272 Abram et al: Land Cover and Summer Cooling Electricity Consumption while holding all other variables constant, which is considerably less than the low end of savings previ- ously reported (80 kWh/tree annually)(Donovan and Butry 2009; Ko et al. 2015). It is likely that the study location and methodology employed are playing a role in how different our find- ings are compared to previous studies. For example, the i-Tree model has been commonly applied in sim- ulation studies to determine energy savings of trees in a variety of urban settings (McPherson and Simpson 1999; Nowak 2002; Nowak et al. 2008; Nowak et al. 2017). In contrast, our methodology included a large sample size at 21,048 single-family households, giv- ing us an extensive pool of data to work with. Addi- tionally, our explanatory variables were calculated using high-resolution spatial data which did not dis- criminate between individual trees, their heights, or their species. These differences in methodology and assumptions are important to consider, as they could be contributing to differences in results. Our study area is a semi-arid city where most trees are not naturally occurring. Central air conditioning in the city has historically been less common due to cooler temperatures and low humidity, which is dis- tinct from other study locations in warmer climates, like Sacramento. In addition to being semi-arid, the study area is located close to the Rocky Mountains, which could have climatic impacts altering the rela- tive importance of UTC on summer cooling electric- ity. In contrast to somewhat older, larger, and more established cities, our study area is a relatively young, midsize city. This age and size difference could be reflected in the age of homes, how they were devel- oped, or the overall design of homes in the study area, ultimately impacting the overall role UTC plays on cooling electricity consumption. Based on our study, which used a large amount of high-resolution data for both the explanatory and response variables, the impact of tree canopy on sum- mer cooling electricity is comparatively less than mod- els and studies that are often referenced for energy savings provided by trees and highly utilized by city managers (McPherson and Simpson 1999; Donovan and Butry 2009; Nowak et al. 2017; Ko 2018). For context, running a 10 W LED lightbulb instead of a 60 W incandescent lightbulb for 5 hours/day over the course of the summer would save you about 23 kWh, just shy of the 26 kWh/tree savings in our study area. Despite low cooling energy savings, it is important to ©2022 International Society of Arboriculture note that UTC provides other ecosystem services such as removing particulate matter from the air, seques- tering carbon, reducing noise, improving water quality, and reducing outdoor water use, enhancing its overall value to urban ecosystems (Herrington 1974; Dwyer et al. 1992; Scholz et al. 2018; Rasmussen et al. 2021). Limitations and Future Research A component that our analysis omits is tree species, which can impact the cooling effectiveness of trees through differences in tree characteristics. Tree growth rates, Leaf Area Index (LAI), and crown size have all been found to impact cooling effects in urban areas (Armson et al. 2013; Rahman et al. 2015; Speak et al. 2020). This type of analysis would be especially help- ful in the study area, where most trees are not natu- rally occurring and good planning and care are required to ensure the health and sustainability of the UTC. Additionally, tree height was not included in our analysis, which results in the omission of a dimen- sion that could further account for the impact of shad- ing throughout the day. Tree height has been used minimally in studies looking at the impact of UTC on energy savings, but when included it has shown sig- nificant results where a greater sum of tree heights in east, south, and west configurations around homes was associated with lower summer cooling electricity use (Ko and Radke 2014). Looking at electricity data seasonally is important to prevent averaging out of trends; however, it leaves out the impact that trees or impervious surfaces have on winter energy use. Our data set is solely for elec- tricity use, so applying a similar analysis to winter would not account for homes that may use natural gas for heating. There is evidence that trees, especially those planted on the south side, can hinder passive solar warming during the heating season and increase energy use (Heisler 1986; Hwang et al. 2015). In addi- tion, climate, latitude, time of day, urban density, and urban form have also been found to impact urban heating and energy savings of trees (Zhou et al. 2014; Myint et al. 2015; Wheeler et al. 2019), and we did not capture these variables in our analysis. These are important considerations for future analysis to fully understand the role that land cover, especially UTC, plays in energy use in single-family homes. Our goals for this study were to evaluate whether UTC and impervious surfaces surrounding single- family homes have an impact on summertime energy
September 2022
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