Arboriculture & Urban Forestry 44(3): May 2018 In the current study, trees that were dead, trees that displayed poor canopy condition, and trees that were planted in pits had soil conditions with significantly higher levels of Na and EC than other trees (Table 3; Table 4; Table 5). The presence of salt ions may have caused osmotic and ionic stress to the trees, dehydrating plant tissue, impair- ing photosynthesis, and accelerating leaf senes- cence (Munns and Tester 2008). Although it could be argued that the choice of tree species for this stretch of commercial-retail street was poor, given that London planetrees have a low to moderate salinity tolerance (Gilman and Watson 1994; Mor- ton Arboretum 2017), it is unclear if another tree species would perform differently in this environ- ment, as in many situations, de-icing salts accu- mulate long-term, and the high soil salinity caused by this accumulation negatively affects tree con- dition regardless of species (Equiza et al. 2017). But de-icing salts alone do not explain the patterns of tree mortality on Bloor Street. Ulti- mately, some of the trees that were still alive before removal had high soil concentrations for Na and EC (Table 4). The characteristics of the planting sites, such as type of planter, may have exacerbated tree decline and mortality for at least some of the 133 trees. Although the type of planter was not significantly associated with mortality rates (Table 2), soil within pit planters had significantly higher levels of salts (Table 5). These planters may have been more sus- ceptible to salt influx, as is commonly the case with street-level planters (Hootman et al. 1994). Although soil compaction can influence salt accumulation (Grabosky et al. 2001), the analysis did not see this relationship directly, as compac- tion remained below the suggested threshold of between 2.0 and 2.6 MPa (Day and Bassuk 1994; Day et al. 2010; Millward et al. 2011). In addition, soil alkalinity along Bloor Street was generally higher than the recommended pH 6.5 for growing trees in urban areas (Jim 1997; Trowbridge and Bassuk 2004), especially in pit planters (Table 5). The associations between pH and Na, Mg, and Ca seen in the data (Table 1), corroborate previous reports on how elevated salt concentrations and pH result in conditions amenable to leaching of soil nutrients, a known stressor of plant health (Gałuszka et al. 2011; 141 Eimers et al. 2015; Kargar et al. 2015). How- ever, given the lower concentration of Mg in pit planters, compared to bed planters (Table 5), Mg concentrations could have been replenished by the application of fertilizers to the flowers in the bed planters. Despite the strong associations this study found between soil chemical variables and tree condition, more research is needed on the interactions among nutrients, salinity, and pH, in urban soils, and their collective and cumulative effects on urban tree performance. Researchers’ analysis identified sunlight avail- ability as an important factor influencing tree mortality patterns. Lack of available sunlight has been suggested as a possible cause for poor tree performance (Jutras et al. 2010). In the current study, trees that were in better condition had more available direct sunlight at the beginning of the growing season, but lower seasonal varia- tion (Table 4). Another important association of this factor is inferred indirectly. Since the north side of Bloor Street received more direct sun- light compared to the south side (Table 6), and this side displayed higher rates of tree mortality (Table 2), it is suggested that too much sun- light may have affected tree health at the north side through heat stress and soil moisture loss. Since elevated soil temperatures in the rooting zone can damage existing roots and inhibit root growth (Gillner et al. 2013a), researchers specu- late that these conditions may have exacerbated soil moisture loss through evaporation and thus influenced tree mortality and condition patterns. Finally, trees with more physical damage (e.g., broken branches, trunk scars, missing bark) displayed higher rates of mortality along Bloor Street (Table 2). This is consistent with the literature on tree condition in urban areas (Nowak et al. 2004; Lu et al. 2010). Nonethe- less, some of this physical damage to tree limbs and bark may have resulted from weather con- ditions or storm events between 2011 and 2015, since bark peel is a common manifestation of heat stress in trees, and ice damage causes bro- ken branches (Trowbridge and Bassuk 2004). However, researchers were not able to quan- tify actual physical damage to trees resulting from meteorological conditions, as weather data were not compatible with the analysis. ©2018 International Society of Arboriculture
May 2018
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