution that can reduce the rate of photosynthesis or increase susceptibility to microbial and entomological stressors (Depietri et al. 2012). Heatwaves lead to crown dieback and mortal- ity among both native and exotic species (Marchin et al. 2022). Flooding reduces nutrient availability in the soil (Depietri et al. 2012). In order to develop and manage urban forests that are resilient in the face of these threats while provid- ing the full range of nature-based solutions to nearby residents, we must be able to project into the future. In this way, we can understand precisely what climatic conditions the urban forest will face over the coming decades, select the tree species that will be able to thrive under these conditions, and detail the full range of ecosystem services that will be provided as a result. The overarching objective of this study is to outline a framework for creating future urban-forest scenario models by clarifying the impact of each stage of the scenario-development process on a model’s outputs
November 2023
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