308 McPherson and Muchnick: Effects of Shade on Pavement Performance to them may affect the curves that were created and the economic implications that resulted. Therefore, the savings found here should not be extrapolated to other types of resurfacing treatments or to pavements in different climatic environments. Another limiting assumption is that the PCI returns to 100 after every resurfacing and follows the same deterioration curve downward. In reality, PCI may not return to 100 each time, and microdefects below the surface layer may cause the PCI to descend more sharply than assumed here. The hypothetical example in this paper did not incorporate variability in tree growth, survival, and management found in the real world. Including these sources of variability could alter the results considerably. Relations between pavement condition and tree shade merit further investigation. Controlled laboratory experi- ments could lead to a comprehensive understanding of the relationship between tree shade and AC pavement perfor- mance. For example, experiments are needed that test identically constructed AC pavements during exposure to different controlled temperature and shade regimes. PCI and TCI calculations may be improved if pavement dis- tresses not affected by tree shade were eliminated (e.g., utility cut patching) and data are used only when air temperature and solar radiation are most intense (e.g., May through September). Further research is needed to refine existing pavement performance models and develop new ones based on both mechanistic and empirical data. Current models are primarily traffic related, focusing only on structural fatigue, or failing to incorporate climate informa- tion. The capability of new models to accurately predict effects of tree shade on distresses, such as cracking, rutting, and roughness, should be studied. Benefits of reducing pavement temperatures by means other than tree shade have been reported (Pomerantz et al. 1997; Akbari and Rosenfeld 2000; Pomerantz et al. 2000b). Benefits include (1) the reduction of volatile asphalt fume emissions, due to a diminished need for asphalt production for pavement repairs, and (2) the ability to use a cheaper grade of asphalt in place of one that is modified, and therefore more expensive, in pavements typically exposed to extreme tempera- ture and solar radiation. Examining and quantifying tree shade’s role in the production of these benefits would engender a more comprehensive urban forest benefit-cost analysis. CONCLUSION Effective allocation of limited street tree and pavement management funds requires a comprehensive understand- ing of their costs and benefits. Although benefits of our urban forests have been shown to outweigh their costs, street trees are still often regarded as liabilities. This study identified a previously unquantified benefit of street trees, their effect on pavement performance. As the cost of constructing new pavements increases, the ©2005 International Society of Arboriculture need to protect current investments grows. Better pavement performance translates into reduced maintenance and repair costs, and results in decreased total life cycle costs. This study found a correlation between tree shade and better pavement performance. It also demonstrated the economic benefits of increased pavement durability and reduced maintenance costs associated with increased tree shade. Although our results are limited to a select group of pavements in a specific location, they are significant enough to warrant further investigation. A comprehensive under- standing of the benefits of tree shade on pavement perfor- mance will allay perceptions of street trees as liabilities and help to justify the retention of healthy urban forests. LITERATURE CITED Akbari, H., and A.H. Rosenfeld. 2000. Cool communities, pp. 305–308. In Zumerchik, J. (Ed.). Macmillan Encyclopedia of Energy. Macmillan Reference USA, New York, NY. Asaeda, T., V. Ca, and A. Wake. 1996. Heat storage of pavement and its effect on the lower atmosphere. Atmos. Environ. 30:413–427. Brusca, J. 1998. Street Superintendent, City of Modesto, California. Personal communication with Greg McPherson 17 Nov. 1998. Buus, Norman. 2002. Assistant Civil Engineer, City of Modesto, California. Personal communication with Jules Muchnick 8 Oct. 2002. City of Modesto. 2001. City of Modesto Pavement Management System Overview. www.ci.modesto.ca.us/ etd/reports/pdf/PMS_Overview_Report.pdf [accessed 10/17/20]. Harvey, J., A. Chong, and J. Roesler. 2000. Climate regions for mechanistic-empirical pavement design in California and expected effects on performance. Draft Report for California Department of Transportation. University of California at Berkeley Pavement Research Center. www.its.berkeley.edu/pavementresearch/ Publications.htm [accessed 3/3/01]. Heisler, G.M. 1977. Trees modify metropolitan climate and noise. J. Arboric. 3:201–207. Hunter, R. 1994. Bituminous Mixtures in Road Construction. Thomas Telford Services Ltd., London. UK. 454 pp. Lohr, V.I., C.H. Pearson-Mims, J. Tarnai, and D.A. Dillman. 2004. How urban residents rate and rank benefits and problems associated with trees in cities. J. Arboric. 30:28–37. Maco, S.E., and E.G. McPherson. 2002. Assessing canopy cover over streets and sidewalks in street tree populations. J. Arboric. 28:270–276. McPherson, E.G. 2003. A benefit-cost analysis of ten street tree species in Modesto, California, U.S. J. Arboric. 29:1–8.
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