Arboriculture & Urban Forestry 38(2): March 2012 and Stokes 1989). Some planted spaces had trees that were re- moved and not yet replaced. Others exhibited poor arboricul- tural practices, such as constricting guy wires, girdling roots, and excessive mulching. While both approaches showed the need for maintenance to improve value, neither was clear- ly optimal for realizing the benefits of Raleigh’s ordinance. Size and Shape of Planting Spaces Among designed planting spaces, islands were the most com- mon in the study area, yet larger trees were growing in rows. The larger basal area and canopy area of trees in rows could be due to numerous factors, including differences in tree density, spe- cies choice, tree age, use of irrigation, or fertilizer applications. While controlled studies would be needed to test these effects, it is suspected that rows provide a better environment for healthy root growth (McPherson 2001; Grabosky and Gilman 2004; Ce- lestian and Martin 2005; Smiley et al. 2006; Wolf 2009). While islands may constrain roots, rows provide more room to grow. There are additional reasons to build parking lots with trees planted in rows: * A row of mature tree canopies, oriented appropriately, can shade a row of parking spaces throughout the day better than trees in scattered islands. This would help to extend pavement life (McPherson and Muchnick 2005) and re- duce the emission of volatile organic compounds from fuel tanks (Scott et al. 1999). * A row of trees can provide a safe, shaded path for pedestri- ans and cyclists (Adams 2006). * The long shape of rows is compatible with bioswales: wide, gently sloping, vegetated ditches that can help to control stormwater while supporting attractive vegetation (McPherson 2001; Wolf 2009). * The cost of concrete curbs, irrigation lines, and mainte- nance can be reduced by using a few large rows instead of many scattered islands. * Rows can support multiple trees, and the spacing between trees can be controlled over time. As trees die or are out- competed by neighbors, they can be removed and neigh- boring trees can make use of this space. This helps to maxi- mize canopy coverage and minimizes the impact of the loss of any individual tree. Therefore, the study authors recommend that park- ing lot ordinances allow and encourage parking lot de- signers to create large, linear planting spaces contain- ing multiple trees, rather than the scattered, raised islands that were the most common planting space in Raleigh. Species Choice Species choice is likely to have a lasting effect on the benefits of this resource. Crapemyrtles and heavily pruned waxmyrtles (Morella cerifera) and hollies (Ilex spp.) were commonly planted in parking lots throughout the study area, but are ineffective at providing most of the environmental benefits enumerated in Ra- leigh’s ordinance. Willow oaks (Quercus phellos) appear to be an outstanding parking lot tree for this area, exhibiting almost no serious health problems and reaching large sizes (Figure 3). Willow oaks comprised more than half of the trees observed in 55 designed planting spaces that had reached 10.7 m tall and 9.1 m in canopy diameter, Raleigh’s goal for mature parking lot trees. Of the 260 willow oaks observed, 135 (52%) met this goal. While red maple (Acer rubrum) was the most commonly planted species, researchers did not observe any outstand- ing characteristics that would explain this popularity. Only 8 of 400 red maple trees observed in this study had reached the mature size goals specified in the ordinance. Further re- search is needed to determine if the average red maple tree is likely to reach the size specified in Raleigh’s ordinance when planted within a parking lot. Red maples are vulner- able to mechanical damage, trunk rot fungi, and stem dis- eases (Hutnik and Yawney 1961), and severe trunk damage was observed in many red maples in this study. Addition- ally, many different insects can reduce red maples’ growth, make the trees vulnerable to decay, or help kill weakened trees (Hutnik and Yawney 1961). Maples are also very good hosts for Asian longhorned beetle (Raupp et al. 2006). A diverse mix of trees could enhance seasonal aesthet- ics and provide landmarks for motorists to use in locating parked vehicles. Several researchers have recommended that each species comprise no more than 5%–15% of street tree populations to protect long-term forest health (Clark et al. 1997; Raupp et al 2006). The study authors recom- mend avoiding monocultures in parking lots and that ordi- nances specify the maximum percentage of parking lot trees that can be met by a single species. The level of diversity recommended for street trees could be met easily in park- ing lots in the Raleigh area using species that are commer- cially available and well-adapted to growth in parking lots. CONCLUSIONS Parking lots in a city may contain tens of thousands of trees, representing a unique and interesting component of the urban forest. Periodic sampling of these trees allows urban forest managers to understand the composition of this resource and can help to shape policy and improve design. While the size and shape of parking lots had no apparent relationship to the composition of trees within them, the decisions made during the design of planting spaces do appear to have a lasting effect on the benefits provided. The effectiveness of a city’s parking lot tree ordinance may be determined largely when designers decide to preserve or plant trees, choose the size and shape of these planting spaces, and select tree species. The study authors suggest that developers work with arborists to identify trees that can be preserved, and where possible, preserve multiple large trees in large spaces rather than individual trees in islands. Where trees must be planted, researchers suggest incorporat- ing a diversity of large-growing species in large contiguous rows instead of isolated islands. By providing information to parking lot owners and designers, urban forest managers can help to ensure that their cities’ goals are being realized. Acknowledgments. The Garden Club of America provided financial support for this research. Louise Alexander, Laura Fieselman, Diana Keto, Joan Louie, Susan McIntyre, Corey Shake, Jasmine Shaw, Megan Shaw, and Elizabeth Wood assisted with data collection. ©2012 International Society of Arboriculture
March 2012
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