8 Jutras et al.: Appraisal of Key Abiotic Parameters Affecting Street Tree Growth should preferably minimize the infiltration of de-icing salts into soil, broadcasting machinery should be frequently calibrated and dosage reduced to the minimum acceptable level. Salt-tolerant species selection is also critical: only the right plant will have the ability to survive and thrive (Watson and Himelick 1997). Trees within the experimental protocol were distributed among three soil types. Clayey soils were associated with 57% of tree locations, sandy soils with 27%, and tills with 23%. With such statistics, one might expect a strong abiotic effect related to clays. Yet, contingency analysis depicted a different result. For species exhibiting significant relationships (refer to Table 2) the most im- portant outcome was the presence/absence of sand and gravel. In all cases, limited growth rates were closely linked to presence of sand/gravel deposits (Table 6). Conversely, distribution of clus- ters within tills and clay soils demonstrated no such tendency. Urban planting site evaluation requires the assessment of local soil types. Generally speaking, professionals primar- ily consider the presence of clays as they are poorly drained soils that frequently limit planting success. It is well known that, in many cases, newly planted urban trees die when roots “drown’’ from too much water (Watson and Himelick 1997). Nonetheless, the contingency analysis results suggest that sand/ gravel textured soils should also be closely looked upon as an important limiting factor. These soils have a low water hold- ing capacity. When they dry out, the rapid decrease in soil water potential places the vegetation under great stress. In ad- dition, they are characterized with a low cation-exchange capac- ity and readily loose nutrients through leaching (Craul 1992). Table 6. Frequencies in the contingency analysis between species-specific tree growth patterns and types of surficial deposits. Species Norway maple Green ash Honeylocust Growth rate Slow Fair Fast Slow Fair Fast Fastest Slow Fair Fast Fastest All-species matrix Slow Fair Fast Fastest Till 7% 28% 34% 2% 16% 19% 16% 11% 13% 16% 43% 10% 27% 23% 29% Surficial deposits Clay 38% 31% 40% 58% 63% 65% 77% 57% 70% 67% 52% 48% 46% 57% 51% SUMMARY AND CONCLUSIONS Frequently, studies of urban effects on street tree stress can become largely descriptive because the geographic separation between study trees results in the confounding of innumerable factors and precludes rigorous hypothesis testing (Cregg and Dix 2001). With- in the scope of this research, use of contingency analysis circum- vented this difficulty and noteworthy conclusions were derived. Analyses with the Pearson χ2 statistic and Fisher’s exact test brought out central parameters related to differential street tree growth. Among them, urban zone type, surficial deposit, irradia- tion level, street width, and distance from tree to curb were the ©2010 International Society of Arboriculture Sand-gravel 55% 41% 26% 40% 21% 16% 7% 31% 17% 17% 5% 42% 27% 20% 20% most important. Unsurprisingly, stressed trees were found pri- marily in intensive commercial and commercial zones. In con- trast, normally-growing or vigorous trees were found not only in intensive residential and residential areas, but also in commercial and institutional zones indicating that the species under study can tolerate harsh environments. In this situation, ample irradiation intensity might be an important factor contributing to satisfac- tory urban tree growth. All other things being equal, contingency analysis led to the identification of a threshold level above which adequate growth was found in commercial zones for Norway ma- ple, silver maple, hackberry, green ash, honeylocust, and Siberian elm. Minimally, 80% of total potential irradiation estimated with the algorithm at the latitude of Montreal (1,285 hours during the active vegetative growth period) is required for optimal growth. Research results also illustrated the importance of the underly- ing surficial deposits. In all cases, slow growth rates were closely linked to the presence of sand/gravel deposits. Finally, higher soil de-icing salt concentrations were found in tree pits where trees were transplanted closer to the curb and on wider streets. The geographic location of Montreal entails particular envi- ronmental conditions and any extrapolation of the findings of this study to other tree species, urban conditions and/or geo- graphic location should be made with caution. Nevertheless, it is suggested that the assessment of these abiotic parameters become an essential component of street-tree management pro- grams in order that pre- and post-transplantation procedures be specifically adapted to the local urban environmental conditions. Acknowledgments. The authors are grateful for the financial support of City of Montreal and the Natural Science and Engineering Research Council of Canada (NSERC). We also express gratitude to anonymous reviewers for their pertinent comments on the original manuscript. LITERATURE CITED Annandale, J.G., N.Z. Jovanovic, G.S. Campbell, N. Du Sautoy, and P. Lobit. 2004. Two-dimensional solar radiation interception mod- el for hedgerow fruit trees. Agricultural and Forest Meteorology 121:207–225. Ball, J., S. Mason, A. Kiesz, D. McCormick, and C. Brown. 2007. As- sessing the hazard of emerald ash borer and other exotic stressors to community forests. Arboriculture & Urban Forestry 33:350–359. Bauerle, W.L., J.D. Bowden, M.F. McLeod, and J.E. Toler. 2004. Model- ing intra-crown and intra-canopy interactions in red maple: assess- ment of light transfer on carbon dioxide and water vapor exchange. Tree Physiology 24:589–597. Berrang, P., D.F. Karnosky, and B.J. Stanton. 1985. Environmental factors affecting health in New York City. Journal of Arboriculture 11:185–189. Boyer, L., A. Bensoussan, M. Durand, R.H. Grice, and J. Berard. 1985. Geology of Montreal, Province of Quebec, Canada. Bulletin of the Association of Engineering Geologists 22:333–394. Brady, N.C., and R.R. Weil. 2002. The Nature and Properties of Soils. 13th Edition. Prentice Hall, Upper Saddle River, New Jersey. 960 pp. Brazeau, A. 2003. Sable et gravier de la grande région de Montréal. Res- sources minérales de la grande région de Montréal. In: D. Brisebois (Ed.). Géologie Québec, Ressources Naturelles. pp. 33–36. Cekstere, G., O. Nikodemus, and A. Osvalde. 2008. Toxic impact of the de-icing material to street greenery in Riga, Latvia. Urban Forestry & Urban Greening 7:207–217.
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