©2023 International Society of Arboriculture 314 Barron et al: Scenario Modelling for Resilient and Diverse Urban Forests in Densifying Cities Mitigation strategies are just as crucial as adaptation solutions, because they aim to stave off the most dras- tic harms of climate change by reducing greenhouse gas emissions or increasing carbon storage (European Commission 2015). One such strategy is urban densi- fication, which has been linked to improvements in building energy efficiency and population-level shifts to lower-emissions transportation methods—such as walking, cycling, and the use of public transit—both of which can reduce per capita greenhouse gas emis- sions (Tonne et al. 2021). Conversely, more dispersed community designs have been linked to significantly larger GHG emissions, due both to a greater reliance on private-vehicle travel over larger distances (Ewing and Hamidi 2015) and to a higher amount of per-capita infrastructure, such as single-family homes rather than multi-unit dwellings (Norman et al. 2006; Hamidi et al. 2015). How urban forests perform with increased density is not well understood (Haaland and van den Bosch 2015). In terms of urban forest health, urban densification can also lead to a lack of permeable sur- faces, insufficient soil volume to grow healthy trees, and less space for large patches of urban greenspace. At the same time, many urban-planning policies that seek to increase density may threaten existing urban forests. In already developed urban cores, these threats occur in 2 principal areas. First, as lots that once held single-family homes or low-rise multi-unit dwell- ings are redeveloped into larger multi-unit develop- ments, existing private trees are often cleared for the larger required footprint. Numerous cities have tree protection ordinances (TPOs) in place that regulate the replacement of lost trees, governing issues such as the size and species of replacement trees, which trees are subject to replacement, and how these rules are enforced (Lavy and Hagelman 2019). However, delv- ing into the TPO for Vancouver, Canada demonstrates a number of factors that may result in replacement trees that provide fewer ecosystem services than the original components of the urban forest. In Vancouver, these issues include the following: (1) the rule applies to trees of at least 20 cm in diameter at removal, but replacements need only be 6 cm at the time of plant- ing; (2) tree species are required to be selected “for disease resistance and hardiness” but not to be chosen with a changing climate in mind; and (3) the require- ment as a whole can be met in lieu by paying a rela- tively small fee (City of Vancouver 2022a). In addition, even carefully selected and well-cared-for Toronto, Canada had the same impact on overall health improvements as an additional $10,000 in yearly household income (Kardan et al. 2015). Likewise, individuals living in areas with the highest tree den- sity within 1 kilometer of their homes in New York City were 23% more likely to report very good or excellent health in comparison to residents of areas with the lowest tree density (Reid et al. 2018). In terms of psychological health, the evidence is even more robust. Greater street tree density was linked to sig- nificant reductions in depressive symptoms among residents of socioeconomically deprived neighbour- hoods across the Netherlands (Gubbels et al. 2016) and to reduced rates of antidepressant prescriptions among lower-socioeconomic status individuals resid- ing in Leipzig, Germany (Marselle et al. 2020). More complex assessments of urban forests also demon- strate benefits. A recent evaluation of the 3-30-300 rule for urban forestry carried out in Barcelona, Spain found that individuals who could see at least 3 trees from their windows, had 30% canopy cover sur- rounding their homes (as assessed via a measure of residential surrounding greenness), and were within 300 meters of a park or other greenspace were 69% less likely to have visited a psychiatrist or psycholo- gist over the prior year (Tonne et al. 2021). Healthy urban forests also indirectly benefit human health by serving as a solution in climate-change resilience efforts. Resilient urban forests seek to reduce the harms to humans associated with high greenhouse gas (GHG) emissions (European Commission 2015) and to increase capacity to help cities recover from disturbances related to climate change (Huff et al. 2020). A systematic analysis of existing reviews that have examined the potential of various urban natural environments to serve as NBS found robust evidence for the substantial cooling effect of urban greenery, offsetting the increased risks of heat stress, chronic -disease exacerbation, and excess mortality in increas- ingly hot cities (van den Bosch and Sang 2017). Addi- tional insights come from modelling efforts: a comprehensive urban greening scenario that inte- grated a 30% increase in street trees along with green roofs and vegetated impervious surfaces (such as car- parks) across greater Manchester in the United King- dom linked this “deep green” development with a reduction of 3 to 4.9 °C in surface temperatures, even in the context a high-emissions future (Carter et al. 2015).
November 2023
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