Arboriculture & Urban Forestry 48(2): March 2022 on surrounding natural areas while promoting acces- sibility and walkability, reducing driving mileages, and creating efficient use of urban land (Haaland and van den Bosch 2015). Yet densification often com- petes against other needs such as urban canopy and green spaces, in particular. Densification intensifies UHI due to increased impervious surfaces, lowered albedo, altered urban geometry, and reduced green spaces (Erlwein and Pauleit 2021). Such develop- ment patterns are evident in this work, and they pres- ent a major threat to urban canopy health such as loss of green spaces and urban canopy shading quality due to insufficient soil volume and competition with utilities below ground and above ground, compacted and degraded soil, and limited access to water (Haa- land and van den Bosch 2015). The outcome of this research can inform planners, urban foresters, and policy makers to better address urban densification and urban green space. Previous research has suggested that dense areas are not neces- sarily less green (Guo et al. 2019). (Re)development of properties occurring alongside the densification process might be one of the main culprits of the urban tree canopy decline due to its impacts on soil quality and volume, surface paving, and aerial and under- ground space available for trees (Haaland and van den Bosch 2015; Jim 2017; Guo et al. 2018; Guo et al. 2019). However, Metro Vancouver (2019) has indi- cated an increasing trend for tree canopy in newer and higher-density residential areas, while tree canopy on low-density existing housing parcels has declined from 36% in 1970 to 18% in 2000 on average. There has also been a declining trend in the levels of impervious surface in high-density housing parcels since the 1950s in Metro Vancouver (Metro Vancouver 2019). Although this study found some good examples of areas that can be both green and dense, it should be noted that the causes of urban canopy decline are complex. Managing urban canopy requires robust policy and community support, proactive planning and design, and adequate management, especially in cities that are going through densification. Densifying cities can also learn from the practices of cities and countries that manage to maintain a high level of both urban canopy and density, such as Hong Kong (Tan Z et al. 2016) and Singapore (Tan PY et al. 2013). In addition, the modeling framework and outcome from this work demonstrate many potential research and planning implementation opportunities. For exam- ple, being able to systematically identify surfaces 107 with limited shading, particularly on façades and roofs, planners and builders can prioritize constructions such as green roofs or walls (e.g., vertical greeneries) to improve overall shading quality and quantity. Green roofs, if applied at the city scale, may reduce the average ambient temperature by 0.3 to 3 °C. Vertical greeneries are another technique that has been increasingly adopted in cities for thermal comfort and energy conservation. Both green roofs and vertical greeneries provide an excellent opportunity to mitigate urban heats, enhance indoor thermal comfort, and potentially reduce build- ing energy consumption for cooling. They also present new ways to increase green areas with various eco- logical and social co-benefits in underutilized spaces like roofs and walls, especially in densely populated areas where space is a major limiting factor (Alexandri and Jones 2008). Urban form, land use patterns, and even local social conditions can also play a role in over- all shading efficacy. We found that although neighbor- hoods with lower building height (e.g., single-family residential areas) appear to benefit from more canopy shading, it was possible to maintain both residential density and tree canopy, and consequently, canopy shading benefits in some neighborhoods. This was mainly caused by the Vancouverism architectural style that features high floor area ratio (i.e., small lot sizes with tall buildings), preserving pervious surface for trees and other vegetation (Metro Vancouver 2019). According to an interview of Vancouver’s staff, neighborhoods like Kitsilano and West End are also older neighborhoods where native (although disturbed) soil has been preserved. However, there are no regu- lations to preserve native soil, which might be a con- tributing factor in some neighborhoods with limited canopy cover. Moving forward, the authors have identified cur- rent caveats and future research directions that can further improve the current project and modeling capability. Firstly, this model currently uses constant weather and climate without considering the seasonal variations or other urban form features that could impact tree shading performance. Middel et al. (2021) presented possible research opportunities for this work and proposed a series of shade performance curves that characterize various urban tree shading efficacy for the City of Tempe, Arizona, offering a more comprehensive assessment of urban tree shade performance. Secondly, the canopy evapotranspira- tion was not included in the current modeling param- eter, which likely caused an underestimation of the ©2022 International Society of Arboriculture
March 2022
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