2 Jutras et al.: Appraisal of Key Abiotic Parameters Affecting Street Tree Growth lism and reduce their growth without causing visible injury. By weakening trees, air pollutants often predispose them to certain diseases (Kozlowski 1986). Finally, there is a growing list of ex- otic insects and pathogens that are current or looming threats to community forests throughout North America (Ball et al. 2007). Trees facing these arrays of man-made and natural stresses may have reduced life spans compared to trees in rural areas or natural habitats (Cregg and Dix 2001). Although estimates vary, life spans of trees in downtown urban areas are often less than 15 years (Nowak et al. 1990). Understanding the re- lationship between urban site conditions and tree growth is es- sential if remedial treatments are to be effective (Hodge and Boswell 1993; Mamy 1997). In order to achieve this goal, re- search is required to analyze and model the peculiar urban en- vironmental conditions in order to identify tree growth patterns. MATERIALS AND METHODS Study Site and Experimental Details The City of Montreal (Quebec, Canada) is located at latitude 45°30’N and longitude 73°34’W. Climate is continental and humid, with hot summers and cold winters. The mean annual precipitation based on an almost 30-year span (1971–2000) is 1,062 mm (42 in) with a mean annual snow cover of 226 cm (88.9 in) (Environment Canada 2008). The mean daily maxi- mum and minimum temperatures are 11°C (52°F) and 3°C (37°F), respectively; extreme yearly minimum and maximum temperatures are typically -34°C (-29°F) and 36°C (97°F). An- nually, there are an average of 1,800 hours of sunshine, 2,335 degree-days below 10°C (50°F), and 1,399 degree-days above 10°C (50°F) (Environment Canada 2008). Frost occurs be- tween October 1 and April 15; the typical snowfall season is from mid-November to the end of March (Boyer et al. 1985). During the ice age, Montreal was completely covered by gla- ciers. When glacial ice retreated north, lowlands were invaded by the Atlantic Ocean, forming the Champlain Sea. Such pro- cesses deposited considerable amounts of clays and silts, which are frequently stratified, neutral to alkaline, and poorly drained; some sandy soils which are acid to neutral, usually character- ized by rapid drainage; and finally tills, which evolved mostly into poorly drained loams, marine gravels, inter-fingered es- tuarine deposits, and coarse-grained alluvium (Brazeau 2003). Generally speaking, tills are the most abundant deposits with an approximate coverage of 46% of Montreal Island, clays are second with 23%, and sands of all types (glacial, marine, and river) are estimated at 18%. Other deposits are peat and marl, and rock (Prest and Keyser 1962). Overall, the City of Montreal has 4,460 km (2,734 mi) of roads, boulevards, and streets, where more than 240,000 public trees can be found. To define the experimentation strategy and sample size, ex- ploratory field work was carried out in 1999 and 2000 in dif- ferent urban ecological zones. Once preliminary results from probing surveys were analyzed, sampling procedures were re- vised. Final data collection took place during summer 2001. Rep- resentative streets of downtown, institutional, commercial, and residential areas were selected following these criteria: height of buildings and geographic orientation (variable irradiance conditions), importance of vehicular and pedestrian use, size of tree pits, and width of street as trees beside a wide, heavily ©2010 International Society of Arboriculture travelled street may be subjected to high salts and high winds (Berrang et al. 1985). Each studied street was assigned one of the following types: 1) intensive commercial, 2) commer- cial, 3) institutional, 4) intensive residential, and 5) residential. Intensive commercial zones have the most severe urban strain and trees are normally slow-growing and very much damaged. Commercial zones exhibit less intense use but tree growth rates are only slow to fair. Tree caliber is normally larger than in the former zone. Institutional zones are mainly used by pedestrians and cars but mostly only to move around districts. Less mechanical damage occurs and trees may have fair to superior growth rates. Intensive residential zones have mid-level pedestrian and vehicle circulation. Trees have nor- mally superior growth. Finally, residential zones are used by and large only by residents, and trees are typically vigorous. For each surveyed downtown street, a systematic sampling was undertaken (total of 961 trees). In districts outside the down- town zone, trees were randomly selected within commercial, institutional, and residential environments (total of 571 trees). All trees were geopositioned on a geographical information sys- tem. The final cohort was composed of 1,532 trees of species that are representative of 75% of the City street trees population: Norway maple (Acer platanoides L.) (312 trees), silver maple (Acer saccharinum L.) (224), hackberry (Celtis occidentalis L.) (187), green ash (Fraxinus pennsylvanica Marsh.) (245), hon- eylocust (Gleditsia triacanthos L.) (301), littleleaf linden (Tilia cordata Mill.) (116), and Siberian elm (Ulmus pumila L.) (147). Trees were sampled using 11 biotic variables identified as significant for urban tree inventories by multivariate statistical analyses (Jutras et al. 2009). These were diameter at breast height (DBH), crown diameter, height, crown diameter / DBH, crown volume / DBH, height / DBH, crown volume, annual DBH in- crement, crown diameter increment, height increment and crown volume increment. The last four incremental variables are associ- ated with size measurements. An annual crown volume increment was calculated as the difference between measured crown vol- ume and crown volume at transplantation, divided by the number of years since transplantation. To obtain transplantation data, 15 City of Montreal nursery trees of ball-and-burlap transplanting size were measured for every species and mean values were cal- culated. Other incremental indices were similarly constructed. Abiotic variables were selected for their ease of collection by end-users (arboriculture and horticulture city workers) once the model and methodology are deployed. The following param- eters were used: street type (five classes as defined earlier) and orientation (calculated from a geographic information system), width of street, distance from tree to the closest nearby building, height of opposite and adjacent buildings (modulation of solar irradiation levels), volume of tree pit, penetration resistance at the surface, at 15 cm (5.9 in), 30 cm (11.8 in), and 45 cm (17.7 in) depths, type of tree pit ground cover (bare ground, rocks, grass, wood chips, etc.), presence of protective metal gratings at the base of trees (theoretical soil compaction reduction), dis- tance from tree to street (indirect measurement of de-icing salt effect), presence of aerial and underground obstacles, and type of surficial geomorphologic deposit obtained from Prest and Keyser (1962). Four soil samples were collected at the edges of 796 tree pits (0–20 cm / 0–8 in depth) then mixed to form a composite for each tree pit. These samples were analyzed for K, Ca, Mg, Na, Zn, Cu, Fe, Mn, P, pH level, and organic matter.
January 2010
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