216 Hwang et al.: Tree Planting Configuration Influences Shade on Residential Structures angles in the early morning and the late aſternoon are less influenced by latitude. Because smaller trees and trees placed on the north aspects of a structure provided negligible levels of shade, discussion in the following sections primarily focuses on the latitudinal nuances of large trees placed on east, south, and west aspects. Shade Provision in the Cooling Season Across study sites, the cooling season ranges from three months (Minneapolis) to nine months (Orlando) (Table 2). During these stretches of time, maximizing tree shade on building surfaces is criti- cal for energy conservation and human health. A more practical measure of sustained shade benefit is the average shade provision, which is the value calculated by summing shade provision (coverage) divided by the number of months over the duration of the cooling season. The shade simulations revealed some notable trends in tree shade (only data for large, nearby trees is shown in Table 3 to best exem- plify the influence of aspect and latitude on shade). First, when considering tree types, coniferous trees provided greater average shade provision (averaged values over the duration of the cooling season) than the deciduous trees—these differences were most pronounced in Orlando where the cooling season is considerably longer, whereby dense, persistent foli- age is more advantageous. Second, tree placement on the east or west aspect of the structure provided greater average tree shade over the course of the season, regardless of latitude. In contrast, shade was consistently lowest for trees on the south or south- west aspect because the sun angle is very high and therefore sunlight passes over the top of the tree dur- ing the cooling season. All of these trends in shade provision also held for the peak of the cooling sea- son in July or August. Average daily shade provision by either tree types during the cooling season (and its peak) diminished when moving from northern to southern cities due to progressively higher sun angles as moving toward more southerly locations. The measure of maximum shade provision (sin- gle-day coverage value) gives an indication of how tree shade could be maximized on a single day. Dur- ing the cooling season, the maximum daily shade provision of all TPC permutations on building sur- faces ranged from 445 m2 (in Charlotte) to 564 m2 (in Orlando) (Figure 5). Large, nearby trees consistently ©2015 International Society of Arboriculture provided the most shade, with the majority of these TPCs exceeding the 90th percentile of all TPC per- mutations. In three of the four localities, tree place- ment on the south aspect maximized daily shade in the cooling season. Minneapolis was the exception, where shade was maximized by an east-positioned tree. Due to the longer cooling season in southern localities, extending well into autumn when the sun angle is low, south-positioned trees maximized sunlight interception. Across all localities, although coniferous trees provided greater average shade pro- vision, deciduous trees tended to provide the maxi- mum shade (of any single-day values) (Figure 5). The differences in shade provision between conif- erous and deciduous trees were primarily due to the elliptical crown shape simulated for deciduous trees, which have their widest diameter positioned at a greater vertical height above the ground than do parabolic coniferous trees. In general, differences in shade provision between tree types diminished when moving from northern to southern locali- ties as the duration of the cooling season increased. Shade Provision During Peak Cooling Demand Historical weather data shows that both average monthly temperatures and cooling degree-days (CDDs) peak in the month of July or August, de- pending on the study area (Table 2). The most severe EHEs in the U.S. since 1980 have typically occurred during July in locations such as Chicago, Illinois (1980, 1983, 1986, and 1995); Kansas City, Missouri (1980); Memphis, Tennessee (1980); Philadelphia, Pennsylvania (1993); and Phoenix, Arizona (2005) (Whitman et al. 1997; U.S. Environmental Protec- tion Agency 2006; Centers for Disease Control and Prevention 2013). The simulation results showed that during the peak of the cooling season both coniferous and deciduous trees placed on the east aspect of the structure cast the most shade on build- ing surfaces, followed by trees placed on the west. Orlando was a notable exception, with trees on the west aspects providing more shade (Table 3). Com- pared to trees on other aspects, south-positioned trees provided lower levels of shade across all study areas due to the high sun angles that occur at mid- day during peak cooling demand. However, differ- ences in shade provision between trees on the south and on other aspects decreased when moving to-
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