Arboriculture & Urban Forestry 36(1): January 2010 Arboriculture & Urban Forestry 2010. 36(1): 1–10 1 Appraisal of Key Abiotic Parameters Affecting Street Tree Growth Pierre Jutras, Shiv O. Prasher, and Guy R. Mehuys Abstract. Street trees must cope with severe environmental conditions. Identification of factors that modulate their survival and growth is a key process for successful management. To estimate these factors, contingency analyses were carried out using abiotic data collected locally for Nor- way maple (Acer platanoides L.), silver maple (Acer saccharinum L.), hackberry (Celtis occidentalis L.), green ash (Fraxinus pennsylvanica Marsh.), honeylocust (Gleditsia triacanthos L.), littleleaf linden (Tilia cordata Mill.), and Siberian elm (Ulmus pumila L.) trees. Also, compar- able analyses were performed on a merged all-species data set. Whereas sharp differential growth was found between commercial zones and resi- dential areas, studied species demonstrated ecological tolerance as individuals were found normally or vigorously growing in severe con- ditions. In this situation, ample irradiation intensity might be an important factor contributing to urban tree growth. Statistical analyses led to the identification of a threshold level (80% of total potential irradiation) above which optimal growth was found in commercial zones. Research results also brought out the importance of the underlying surficial deposits: slow growth rates were linked to the presence of sand/gravel de- posits. Finally, higher soil de-icing salt concentrations were found in tree pits where trees were transplanted closer to the curb and on wider streets. Key Words. Compaction; Contingency Analysis; De-icing Salt Effect; Light Irradiation; Street Trees; Surficial Deposit; Urban Zone Type. Trees provide a basic contact with nature, heighten pleasure in our surroundings, and directly improve human health and the quality of life (Lohr et al. 2004). They soften architectural lines and accentuate structural details, relieve the monotony of pave- ment and masonry, make enticing play areas, and unify and give coherence to visually chaotic scenes (Gorman 2004). In retail districts, visitors perceive the streetscape canopy to be an integral amenity of the city’s shopping environment and well-planned canopy-covered streets are highly appreciated (Wolf 2004). Yet, urban landscapes are environmentally detrimental to the biology of trees (Percival 2005). During summer months, high ambient temperatures may trigger water deficits and exceed the optimum range for photosynthesis (DeGaetano and Hudson 2000). Soil water resources in the street planting pits are generally reduced during this period as well (Nielsen et al. 2007). Severe physi- ological problems then develop: impaired respiration, protein synthesis, secondary carbohydrate metabolism, disintegration of leaf structure, and reduced growth of stems and twigs, followed by death of terminal buds and branches and by production of small and chlorotic leaves (Kozlowski and Pallardy 1997). Ad- ditionally, temperatures exceeding 30°C (86°F) in root zones at planting sites are not uncommon and may exceed 40°C (104°F) (Celestian and Martin 2004). Freezing temperatures have also been identified as a major detrimental condition (Collins et al. 2000). Spring and autumn frosts may injure trees and induce frost cracks or sunscald on stems (Harris et al. 1999). Low soil tem- perature inhibits both root initiation and growth of existing roots, decreases absorption of water by increasing the resistance to wa- ter movement across living root cells, and reduces both passive and active uptake of minerals (Kozlowski and Pallardy 1997). Urban soil conditions may also be adverse for adequate tree development. Common conditions are deficiency in nutrients (De Kimpe and Morel 2000); compaction in and around tree pits and construction sites which lowers soil aeration and water movement and impairs root growth (Smith et al. 2001; Percival et al. 2004); restricted water drainage and water-holding capac- ity; interrupted organic matter and nutrient cycling; modified soil organism activity; significant concentration levels of heavy met- als (Lu et al. 2003); and modified soil reactions consequential to use of de-icing salts (Zimmerman et al. 2005). Indeed, dur- ing winter, large volumes of de-icing salts are applied in some cities. Total de-icing salt use in cold climates has been rising since 1970 due to increasing urbanization, roadway construc- tion, and public safety concerns (Paludan-Miller et al. 2002; Percival and Fraser 2007). High salt concentration induces iron and potassium chlorosis (Lemaire and Rossignol 1999), and severe dehydration of leaves that closes stomata and reduces photosynthesis (Zimmerman and Jull 2006). It is one of the most serious causes of streettree stress (Cekstere et al. 2008). Frequently, trees are confronted with insufficient irradiation 1999). Pollutants not only injure trees but influence their metabo- levels. Certain downtown environments are known as urban can- yons because the shade patterns cast by buildings are somewhat analogous to the shade cast by canyon walls (Harris and Bassuk 1993). These shady urban sites can impose severe stress on trees. Low light intensity levels bring in leaf necrosis and significant root growth reduction (Kjelgren 1995). Additionally, trees are adversely affected by a wide variety of environmental pollut- ants. The most important are sulfur dioxide (SO2 fluorides (F), oxides of nitrogen (NOx ), and particulates (Garrec ), ozone (O3 ), ©2010 International Society of Arboriculture
January 2010
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