220 Hwang et al.: Tree Planting Configuration Influences Shade on Residential Structures Even though larger trees provide high levels of shade, they can also create possible hazards or conflicts when placed too close to a structure. The resulting problems, such as structural damage, could possibly negate the benefits of energy conservation (McPher- son et al. 2005; McPherson et al. 2006; Peper et al. 2009; Peper et al. 2010). Therefore, the compromise between tree size and distance should be carefully considered when selecting and planting shade trees. The use of a strategically placed single tree is ampli- fied when one considers that shade trees can contribute to “spillover” shade benefits to neighboring structures, especially in dense urban developments (Nikoofard et al. 2011). These trees may also contribute to energy conservation by shading low albedo ground covers (e.g., streets and driveways) and by evapotranspira- tion cooling (Huang et al. 1987) and windbreak effects (Heisler 1986a). In addition, tree selection and place- ment must also be considered in the full context of the landscape situation and across the full range of potential benefits. For example, a greater environmental benefit may occur from placing a tree on a low-shade north aspect if its canopy projects over an impervious surface, thereby intercepting rainfall and delaying stormwater runoff. The key is to understand how tree form and tree placement interact with the built environment to derive a multitude of envi- ronmental benefits and then select the planting configuration that puts the tree to its highest use. CONCLUSION This study has demonstrated a simulation method to assess the impact of a single-tree planting configura- tion on shade provision for a prototypical residential structure in various geographic locations. To isolate some of the key variables, the simulations considered a contrived situation that simplified the geometry of the structure and placement of specific trees in selected geographic settings. As a result, it demonstrated that shade provision is influenced by not only tree form and placement, but also daily, seasonal, and latitudinal vari- ability in sunlight exposure. These simulations support the general recommendation that large trees placed adjacent to buildings, on their east or west aspects, pro- vide high levels of shade during the cooling season while minimizing unwanted shade during the heating season. However, the simulations indicate that in addition to the tree direction relative to the structure, tree distance should be considered in conjunction with tree size. ©2015 International Society of Arboriculture Quantity, quality, as well as timing of shade cast upon building surface areas impact the magnitude of shading benefits with regard to energy conservation and human health (Heisler 1986b). However, because this study only quantified shade provision, the authors cannot draw conclusions about how the quality of shade (e.g., shade of similar magnitude cast by an east versus a west aspect tree) impacts energy conserva- tion. Through simulating shade effects specifically on building energy consumption, further studies could address this limitation. In addition, shade provision is not the only factor that influences building energy performance; for example, weather, building charac- teristics, and occupant behavior have notable impacts (Livingston and Cort 2011). Despite these limitations, this study has demonstrated the nuanced relation- ship between TPC variables, geographic latitudes, and shade provision across a broad spectrum of urban settings in the U.S. With these findings, researchers can move closer toward providing recommendations for optimal tree selection and placement based on geographic location in order to maximize shade ben- efits for both energy conservation and human health. Researchers’ ability to make precise tree planting recommendations takes on even greater significance for urban neighborhoods where UHI effects are more acute, and potential tree planting space is more limited. Acknowledgments. We would like to thank Dr. James R. Simpson for providing the Shadow Pattern Simulator (SPS) program and technical support, and Dr. Susan D. Day, Dr. James B. Campbell, and Tammy E. Parece at Virginia Tech for their valuable perspective for preparing the manuscript. We also acknowledge anonymous reviewers and the editors for their constructive comments. LITERATURE CITED Akbari, H. 2002. Shade trees reduce building energy use and CO2 emissions from power plants. Environmental Pollution 116: S119–S126. Akbari, H., H. Pomerantz, and H. Taha. 2001. Cool surfaces and shade trees to reduce energy use and improve air quality in ur- ban areas. Solar Energy 70(3):295–310. Baechler, M.C., J. Williamson, T. Gilbride, P. Gole, M. Heſty, and P.M. Love. 2010. High-performance home technologies: Guide to determining climate regions by county. Accessed 12/04/2013. Centers for Disease Control and Prevention. 2013. Climate change and extreme heat events. Accessed 12/04/2014. Donovan, G.H., and D.T. Butry. 2009. The value of shade: Estimating the effect of urban trees on summertime electricity use. Energy and Buildings 41:662–668.
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