Arboriculture & Urban Forestry 40(4): July 2014 root cells are altered (Pearson 1965). Differences among species in their ability 199 to penetrate strong soil layers appear to be due to differ- ences in root diameter (Clark et al. 2003). Temperature Urban soils can be warmer due to surround- ing pavements and lack of vegetation cover. Unvegetated playground soils in Central Park (New York City, New York, U.S.) were 3.13°C warmer than an adjacent wooded area (Mount et al. 1999). Maximum summer soil tempera- tures under pavement in the northern United States were 32°C–34°C, and up to 10°C warm- er than nearby unpaved areas (Halverson and Heisler 1981; Graves and Dana 1987). In Texas, U.S., summer soil temperatures under pave- ment exceeded 48°C, 10°C warmer than unpaved areas, and remained above 35°C for all but a short time at night. Temperatures are highest under dark pavements (Arnold and McDonald 2009). Effect on Root Growth Biological activity in the soil, and therefore root growth, varies with temperature (Lloyd and Taylor 1994). Root growth occurs over a wide range of temperatures, but is much slower at low and high temperatures. Reported minimum temperatures for root growth range from 2°C to 11°C (Lyr and Hoffmann 1967; Solfjeld and Pedersen 2006). Sugar maple (Acer saccharum) roots began to grow in spring as soils warmed to 5°C, but initial root growth may be quite slow at such low tem- peratures. Active root growth has been reported to begin when soil temperatures reach 10°C–15°C (Nambiar et al. 1979; Carlson 1986; Harris et al. 1995; Solfjeld and Pedersen 2006). Optimum tem- peratures for root growth have been reported at 18°C–32°C (Lyr and Hoffman 1967; Larson 1970; Nambiar et al. 1979; Struve and Moser 1985; Head- ley and Bassuk 1991; Harris et al. 1995; Solfjeld and Pedersen 2006; Richardson-Calfee et al. 2007). The high temperature at which root injury begins to occur is around 34°C (Graves and Wilkins 1991; Graves 1994; Graves 1998; Wright et al. 2007). Roots of most woody species are killed at 40°C–50°C (Wong et al. 1971). Maximum temperatures for active growth have been reported at 25°C–38°C, depending on the species (Proebsting 1943; Wong et al. 1971; Gur et al. 1972; Graves et al. 1989a; Graves et al. 1989b; Graves 1991; Martin and Ingram 1991; Graves and Aiello 1997; Arnold and McDonald 2009). Direct heat injury of roots can occur when the soil remains above 32°C for extended periods of time (Graves 1998), and the longer the duration of high temperatures, the more root growth is reduced (Graves et al. 1989b; Graves and Wilkins 1991). Hon- eylocust (Gleditsia triacanthos) is the only temperate tree species reported to sustain growth at root-zone temperatures above 32°C (Graves et al. 1991). The root tissues of most woody plants can be killed at soil temperatures of -5°C to -20°C (Havis 1976; Studer et al. 1978; Santamour 1979; Pellett 1981; Lindstrom 1986; Bigras and Dumais 2005), although roots of black spruce (Picea mariana) were not affected by temperatures as low as -30°C (Bigras and Margolis 1996). Young roots are less freeze-tol- erant than mature roots (Bigras and Dumais 2005). Soil pH Plant performance is strongly affected by nutrient availability, which in turn is influenced by soil pH (acidity or alkalinity). Most nutrients are available at optimal levels in slightly acid to neutral soils (pH between 5.5 and 7.2), and trees generally grow best in this pH range. Soil pH can be mea- sured with electronic meters or colorimetric tests based on color of Urban soils tend to have higher soil pH than their natural counterparts. In Berlin, Germany, a pH of 8 was observed streetside, compared to a pH of less than 4 within a forest a short distance from the street (Chinnow 1975). Over half of soils sampled in Hong Kong, China, were rated strongly (pH 8.5–9) to very strongly (pH 9–9.5) alkaline, while surrounding soils were acidic at pH 4–5 (Jim 1998b). Streetside soils of Syracuse, New York, U.S., had a pH range of 6.6 to 9.0 with an average of about 8.0 (Craul and Klein 1980). Urban soils of Philadelphia, Pennsylvania, U.S., ranged from 3.7 to 9.0 with a mean of 7.6 (Bockheim 1974). Elevated pH values have been attributed to the application of calcium or sodium chloride as road and sidewalk deicing compounds in northern latitudes, irrigation with calcium-enriched water (Bockheim 1974), and the surface weathering of concrete and limestone buildings and sidewalks (Bockheim 1974; Messenger 1986; Okamoto and Maenaka 2006). ©2014 International Society of Arboriculture solutions or strips.
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