©2023 International Society of Arboriculture 318 on trees not included in the OSU database were col- lected from Kwantlen Polytechnic University (Kwan- tlen Polytechnic University 2015), Missouri Botanical Gardens (Missouri Botanical Gardens [date unknown]), and Plants For A Future (Plants for a Future [date unknown]). Because only mature tree heights were consistently provided within the consulted online plant databases, 30-year canopy spread (i.e., diame- ter) was estimated as 45% of average mature tree height. Details about growing trees of each species to 30-year sizes for this project, and further information about scenario data and models, are provided in Cze- kajlo et al. (in press). RESULTS The results of the virtual planting process carried out for our case study offer indications of the potential of the urban-forest scenario modelling process to inform the development of salubrious, resilient, and diverse urban forests in the context of a changing climate and densifying populations. Table 1 lists the top 15 species planted across public and private settings, demon- strating significant differences in the composition of these 2 components of the overall urban forest. Most notable is the absence of coniferous trees as common planting choices in the public realm, and their relative abundance in the private realm, with only Prunus found as a top planting choice in both. This distinction in planted species between the public and private realms is particularly important in light of a separate finding from the case study: resil- ience and diversity require an increasing reliance on trees planted on private property in order to achieve a substantial increase in canopy cover. At our study’s baseline, the total canopy cover within the sandbox was 6.6%, with 74% of this coverage coming from publicly maintained trees and 26% from private trees. By 2050, the canopy cover had grown substantially to 15.6% (which was just over half of the scenario’s stated aim of 30%) and the tree ownership ratio had shifted dramatically: 57% of coverage was now pro- vided by private trees. This change in tree ownership was accompanied by a shift in the average size of trees, both with respect to height and crown radius. In the baseline tree inven- tory, tall trees (those greater than 15 m) comprised 25% of the canopy, but this fell to 15% of the canopy in 2050, due primarily to the replacement of fully mature trees (at least 50 years old) with the Barron et al: Scenario Modelling for Resilient and Diverse Urban Forests in Densifying Cities inputs, processes, and outputs used to create, run, and analyze the scenario models. Scenario data assign- ment and model development were conducted sys- tematically using a rules-based approach via R scripts (R Core Development Team 2017), including the fol- lowing packages: ggplot2 (Wickham 2016), tidyverse (Wickham et al. 2019), rgdal (Bivand et al. 2022), raster (Hijmans et al. 2015), and sf (Pebesma et al. 2022). Additionally, some minor manual editing was performed using ArcGIS Pro (Version 2.9.2). Inputs used to create the 2050 scenario models included information about which trees were directly sourced from the baseline 2020 model (i.e. “existing” trees). Trees that were not selected for the 2050 models due to pre-determined mortality rates, based on rates pro- vided by Hilbert et al. (2019) and species prioritiza- tion rules, were removed and considered “aged out”. For replacement and additional trees that were new in the 2050 models, quantities were determined through analysis of available spaces in the neighbourhood. Representative species were selected across small, medium, and large categories from the climate-adapted tree species list (Metro Vancouver 2019a), ensuring that diverse families and genera were represented. In alignment with the blue-green streets concept, trees that are known to tolerate the conditions in stormwater swales were prioritized. Trees from each size cate- gory were then selected for a site based on the loca- tion of the planting area. For example, we “planted” new trees to reach a goal of 2 trees per parcel in pri- vate yards, selecting small and medium species in recognition of the fact that most yards are unable to accommodate large trees. To efficiently run the scenarios, all trees were “planted” in the modelled environment during the year 2020 so they would be of exactly the same age 3 decades later. Data on species’ average 30-year growth curves were unavailable or inconsistent across the range of online plant guides and databases appropri- ate for the study area’s climatic region, which included those produced by Oregon State University (OSU), the University of British Columbia (UBC), and the University of Florida (Breen 2022; University of Florida 2022; UBC Botanic Garden 2022). The OSU data provided the greatest extent of coverage about mature tree height for the species included on our planting list, however, so those details were used to estimate each planted tree’s height following 30 years of growth (estimated as 80% of mature height). Data
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