©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(6): November 2023 323 programs a cornerstone of their sustainability plans (City of Los Angeles 2018; City of New York 2015; Vancouver Board of Parks and Recreation 2018). However, less attention has been paid to the manage- ment costs that are required to address issues of long- term vitality among newly planted trees (Roman et al. 2021). Our case study identified a greater lack of resiliency among public trees than private trees due to the impact of historical planting decisions, making adequate planning and funding for the maintenance of the existing urban forest an essential complement to tree-planting programs. Similarly, the lack of diversity in the baseline urban forest, which was primarily composed of pub- lic trees, calls attention to a challenge that is common to numerous locations beyond Vancouver: few spe- cies currently planted are on the “highly suitable” list for Metro Vancouver, as demonstrated by the findings captured in Figure 2. These findings make it clear that some species of trees should be put on a pause list for planting, both because of their unsuitability in pro- jected future climates and because they have been planted in such large numbers over prior decades, resulting in a less diverse urban forest that is more vulnerable to the effects of urbanization and to climate- related disasters such as droughts, flooding, and pest outbreaks. Promoting high biodiversity in urban ecosystems is a cornerstone of preserving global biodiversity (Alvey 2006), and there is significant evidence that urban landscapes can achieve high biodiversity (Ordóñez and Duinker 2012; Sushinsky et al. 2012). Tratalos et al. (2007) argue that higher-density nodes in large areas of greenspace create a complex form that improves biodiversity, if designed well. In a modelling study comparing compact and sprawling development patterns, researchers found that com- pact developments had comparatively low ecological impact, but that this “depends crucially on maintain- ing high-quality interstitial greenspaces between high density developments” (Sushinsky et al. 2012, p. 407). This final point also draws attention to the impor- tance of public-private partnerships, especially in light of the shift in trees planted on public property to those on private property that our results demonstrate is essential to achieving anything close to the City of Vancouver’s overall canopy coverage aims. Although in the private realm, it’s critical that these trees be selected from our list of identified climate-resilient species, which can only be accomplished via a public awareness campaign accompanied by the provision of saplings of these species to private property-holders. Such an effort is already underway in London, England, where the Mayor’s Trees for London pro- gram is working with a nationwide greenspace advo- cacy group to provide packs of 50 carefully selected 2-year-old saplings at no cost to organizations such as housing associations and schools, along with a detailed planting guide that covers issues of aftercare and long-term maintenance (The Conservation Volunteers 2022). As the inclusion of aftercare instructions acknowledges, supporting private trees immediately after planting is vitally important, particularly to off- set the increased vulnerability of newly planted trees to both drought and flooding. Maintenance issues were also one of the largest concerns expressed by residents who were invited to participate in The Greening of Detroit’s street tree-planting program, along with frustrations about the limited opportuni- ties to select trees they found aesthetically pleasing and a good fit for their properties (Carmichael and McDonough 2018). In light of the fact that achieving the diversity aims of our urban tree trifecta will nec- essarily constrict the types of trees available to plant on private property, this last finding further highlights the importance of both broad-based public education campaigns and more detailed guidance for participat- ing planters. As with any modelling study, this effort has a num- ber of limitations. Principal among these limitations is the fact that any model is only as robust as its under- lying inputs, and our inputs relied on a number of assumptions. These include the extrapolation of base- line species based on what could be visually assessed via Google Street View in a sample of 100 randomly chosen locations across the case study area, rather than an actual inventory of all trees on private prop- erty. For future efforts, this limitation could be over- come by the incorporation of a survey sent to neighbourhood residents asking them to count and identify the trees on their property or by the integra- tion of a citizen-science approach in which individu- als are trained to conduct ground-truthing of species data by taking photographs of private trees. Alter- nately, novel machine-learning approaches may suc- cessfully detect more than 70% of all public and private trees identified via ground-truthing (Lumnitz et al. 2021), allowing for less time-intensive
November 2023
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