200 DISCUSSION The cooling effect of trees is most important during the period of maximum temperature and solar radiation (Parker 1983). Therefore, shade trees can be plausibly considered as a possi- ble demand side management resource to provide cost-effective energy savings to homeowners and business owners (Simpson and McPherson 1996). These findings suggest that such sav- ings may be sizable. As noted previously, Apollo required 2.6 times the electricity for cooling than did Hades. The estimated additional cost to cool Apollo ($7.84 per month) may not seem like much, however the annual cost is approximately $95. Fur- ther, this is for a very small area (160 ft2 /14.9 m2 ). The cost savings associated with the energy required to cool a space 10 times this size, which closely approximates the size of many actual dwellings, would surely be well above $500 per year, even acknowledging the relative cost efficiency of central air conditioning over window units (Pandit and Laband 2008). Moreover, as has been demonstrated previously (DeWalle et al. 1983; McPherson and Dougherty 1989; Laverne and Lewis 1996; Simpson and McPherson 1996; Pandit and La- band 2008), shade that hits a building in the late afternoon, when temperature build-up peaks, has a greater impact on en- ergy used for cooling than shade at other times of the day. Thus, 25% shade on a dwelling during the hottest time of the day likely would have a greater impact on energy consumption than the 13% reduction suggested earlier, since the figure was derived from mean shade coverage irrespective of time of day. Other aspects of the data reported in Appendix 1 are notewor- thy. For example, the maximum inside temperatures recorded in Apollo consistently exceed those recorded in Hades by 5–6°F. Moreover, the standard deviations around those monthly means are considerably higher in Apollo than Hades. In both cases the recorded high temperatures exceed the 72o Laband and Sophocleus: Impact of Tree Shade on Electricity Consumption estimates of these impacts are within reach and it is hoped that future researchers will continue to pursue this line of research. Taken together, this growing body of research demonstrates that owners of residential and commercial properties located in hot regions can reap sizable monetary savings from shade trees that serve as natural complements to their artificial air-condition- ing. The industry now needs to find ways to get this scientific information to the builders and owners of residential and com- mercial properties who make decisions about those properties. If their perception of mature trees located in close proximity to their property is that they increase risk with little offsetting benefits, they are unlikely to take advantage of the energy savings avail- able from the shade provided. Knowledge of the possible size of the benefits should at a minimum permit more informed decision- making with respect to whether or not mature trees are stripped off a residential construction site and/or whether a homeowner cuts down mature trees that are close enough to fall on his residence. Acknowledgments. This research was supported by a competitive McIn- tire-Stennis grant awarded to Laband through the SFWS at Auburn Uni- versity. We appreciate the assistance and shared expertise of Ed Loewen- stein and helpful comments received from the Associate Editor and two anonymous reviewers. We are solely responsible for any shortcomings in the analysis or presentation. LITERATURE CITED Akbari, H., D.M Kurn, S.E. Bretz, and J.W. Hanford. 1997. Peak power and cooling energy savings of shade trees. Energy and Buildings 25:139-148. DeWalle, D.R., G.M. Heisler, and R.E. Jacobs. 1983. Forest home sites influence heating and cooling energy. Journal of Forestry 81(2):84–88. thermostat setting. In some measure, this likely reflects air circulation patterns. But the quite large differences between the maximum temperatures re- corded in Apollo (the numbers reported in Table 1 are average high temperatures each month) and the thermostat setting strong- ly suggest that the AC units were not able to fully handle the cooling job required. This implies that if the study had AC units in place that were able to properly cool to the thermostat setting, the measured difference in electricity used in Apollo as compared to Hades would have been even greater than reported. That is, the findings almost certainly understate the true impact of tree shade. Obviously, this experimental design contrasts electricity used for cooling a building in full sun against a building in essentially full shade – the two ends of a wide spectrum. Few people live in, or want to live in, full shade. Therefore, future experiments focusing on buildings with partial shade will presumably com- mand a good deal of interest. In addition, it seems possible, if not likely, that these findings will be sensitive to the thermostat setting, the color of the roof, how well insulated the buildings are, and a host of other conditions that were held constant. The experimental methodology used is flexible enough to permit researchers to identify quite precise impacts of specific factors under control that influences electricity consumption used for cooling, by systematically varying a single variable at a time; such as the thermostat level, roof colors, the impact of insula- tion with different R-values, the impact of low-E and/or tinted glass, or the impact of blinds/shades. Tightly-calibrated empirical Huang, Y.J., H. Akbari, H. Taha, and A.H. Rosenfeld. 1987. The potential of vegetation in reducing summer cooling loads in residential build- ings. Journal of Applied Meteorology 26(9):1103–1116. Jensen, R. J.R. Boulton, and B.T. Harper. 2003. The relationship between urban leaf area and household energy usage in Terre Haute, Indiana, U.S. Journal of Arboriculture 29:226–230. Laverne, R.J., and G.M.D. Lewis. 1996. The effect of vegetation on resi- dential energy use in Ann Arbor, Michigan. Journal of Arboriculture 22:234–243. McPherson, E.G., and E. Dougherty. 1989. Selecting trees for shade in the Southwest. Journal of Arboriculture 15(2):35–43. McPherson, E.G., D. Nowak, G. Heisler, S. Grimmond, C. Souch, R. Grant, and R. Rowntree. 1997. Quantifying urban forest structure, function, and value: the Chicago Urban Forest Climate Project. Urban Ecosystems 1:49–61. Pandit, R., and D.N. Laband. 2008. A hedonic analysis of the impact of tree shade on summertime residential energy consumption, unpub- lished manuscript. Rudie, R.J., and R.S. Dewers. 1984. Effects of tree shade on home cool- ing requirements. Journal of Arboriculture 10:320–322. Simpson, J.R., and E.G. McPherson. 1996. Potential of tree shade for reducing residential energy use in California. Journal of Arboricul- ture 22:10–18. ©2009 International Society of Arboriculture
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