134 or Ordóñez et al.: Influence of Abiotic Factors in a Commercial-Retail Streetscape died. Eventually, all the trees, living and deceased, were removed in 2015 and replaced. At present, arboricultural science contains insuf- ficient clues that could explain why so many street trees declined so rapidly at the Bloor Street loca- tion. Soil-cell technology is growing in popular- ity, and projects that involve structural soil cells to grow trees in urban areas are viewed in a generally positive light (e.g., Page et al. 2015; Brockbank and Slater 2016). However, to date, there are no investi- gations into factors contributing to tree decline and mortality under the conditions of commercial-retail streetscapes revitalized with structural soil cells. Moreover, although it is well-known that trees growing close to roadsides and highly urbanized streets suffer disproportionally from high levels of mortality (Hodge and Boswell 1993; Nowak et al. 2004; Roman and Scatena 2011), most research on urban tree growth, condition, and mortality is carried out at broad spatial scales, such as at the scale of an entire city. Tree perfor- mance at small spatial scales (streetscapes) and in commercial-retail settings is not usually assessed in-depth (Steenberg et al. 2017). Therefore, it is unclear how trees planted in commercial- retail spaces perform given the changing soil, microclimatic, and built environment conditions. Extrapolating from existing research would not be enough to explain precisely what variables may be contributing, as well as potentially inter- acting, to influence tree performance in specific urban planting sites, such as along Bloor Street. This study seeks to fill some of these research gaps by reporting on an investigation of the abi- otic factors that have influenced tree mortal- ity and condition along Toronto’s Bloor Street. To contextualize the environment of the Bloor Street trees and determine the point of depar- ture for this examination, researchers will review here the factors that influence tree performance in highly urbanized streetscapes. More details about the current research study are then provided. Abiotic Factors Influencing Tree Performance in Urban Areas The most important abiotic factors influencing tree performance in urban areas include: soil conditions; environmental conditions, such as microclimate and air pollution; physical characteristics of the built ©2018 International Society of Arboriculture environment (e.g., streetscape morphology); and tree maintenance practices (Steenberg et al. 2017). Soil provides the rooting medium and essen- tial nutrients for the aboveground growth of trees (Trowbridge and Bassuk 2004). However, soils found in highly urbanized streetscapes are generally not conducive to optimal tree growth (Bassuk and Whitlow 1987). Some of the most important rea- sons for this include: 1) a lack of soil volume avail- able for adequate root growth (Lindsey and Bassuk 1991); 2) low soil nutrient and organic matter con- tent (Cekstere and Osvalde 2013); 3) soil compac- tion, which hinders root development and water availability (Day et al. 2010); 4) elevated soil salinity (Czerniawska-Kusza et al. 2004), which causes osmotic stress to trees and is frequently manifest in leaf chlorosis (Munns and Tester 2008); 5) high soil alkalinity, which can influence nutrient avail- ability (Gałuszka et al. 2011); and 6) either poor drainage or low soil water-holding capacity (Nielsen et al. 2007). Frequently, many of these condi- tions coexist in urban streetscapes and collectively influence tree growth, condition, and mortality. Urban areas oſten have elevated summer tem- peratures due to the urban heat island effect (Souch and Grimmond 2006). Because of this, cold-adapted tree species may not perform as expected in certain cities that would otherwise be characterized as having a northern climate (Yang 2009). Similarly, elevated temperature in conjunc- tion with minimal precipitation can exacerbate plant water stress (Nielsen et al. 2007; Gillner et al. 2013a). Finally, although not usually a signifi- cant influence on tree decline, air pollution, such as tropospheric ozone, can cause some damage to tree leaves and reduce biomass production. This effect is species dependent (Xu et al. 2015). The urban built environment can also be a source of stress for trees. The geometry and density of buildings and other urban structures affect the irradiation essential for photosyn- thesis and plant growth, as well as the urban microclimate (Oke et al. 1989). This can further exacerbate the negative influence of urban micro- climatic conditions on trees, reviewed above. Although some trees can adapt to the low light environment of urban areas (Harris and Bas- suk 1993), a decreased exposure to sunlight hours can significantly affect urban tree growth
May 2018
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
