142 Ordóñez et al.: Influence of Abiotic Factors in a Commercial-Retail Streetscape Limitations There are several limitations to this study, the most important of which was researchers’ inability to process the collected climatic data, as the data could have provided strong analytical insight. This is due to the temporal variability of these data and requirement for microclimate-scale monitoring data in dense urban environments. It is recog- nized nonetheless that this is an important factor that deserves future exploration. Toronto, the lo- cation of the Bloor Street trees, is at the northern edge of London planetrees’ suggested range (GBIF 2017). Therefore, climatic conditions that could not be analyzed in this study may have also con- tributed to tree decline and mortality. Regard- less, these conditions would have co-occurred and cumulatively impacted tree mortality and de- cline along with the other factors described here. The associations between trees with larger DBH values and less mortality, better condition, and lower salinity levels are not conclusive (Table 1; Table 3; Table 4). This is because the historical data collected on the Bloor Street trees at this point did not provide enough information about tree replacements and replanting (e.g., which trees were replaced; when where they replaced; and what size of tree was planted initially and as a replacement), which is necessary to provide baseline conditions for a more adequate evalua- tion of tree performance. Tree size at the time of planting could have influenced a tree’s capacity to adapt to the growing conditions at Bloor Street. A further examination of this issue is warranted. Other limitations include the fact that soil analy- ses only provided a cross-sectional snapshot in time of the soil conditions, and these conditions will vary seasonally and from year to year. Salt concen- tration in soils is influenced by de-icing salt appli- cation protocol in relation to weather patterns, maintenance regulations specific to a property close to a tree planting site, as well as salt flushing rates, which were not considered in the analysis. Also, researchers recognize that leaf analysis may be necessary for an adequate examination of salt impact on trees, and that leaf damage may have been both a potential cause of decline and mortal- ity, as it could be a result of this process along Bloor Street. Finally, while the relatively small sample size of some variables was analytically restrictive, the multimodal analysis, based on a variety of simple ©2018 International Society of Arboriculture but sound multivariate statistics, did allow for a deeper examination of a greater number of fac- tors characterizing the Bloor Street environment. CONCLUSION The investigation of the Bloor Street trees high- lighted several important ways in which abiotic factors can influence tree condition and mortality in highly-urbanized settings, such as in down- town commercial-retail streetscapes. These factors include soil salinity and alkalinity, built environ- ment characteristics (e.g., planter type, such as pit or bed); sunlight exposure, and physical damage to trees. Importantly, researchers showed that tree performance in these settings cannot be explained solely by the influence of one variable, but rather by the co-occurring interaction among several vari- ables with a likelihood of cumulative influence. Structural soil cells are useful for providing soil volume and quality necessary to support the growth of trees along commercial-retail streetscapes. Their presence along Bloor Street allowed researchers to investigate an urban growing environment for trees that controlled for soil volume and several soil quality characteristics at the time of tree plant- ing. Nonetheless, investigating the movement of salts in structural soil-cell installations, and inves- tigating the mechanisms for restricting de-icing salt access to, and accumulation within, these cells, are useful topics for future research. This is instru- mental for developing strategies to reduce the negative impacts of salts on trees growing in these cells. Finally, with climate change increasing the incidence of extreme weather events, such as ice storms and freeze-thaw cycles (Chiotti and Lav- ende 2008), the application of de-icing salts could increase in the future and challenge urban tree per- formance in areas with low tolerance for ice accu- mulation, such as commercial-retail streetscapes. Acknowledgments. This research project was made possible by funding from the Ontario Centres of Excellence, the Tree Research & Education Fund, the Natural Sciences and Engineering Research Council of Canada, and DTAH. Funding to hire research assistants was provided by the Dean’s Office, Faculty of Arts, Ryerson Univer- sity. The authors thank Tanya Brown and Brian Brownlie of DTAH, Peter Simon of the City of Toronto, as well as Arsh Grewal, Chris Scarpone, Danielle Marcoux-Hunter, Jaclyn Winkler, Mihai Grosu, and Samantha Tangir of Ryerson’s Urban Forest Research and Eco- logical Disturbance (UFRED) Group. We also thank Ontario Line Clearing, for their accommodation of our data collection during tree removals on Bloor Street.
May 2018
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