Journal of Arboriculture 31(6): November 2005 303 EFFECTS OF STREET TREE SHADE ON ASPHALT CONCRETE PAVEMENT PERFORMANCE By E. Gregory McPherson1 and Jules Muchnick2 Abstract. Forty-eight street segments were paired into 24 high- and low-shade pairs in Modesto, California, U.S. Field data were collected to calculate a Pavement Condition Index (PCI) and Tree Shade Index (TSI) for each segment. Statistical analyses found that greater PCI was associated with greater TSI, indicating that tree shade was partially responsible for reduced pavement fatigue cracking, rutting, shoving, and other distress. Using observed relations between PCI and TSI, an unshaded street segment required 6 slurry seals over 30 years, while an identical one planted with 12 crape myrtles (Lagerstroemia indica, 4.4 m [14 ft] crown diameter) required 5 slurry seals, and one with 6 Chinese hackberry (Celtis sinensis, 13.7 m [45 ft] crown diameter) required 2.5 slurry seals. Shade from the large hackberries was projected to save $7.13/m2 ($0.66/ft2 Key Words. Avoided repaving costs; pavement distress; tree benefits; urban heat island. on climatic conditions that affect pavement performance (Heisler 1977). “Oasis effects” of 5°C to 7°C (9°F to 12.6°F) have been measured as a result of direct shading of the ground surface and transpiration of water through leaves (Asaeda et al. 1996). ) over the 30-year period compared to the unshaded street. As pavement temperatures rise, volatilization of the asphalt binder and oxidation lead to a progressive harden- ing of the pavement, which results in increased fatigue cracking and reduced durability (Harvey et al. 2000; Srivastava and van Rooijen 2000). Higher surface tempera- tures make the pavement more prone to rutting (Pomerantz et al. 2000a). Better pavement performance due to extensive tree shade could translate into a less frequent repaving schedule and cost savings. The street superintendent in Modesto, California, U.S. estimated that repaving could be deferred 10 years on a well-shaded street and potentially up to 25 years on heavily shaded streets (Brusca 1998). The goal of this study was to determine whether tree Street tree populations provide many benefits that in total often exceed their management costs (McPherson et al. 1999; Maco and McPherson 2002). Yet street trees are often perceived as liabilities due to litter drop, root damage to sidewalks, and visibility and security problems created by blocking signs and lighting (Lohr et al. 2004). Attitudes that trees are “bad.” limited municipal funds, and competing interests make justifying budgets for tree maintenance difficult. One benefit of urban trees that has not been examined is the relationship between tree shade and pavement perfor- mance. Performance relates to a pavement’s ability to maintain its design standards and intended functional and structural condition (Scholtz and Brown 1996). Asphalt pavements are a combination of an aggregate, known as the filler, and asphalt cement, referred to as the binder, varying in both thickness and type (Wallace and Martin 1967). A typical flexible pavement consists of several layers—a hot-rolled asphalt wearing surface supported by a combination of one or several granular base courses on a subgrade foundation of unbound aggregates and compacted soils (Hunter 1994). Asphalt concrete pavements on streets and parking lots occupy about 30% of the land in our cities and can be characterized as miniature heat islands and sources of motor vehicle pollutants (Scott et al. 1999; Pomerantz et al. 2000b). By attenuating irradiance and lowering air and surface temperatures, street trees have a moderating effect shade produces a beneficial effect on pavement perfor- mance. We compared pavement condition data from similar streets receiving different amounts of tree shade. Because a longer-lived pavement can be an economic asset, we used empirically derived relationships between tree shade and pavement condition to project potential savings for two identical street segments, one shaded by large trees and one by small trees. METHODS Study Area The city of Modesto is located approximately 124 km (77 mi) south of Sacramento in California’s San Joaquin Valley. At an elevation of 26.5 m (87 ft) above sea level, the climate is characterized by hot, dry summers and cool, wet winters. Average annual rainfall is 280 mm (11 in.) and most occurs from November to March. Summer temperatures commonly are above 29°C (85°F) and may exceed 38°C (100°F) but rarely exceed 41°C (105°F) (Weatherbase 2001). Winter temperatures commonly fall below 0°C (32°F) but rarely are lower than –4°C (25°F). Modesto’s street system includes 925 street km (575 mi, or about 1,130 lane miles) and about 60% of streets are residential (City of Modesto 2001). The city uses the Metropolitan Transportation Commission’s (MTC) Pavement Management System (PMS) to help evaluate the condition of its pavements. Inspection of collector and arterial streets occurs every 2 years, while ©2005 International Society of Arboriculture
November 2005
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