Arboriculture & Urban Forestry 48(5): September 2022 Most Impactful Orientations The most impactful orientations for both tree canopy and impervious surfaces did not align with our hypotheses and showed different patterns when com- pared to previous studies. A well-established body of literature on the impact of tree canopy on cooling electricity savings has consistently shown that tree canopy on the west side of homes produces the larg- est savings, followed by the east and south sides (Simpson and McPherson 1996; Donovan and Butry 2009; Ko and Radke 2014; Hwang et al. 2015). Addi- tionally, it has been assumed that trees planted beyond 18 m of a home do not impact electricity use directly through shading (McPherson et al. 1988; McHale et al. 2007; Donovan and Butry 2009; Nelson et al. 2012). In our analysis, both the tree canopy model and the combined tree canopy/impervious surface model challenge these assumptions. Our results showed that the most impactful variables in order of magnitude were the 6-m east zone, followed by the 24-m east and 12-m east zones in our tree model, and the 24-m east zone, followed by the 24-m north and 6-m east zones in our combined model. The impact of impervious surfaces around homes on cooling electricity consumption is not well- documented in the literature, but the role of impervi- ous surfaces in urban heating is well-researched, specifically in research on the UHI effect (Chithra et al. 2015; Estoque et al. 2017). Our results showed a clear pattern that impervious surfaces within 6 m of the home at all orientations were the most impactful in our impervious surfaces model. The combined model showed slightly more variation, but impervi- ous surfaces within 6 m west and south of the home still showed high impact and significance. All signifi- cant impervious surface variables with a positive relationship were within 18 m of the home in both the impervious surfaces model and combined model. Since increases in impervious surfaces in cities can result in higher ambient temperatures (Weng 2001), it is possible that impervious surfaces closer to the home would have a more-pronounced impact on the microclimate than those located at a further distance. Comparison to Other Studies While previous studies on the orientation of impervi- ous surfaces around homes and the resulting impact on cooling electricity are scarce, there is ample evi- dence that UHI is associated with increased cooling 271 energy consumption in warmer months (Li et al. 2019; Su et al. 2021). However, there is high variation in this impact, with the increase in electricity demand per increase in degree of temperature falling between 0.5% and 8.5% (Santamouris et al. 2015). More specifi- cally, in Colorado, data showed that daily electricity demand in the Colorado Springs utility district increased 4,000 kW, or about 1%, for every 1 °F increase in temperature (Akbari et al. 1992). Because our analy- sis did not have access to ambient air temperature around homes, we cannot directly compare our results to those that have studied urban heating and electric- ity demand. However, our results do support the notion that impervious surfaces around homes can contribute to increased cooling electricity consumption and are influencing microclimate around buildings. The study of the impact of tree canopy on energy use has taken many forms in the literature with varia- tions among sample size, residential building type, location, explanatory and response variables, and the method of analysis. For example, in simulation stud- ies, annual cooling has been found to be as high as 180 kWh/tree (Simpson and McPherson 1996) and as low as 80 kWh/tree (Ko et al. 2015). The conserva- tive prediction of 80 kWh/tree is comparable to other empirical results found by Donovan and Butry (2009). On the other hand, there are also empirical studies that have found little-to-no impact of tree can- opy on summertime energy savings (Clark and Berry 1995; Abbott and Meentemeyer 2005; Nelson et al. 2012). This variation and subsequent spread in the magnitude of results, as well as how results were reported, makes it difficult to draw direct compari- sons between our results and other studies. However, our results seem to show a much smaller impact than is typically predicted, even compared to more conser- vative estimates of savings found in both simulation and empirical studies. Our most impactful tree canopy variable common to our tree model and combined model was positioned in the 24-m east zone. The average tree canopy in this zone was 22%, which would be equivalent to about 63 m2 , based on the aver- age 24-m east zone size. This area roughly translates to a tree with a 30-ft (9.14-m) crown, which would be a common size for a large, deciduous tree in the city, such as a green ash (Fraxinus pennsylvanica). Using these calculations, we estimated savings of 27 kWh/ tree in our combined model and 26 kWh/tree in our tree canopy model over the course of peak summer ©2022 International Society of Arboriculture
September 2022
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