Arboriculture & Urban Forestry 44(3): May 2018 in urban canyons (e.g., a streetscape flanked by multi-story buildings) (Jutras et al. 2010). Moreover, urban trees are also regularly exposed to contact with humans that includes vandalism, as well as improper handling and maintenance at the time of planting or prun- ing, all of which can negatively and dispropor- tionately affect street trees because of their proximity to highly trafficked sidewalks and roads (Nowak et al. 2004). Decision-making processes during design projects, such as nurs- ery stock selection, the timing of tree planting, the use of watering bags for newly planted trees, the presence of metal gratings in the planting pit, among other factors, can also be major con- tributors to street tree condition and propensity for survival (Jutras et al. 2010; Lu et al. 2010). The consequences of these urban conditions to street trees include failure to establish (i.e., point at which trees start to grow again after trans- plantation; Sherman et al. 2016) and retarded growth rates (Jutras et al. 2010). An annual slow- ing of tree growth can be a precursor to mortality (Gillner et al. 2013b). Downtown commercial urban areas present some of the most challeng- ing conditions for trees to establish and grow (Roman and Scatena 2011). It has been reported that, in North American cities, trees growing along commercial-retail streetscapes survive for between 5 and 20 years, a considerably abbrevi- ated existence providing that many tree species can live for 75 years or more in good condi- tions (Nowak et al. 2004). Short-lived trees not only provide fewer benefits, but they ultimately cost more to maintain and replace, thus strain- ing municipal budgets and never manifesting their true potential when considering aesthetic and ecological benefits (Roman et al. 2013). The Current Study Bloor Street is a major east-west thoroughfare in Toronto. The area between Church Street and Avenue Road is one of the most commercially important urban spaces in Canada, housing high- profile retail businesses. This section of Bloor Street went through a period of revitalization, finishing in 2011 when the streetscape was rede- signed to include structural soil cells for planting trees. One hundred thirty-three London plane- 135 trees (Platanus × acerifolia) were planted in 2010 and 2011 in a combination of raised flowerbed and at-grade pit planters interconnected by the soil cells. London planetrees were chosen by the design team. This species is usually chosen be- cause it is tolerant of urban growing conditions, including soil compaction and drought (Gilman and Watson 1994; Sherman et al. 2016). Aſter five years, many of the trees manifested signs of severe canopy decline. Consequently, all the trees were removed, including both dead and living trees, in a re-planting effort from May to June of 2015. The aim of the study was to explain why the 133 London planetrees planted along Bloor from 2010–2015 performed so poorly. The objectives were to 1) collect and analyze environmental and ecological data describing the Bloor Street trees; and 2) identify which factors contributed the most to tree performance, explained by tree mortality and condition patterns. The focus of the study was to explain tree mortality pat- terns in a commercial-retail space revitalized with structural soil cells, rather than a compara- tive study of tree performance with and without structural soil cells. This study allowed research- ers to dig deeper into other characteristics that may contribute cumulatively to tree perfor- mance in these spaces. The researchers believe the results of this study will help guide future soil-cell research and the refinement of tree- planting projects along downtown streetscapes. MATERIALS & METHODS Site Description Toronto, a city located on the northwestern shore of Lake Ontario, is the provincial capital of Ontario and is the largest city in Canada (Statis- tics Canada 2011). The city has a mean annual rainfall precipitation, based on a 30-year span (1981–2010), of 831.1 mm, and a mean annual snow cover accumulation of 121.5 cm. The annual mean daily maximum and minimum tempera- tures are 12.9°C and 5.9°C, respectively, and the extreme maximum and minimum temperatures are 40.6°C and -32.8°C, respectively. The annual average number of hours of sunshine in Toronto is 2066.3, and there is an average of 170 degree- days above 10°C (Environment Canada 2014). ©2018 International Society of Arboriculture
May 2018
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