Keto et al.: Urban Forest Characteristics in Raleigh Parking Lots ©2012 International Society of Arboriculture 50 Evan M. Keto, Melissa R. McHale, George R. Hess, Bronson P. Bullock, and Gary B. Blank Design Choices and Urban Forest Characteristics in Raleigh, North Carolina, U.S. Parking Lots Abstract. Trees provide important environmental, economic, and social benefits that can help to offset the negative effects of parking lots. Many cities recog- nize that adding space for trees in parking lots is beneficial and have created regulations that dictate minimum requirements for tree planting. However, it is not clear if tree plantings in parking lots achieve the urban tree canopy goals initially imagined by these communities. The study authors sampled parking lot trees in Raleigh, North Carolina, U.S., to determine how species composition and urban forest structure vary with respect to parking lot size, shape, and design. Using a two-stage cluster sampling scheme, Raleigh’s parking lots were found to contain 44,000 ± 24,000 trees (95% confidence interval). No differences in tree composition were explained by the size or shape of the parking lots. Planting spaces within the parking lot that were preserved during construction were found to have more trees, canopy, and basal area per hectare than designed planting spaces in which the number, spacing, and species of trees were pre- scribed. Among designed planting spaces, large, linear rows had greater canopy and basal area per tree but fewer trees per hectare than smaller, circular islands. These results suggest that decisions made during the design process may have lasting effects on the structure and function of this portion of the urban forest. Key Words. Automobile; Car Park; Environmental Impacts; Parking Lot Design; Transportation; Tree Planting; Tree Preservation; Urban Forest Structure. Arboriculture & Urban Forestry 2012. 38(2): 50–57 Impervious cover in cities has been shown to have detrimental effects on urban watersheds; as little as 10% impervious cover can negatively affect water quality, with 30% contributing to sig- nificant degradation of the watershed (e.g., Arnold and Gibbons 1996). Although there are many forms of impervious cover—in- cluding buildings, sidewalks, and driveways—parking lots alone can account for a large percentage of urban areas. Parking lots were estimated to comprise 4.97% of urban land in the Upper Great Lakes region while total area of parking has been shown to cover from 5% to 18% of many cities across the U.S., includ- ing Sacramento, California; Salt Lake City, Utah; Chicago, Il- linois; and Houston, Texas (Akbari et al. 1999; Akbari and Rose 2001a; Akbari and Rose 2001b; McPherson 2001; Rose et al. 2003; Davis et al. 2010;). In areas of high coverage, parking lots are a significant source of oil, grease, nutrients, and carci- nogenic sealing compounds (Stenstrom et al. 1984; Hope et al. 2004; Mahler et al. 2005), and contribute to stream sedimentation and bank erosion (Hammer 1972; Albanese and Matlack 1998). Besides water quality effects, parking lots have many other negative environmental, social, and economic effects. For in- stance, pavement contributes to the urban heat island effect (Asaeda et al. 1996; Grimmond and Oke 1999; Celestian & Mar- tin 2004), and automobiles create air pollution while moving through parking lots as well as when they are parked (Hahn and Pfeifer 1994; Scott et al. 1999; EPA 2007). Furthermore, large amounts of parking and sprawling patterns of development have been blamed for reducing the density of cities and subsequently impairing the street life and the social and economic charac- ter of urban areas (Jackson 1996; Kunstler 1996; Shoup 2005). Trees can provide a wide range of benefits that may help offset the negative effects of parking lots. Shade from trees reduces dam- aging ultraviolet rays (Heisler and Grant 2000), solar degradation of pavement, and thus the need for resurfacing (McPherson and Muchnick 2005). Trees moderate temperatures of pavement, air, and automobile cabins and gas tanks, reducing precursors of ozone formation (Scott et al. 1999). Canopies absorb ozone and inter- cept particulate matter (Beckett et al. 1998; Nowak et al. 2006), sequester carbon dioxide (Rowntree and Nowak 1991), and inter- cept and retain stormwater (Xiao et al. 2000). Anderson and Stokes (1989) showed that well-maintained trees and other vegetation in parking lots improved people’s perceptions of an area’s comfort as well as the lot’s attractiveness and security. Furthermore, custom- ers preferred shaded parking spaces in a California lot during the summer (Scott et al. 1999) and were willing to pay more for goods from, return more frequently to, and stay longer in shopping cen- ters with mature trees and shrubs in their parking lots (Wolf 2009). Many of these benefits depend on the number, size, and spe- cies of trees used (Nowak 2008). Trees with larger canopies have greater effects on aesthetics, water quality, shading, and air quality (Schroeder et al. 2009). Additionally, larger trees have a higher benefit-to-cost ratio than smaller trees (McPherson et al. 2005; Schroeder et al. 2009). Tree species can vary dramatically in mature size, lifespan, aesthetic characteristics, resistance to pests and diseases, and emissions of ozone-forming volatile or- ganic compounds and pollen. The growth of trees in parking lots is affected by many factors, and recent studies have focused on the importance of soil volume and quality (Grabosky and Gilman 2004; Celestian and Martin 2005; Smiley et al. 2006). There-