6 Jutras et al.: Appraisal of Key Abiotic Parameters Affecting Street Tree Growth Table 3. Contingency analysis of presence/absence of metal grating and penetration resistance (frequencies per class). Sampling zone Metal grating Surface level 15 cm depth (5.9 in) 30 cm depth (11.8 in) 45 cm depth (17.7 in) Absence Presence Absence Presence Absence Presence Absence Presence Class 1 29% 36% 7% 11% 2% 6% 2% 3% Penetration resistance Class 2 26% 25% 16% 15% 8% 8% 3% 4% z Ho: independence of variables not rejected (significance level α is > 0.01). tive metal grating was a highly significant parameter for most species. A possible explanation is that, as gratings are found only on commercial streets, their influence is confounded by the strong relationship between urban tree growth and type of urban zone. Urban zones are defined by unique patterns of growing space, tree canopy cover, and housing density (Nilon 1991). In a Mis- souri, U.S. study, trees in commercial zones averaged 24 cm (9.5 in) DBH, but mixed commercial/residential trees (29 cm/11.5 in) and residential trees (31 cm/12.2 in) were larger (Gartner et al. 2002). Comparable figures were obtained during the course of this study. For species taken together, annual mean DBH incre- ments for intensive commercial zones (Type 1) and commercial zones (Type 2) were respectively 0.53 cm/year (0.21 in/year) and 0.78 cm/year (0.31 in/year), while values for institutional (Type 3), intensive residential (Type 4), and residential (Type 5) were 1.18 cm/year (0.46 in/year), 1.03 cm/year (0.41 in/year), and 1.02 cm/year (0.40 in/year), respectively. More particularly, for each significant species contingency result (refer to Table 2), stressed trees were primarily found in commercial zones 1 and 2 (details in Table 4). For example, when examining the distribu- tion of trees in the all-species matrix, it can be seen that 82% of the poorly growing trees were located in these sectors. Like- wise, normal-growing or vigorously-growing trees were found in intensive residential and residen- tial areas. Yet, for most species, such normally or vigorous trees were also found in Zones 2 and 3, indicating trees can withstand harsh environmen- tal conditions. The all-species fast-growing class typifies this last assertion: a total of 45% of the studied trees are associated with commercial and institutional areas (Table 4). All other things being equal, this phenomenon might be explained by the influence of irradiation levels on growth patterns. Uneven irradiation intensity is a trait of the urban environment. When comparing studied resi- dential and commercial streets, differences were found in the distribution of irradiation levels. Residential areas in Montreal are typically one to four-story buildings. For this reason, most residen- tial streets receive relatively important light levels (mean irradiation value of 1,495 hours during the active vegetative growth period). On the contrary, irradiation of commercial zones varies and trees can be exposed to total light levels as low as 205–480 hours. Previous research from Kjelgren (1995) ©2010 International Society of Arboriculture estimated that 70%–85% potential irradiance is a threshold level for increased or decreased leaf area, trunk growth, and crown density of Norway maple ‘Emerald Queen’ trees. Results from our study support this affirmation not only for this species but also for silver maple, hackberry, green ash, honeylocust, and Siberian elm. When growth patterns of these species were pro- jected with irradiation levels in commercial zones, it was found that light levels influenced class membership (Figure 1). For instance, the best growing classes for Norway maple, silver maple, green ash, and honeylocust were respectively associated with 81%, 83%, 80%, and 88% of total potential irradiation es- timated with our algorithm at the latitude of Montreal and with no physical barriers to light interception (a maximum of 1,615 hours during the active vegetative growth period). Similarly, hackberry and Siberian elm best growth classes were associated with 99% and 98% of total potential irradiation, respectively. In order to compare these results with the potential light con- ditions at transplantation time, irradiation levels were comput- ed at a height of 4 m rather than mid-crown elevation for every sampled tree pit in commercial zones. Tree pits associated with slow growing trees had a 4 m mean irradiation value of 1,182 hours (73% of total potential irradiation), while those related Class 3 22% 13% 15% 16% 12% 15% 6% 7% Class 4 23% 26% 62% 58% 78% 71% 89% 86% χ2 Pearson statistic 10.26z 3.45z 7.26z 9.59z Figure 1. Irradiation levels per species and growth rate in commercial zones (mean values ± standard error).
January 2010
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