Arboriculture & Urban Forestry 38(2): March 2012 fore, to understand how to maximize the benefits of a city’s parking lot trees, analysts can begin by understanding the num- bers, species, sizes, and arrangement of trees in parking lots. Like many cities, Raleigh, North Carolina, U.S., has enacted regulations that specify how trees are to be grown in and around parking lots, with the intent of providing aesthetic, economic, and environmental benefits (City of Raleigh 2009). The goal of the study authors was to characterize the overall structure (spe- cies, size, number per acre) of trees within the interiors of park- ing lots in Raleigh, and determine how this composition varies * among parking lots of differing size and shape, * between planting spaces that contain trees that appear to have been preserved during lot construction and those that were planted afterward, and * among planting spaces of differing size and shape. Any significant differences in composition would then al- low identification of design approaches that are more likely to achieve the goals of the city’s parking lot tree ordinance. MATERIALS AND METHODS Study Area The 472 km2 study area consisted of the City of Raleigh, North Car- olina and its extraterritorial jurisdiction (Figure 1). Raleigh is the 45th largest city in the United States, with a population of more than 400,000 (U.S. Census Bureau 2010). Raleigh passed an ordinance requiring trees to be planted in and around parking lots in 1987, for purposes including air purification, stormwater retention, ground- water recharge, aesthetic and economic improvement, energy con- servation, and reduction of heat and noise (City of Raleigh 2009). 51 Site Selection and Sample Design Raleigh has made publicly available a series of digital plani- metric maps of the parking lots as they existed in 2006 (Wake County GIS 2010). Researchers verified the accuracy of these maps using six-inch resolution color aerial orthophotos. En- trances and exits, parking spaces, the top levels of some parking decks, and loading and unloading areas were included in these maps; non-paved areas within each parking lot were accurate- ly delineated as empty spaces within the parking lot surface. The authors used a two-stage cluster sampling scheme to estimate characteristics of trees in Raleigh’s parking lots (Avery and Burkhart 2002). In the first stage, the au- thors classified and selected parking lots that contained non- paved areas that were likely to contain trees, and in the sec- ond stage these nonpaved areas were selected for sampling. The majority of trees in parking lots in the study area were found in discrete, nonpaved areas, which were identified in the digital planimetric maps using ArcGIS 9.2 (ESRI 2009). Non- paved areas containing buildings were removed from further con- sideration; the remaining areas were classified as planting spaces. Most small parking lots can satisfy Raleigh’s ordinance without providing space for trees within the lot interior. Be- cause the focus of this study was on the trees in the inte- rior of parking lots, the more than 8,000 (80%) parking lots that did not contain at least one planting space were as- sumed to contain no trees and were not sampled. For the re- maining 1,982 parking lots, six quantities were calculated. 1. Total area of the parking lot (m2 ). 2. Compactness ratio of the parking lot [K, defined as K = 2√π(Area)], Perimeter where Area is the area of the parking lot and Perimeter is the perimeter of the lot (Forman 1995). This ratio compares the pe- rimeter of a parking lot (the denominator) to the perimeter of a circle with the same area (the numerator), and decreases from one for circular shapes toward zero for increasingly long, thin shapes. 3. Number of planting spaces in the parking lot. 4. Total area of all planting spaces in the parking lot (m2 ). 5. Ratio of parking lot area per planting space (i.e., #1 / #3). 6. Ratio of parking lot area to planting space area (i.e., #1 / #4). These six values were used to perform a multivariate cluster- ing using SAS JMP 7 (SAS Institute 2007). The authors iden- tified four classes of parking lots: simple (small lot size with an average number of planting spaces), intensive (small lot size with more or larger planting spaces), extensive (large lot size with a compact shape), and elongated (large lot size with less compact shape). Parking lots were selected ran- domly from each of the four resulting classes for field sam- pling, with a total 110 parking lots sampled (Stage 1, Table 1). For all planting spaces, the authors calculated the area, perim- Figure 1. Map of parking lots in the study area (black/shaded). Parking lots cover approximately 28.2 km2 in the City of Raleigh’s planning jurisdiction. For comparison, Umstead State Park (22 km2 ) is shaded in gray. Inset: The state of North Carolina. The darkened spot represents the City of Raleigh. eter, and a compactness ratio (K), as previously defined. The area of each planting space was plotted against its compactness ratio and classified into four types on the basis of its size and shape (Fig- ure 2). Using this plot, researchers defined four types of planting spaces: islands, rows, chunks, and slivers (Figure 3). In each sam- pled parking lot, 12 planting spaces or 20% of all planting spaces, whichever was greater, were subsampled, stratified by planting space type. At least one of each type of planting space present in a ©2012 International Society of Arboriculture
March 2012
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