Screening of Tree Species for Improving Outdoor Human Thermal Comfort in a Taiwanese City

sustainability

Article Screening of Tree Species for Improving Outdoor Human Thermal Comfort in a Taiwanese City

Yu-Hao Lin and Kang-Ting Tsai * Program of Landscape and Recreation, National Chung Hsing University, 145 Xingda Rd., Taichung 402, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-4-2285-0513-108

Academic Editor: Karsten Grunewald Received: 24 December 2016; Accepted: 21 February 2017; Published: 24 February 2017

Abstract: Tropical cities can use urban greening designs featuring trees that provide shade and cooling in hot outdoor environments. The cooling effect involves numerous tree characteristics that are not easy to control during planting design, such as the canopy size and the optical properties of leaves. Planting the appropriate tree species dominates the cooling effects and the human thermal environment. Based on environmental and plant data, including the tree species, crown diameter of trees, physiologically equivalent temperature (PET), and sky view factor (SVF) in an outdoor space, a series of hierarchical cluster analysis (HCA) procedures was implemented to identify the tree species that are appropriate for improving thermal comfort. The results indicated strong correlations between SVF, average crown diameter, and PET. SVF decreased as the average crown diameter increased. For the average crown diameter of trees in an area wider than 1.5 m, the cooling effect was especially dominated by the tree species. Therefore, 15 species were screened by HCA procedures, based on a similar cooling effect. These species had various cooling effects, and were divided into four categories. Tree species, such as Spathodea campanulata and Cinnamomum camphora, had the appropriate crown diameter and cooling effect for the most comfortable thermal environment.

Keywords: tree crown diameter; sky view factor (SVF); physiologically equivalent temperature (PET); thermal comfort; tree planting design; urban greening

1. Introduction Because outdoor environments in tropical areas are hot, people who engage in outdoor daytime activities tend to prefer outdoor areas shaded by trees, to outdoor areas that are not shaded by trees [1]. A favorable tree planting design provides appropriate shade and cooling functions [2–5]. These functions not only influence microclimate modification, especially for tropical and subtropical regions [6–8], but also improve thermal comfort [9–11]. For example, the structures of tree crowns, and the shapes and colors of the leaves, influence the levels of visible light and solar infrared waves [6]. Small-leaved tree species tend to be more effective at cooling than larger-leaved species [12,13]. Therefore, the cooling effect and shading of plants is mainly determined by the canopy density, leaf arrangement, height, branch spread, bole height, solar radiation, and optical properties of the leaves [8,14,15]. Mature trees dominate the crown characteristics. Other tree characteristics, such as the leaf density, are either involved in complex processes or subject to considerable natural variability. The influence of different species on the outdoor thermal environment also differs by area [16]. A database of urban trees should be created to support landscape designers, and tree species should be evaluated for cities’ tree planting plans and the maintenance of an urban microclimate that does not impair humans [6,17]. Several human-biometeorological variables, such as the standard effective temperature, predicted mean vote, mean radiation temperature (Tmrt), and physiological equivalent temperature

Sustainability 2017, 9, 340; doi:10.3390/su9030340 www.mdpi.com/journal/sustainability Sustainability 2017, 9, 340 2 of 12

(PET), are used to evaluate human thermal comfort [18]. These variables for estimating outdoor thermal sensation are presented as functions of air temperature (Ta), wind speed, solar radiation intensity, and absolute humidity [19]. In European countries and Hong Kong, wind has a negative effect on thermal sensation, and high wind speed only becomes signiﬁcant in thermal sensation in winter [19,20]. The sky view factor (SVF) describes the impact of canopy density on air circulation [8] and the irradiance reduction of plant canopies [9,10,21]. Because SVF can be measured conveniently, some studies have used SVF to discuss variations in shading levels and air temperature [22–25]. Areas with relatively dense broadleaf trees tend to have low SVF values [26]. High-SVF areas with thin broadleaf trees were shown to facilitate improved ventilation [27], but these areas also have low shading levels and are exposed to relatively high-air-temperature environments on summer days [28]. Therefore, SVF may be an integrated indicator to describe the crown characteristics. Hierarchical cluster analysis (HCA) is suitable for clustering targets with an unknown number of groups [29], and has been applied to identify pollution sources and assess rice production risks on the basis of the similarity of observations [30–32]. In addition, previous studies on the planting design of green spaces have investigated the visual effects, but not the functions, of plants. The aim of this study was to improve outdoor human thermal environments through the appropriate choice of tree species for planting in open spaces, especially in the tropics. This study included a survey of environmental and planting parameters of outdoor spaces in Taiwan; parameters included the tree species, crown diameter, SVF, and meteorological and human-biometeorological variables. A database was created to support landscape designers. The relationships between these parameters were also discussed. Finally, HCA was used to choose appropriate tree species for tree planting in open spaces. This method could facilitate the improvement of thermal environmental comfort and climate adaptation in tropical urban areas.

2. Materials and Methods

2.1. Thermal Environment and Planting Design of Outdoor Space Our study was conducted in the outdoor space of Chiayi Park (23.48◦N, 120.47◦E), which is located in southern Taiwan, and experiences subtropical and tropical climates (Figure1). July is the hottest month, with a maximum air temperature of 33.1 ◦C and an average air temperature of 25.2 ◦C. The monthly average relative humidity (RH, %) is 75%–85% and the average vapor pressure during 2011–2015 was 27 hPa–32 hPa, indicating that this outdoor space experiences hot summers and high year-round humidity. Chiayi Park features a rich ecology and abundant relaxation facilities. Additionally, various shaded spaces provide park visitors with space for outdoor activities. In Figure1, a total of 12 sites, including A1, A2, A3, B1, B2, B3, C1, C2, C3, D1, D2, and D3, can be seen, that could encourage more people to participate in activities and which have a low wind speed. A wind meter (R M Young 81000) was used to record the wind speed at a height of 1.1 m above the ground, and the average wind speed for each measurement was <0.5 m/s. For evaluating outdoor human thermal environments and reducing the interference of wind, the 12 sites were chosen as measurement sites. Environmental and planting parameters, such as plant characteristics, SVF, and meteorological and human-biometeorological variables, were measured at the 12 sites (Figure1). Measurements were taken during the summer (July–October 2014) because this season witnesses the maximum use of shaded spaces. Meteorological and human-biometeorological variables, including Ta,Tmrt, globe temperature (Tg), and PET, were measured to evaluate the outdoor thermal comfort [33]. RH, Ta, and Tg were sampled at a height of 1.1 m above the ground. A black globe thermometer was used to record Tg, and a Center 314 humidity and temperature meter was used to record the humidity and Ta. Measurements were recorded automatically and simultaneously at 1-min intervals, from 10:00 to 16:00, as the Ta of this period is hot; the temporal resolution of 2 h of recorded data will be adopted for further analysis. The recorded meteorological data per day was selected, according to similar weather conditions; Sustainability 2017, 9, 340 3 of 12

for example, the recorded data was not selected for analysis when it rained during the day. Tmrt was calculated using Ta,Tg, and wind velocity, on the basis of ISO standard 7726 [34,35]. To obtain the values of PET [36,37], parameters including Ta, relative humidity, wind speed, and Tmrt, were entered into RayManSustainability software 2017, 9, 340package [38]. 3 of 13

FigureFigure 1. Measurement 1. Measurement sites sites of of environmentalenvironmental parameters parameters in inthe the study study area. area.

Meteorological and human-biometeorological variables, including Ta, Tmrt, globe temperature The studied area included 95 types of plant. Plant characteristics, such as tree species, tree height (Tg), and PET, were measured to evaluate the outdoor thermal comfort [33]. RH, Ta, and Tg were (m), crown diameter (m), and diameter at breast height (m), were recorded. Tree height, measured sampled at a height of 1.1 m above the ground. A black globe thermometer was used to record Tg, fromand the a base Center to the314 tiphumidity of the tree,and temperature was measured meter using was aused Christen to record hypsometer the humidity [39]. and The T crowna. diameterMeasurements of each tree were was recorded obtained automatically by taking theand arithmeticsimultaneously mean at of1-min the horizontalintervals, from crown 10:00 diameter to measured16:00, onas the the T north-southa of this period axis, is hot; and the again temporal on the resolu east-westtion of axis2 h of [40 recorded]. The diameter data will atbe breastadopted height was obtainedfor further by analysis. directly The measuring recorded meteorological the diameter ofdata the per trunk day withwas selected, a tape. GPSaccording was usedto similar to locate the plantweather which conditions; had been for measured,example, the and recorded photographs data was of not the selected plant were for analysis recorded. when Photographs it rained or leavesduring were the collected day. Tmrt and was compared calculated using with theTa, T literatureg, and wind to velocity, confirm on the the tree basis species of ISO standard [41], and 7726 the data [34,35]. To obtain the values of PET [36,37], parameters including Ta, relative humidity, wind speed, were entered into a geographic information system database. In addition, SVF was determined using and Tmrt, were entered into RayMan software package [38]. a fish-eye lens at a height of 1.1 m above the ground and RayMan software package [6,38]. The studied area included 95 types of plant. Plant characteristics, such as tree species, tree height (m), crown diameter (m), and diameter at breast height (m), were recorded. Tree height, 2.2. Screening of Appropriate Tree Species Using HCA measured from the base to the tip of the tree, was measured using a Christen hypsometer [39]. The HCAcrown isdiameter a popular of each method tree was for discoveringobtained by taking patterns the inarithmetic large observed mean of datathe horizontal and clusters crown targets accordingdiameter to themeasured similarity on the of north-south observations axis, (or and variable) again on [42the]. east-west Groups axis of tree[40]. speciesThe diameter with similarat characteristicsbreast height were was identified obtained by using directly HCA. measurin The distanceg the diameter between of the two trunk tree with species a tape. was GPS defined was as the Euclideanused to locate norm the of plant the difference which had between been measured, the centered and photographs and scaled of vectors the plant of theirwere characteristics.recorded. Photographs or leaves were collected and compared with the literature to confirm the tree species The dissimilarity between clusters, based on this distance, was defined using Ward’s method; the [41], and the data were entered into a geographic information system database. In addition, SVF was m standardizeddetermined-space using a Euclidian fish-eye lens distance at a isheight given of by 1.1 the m followingabove the equationground and [43 RayMan]: software package [6,38]. v u 2 2.2. Screening of Appropriate Tree Species Usingu HCAm − t ∑k=1 Xik Xjk d = HCA is a popular method for diijscovering patternsm in large observed data and clusters targets according to the similarity of observations (or variable) [42]. Groups of tree species with similar wherecharacteristicsXik denotes were the kidentifiedth measured using variable HCA. The for dist observationance betweeni andtwo treeXjk isspecies the k wasth measured defined as variablethe for observationEuclidean normj. In of the the HCA difference dendrogram, between athe short centered distance and scaled indicates vectors that of the their two characteristics. observations are similarThe or dissimilarity “close together”, between and clusters, vice versa.based on Detailed this distance, calculation was defined processes using of Ward’s HCA method; can be found the in standardized m-space Euclidian distance is given by the following equation [43]: a classical paper conducted by Punj and Stewart (1983) [44]. Some common decision rules, such as Sustainability 2017, 9, 340 4 of 12 the R-squared (R2) and semipartial R-squared (SPR) rules, were recommended for screening the appropriate observations and evaluating the numbers of groups; the number of groups should be chosen such that a further reduction results in a substantial decrease in the SPR [45]. Within the analysis, when the rule values were bad, the inappropriate observations were removed, and the HCA was implemented progressively, until better rule values were obtained. In this study, the data were analyzed using SPSS 18 software. Specifically, tree species were classified into the same group if they had a similar cooling effect; the variables, including crown diameter, SVF, and PET, were the inputs of HCA. The process was repeated to identify the appropriate tree species and determine the number of groups, according to the R2 and SPR rules. However, because trees with low heights did not cause shading and cooling effects, when they were located near other trees with higher heights, the HCA procedures for screening the appropriate tree species were conducted on the basis of the survey data for the trees near the SVF measurement sites that were higher than 2 m. In addition, the discovered patterns depended on the observations, climatic conditions, and geological environment.

3. Results and Discussion

3.1. Tree Database and the Relationships between Average Crown Diameter, Plant Height, Diameter at Breast Height, and SVF The tree database comprised 614 trees that were surveyed in the study area, and Table1 shows the classification of tree species according to the average crown diameter. Ten plant species were identified with crown diameters exceeding 10 m; twenty-five plants species were identified with crown diameters between 5 and 10 m; and twenty-one plants species were identified with crown diameters between 3 and 5 m.

Table 1. Plant species in the database.

Average Crown Diameter Plant Specie Tectona grandis, Peltophorum pterocarpum, Cerbera manghas, Hibiscus tiliaceus, Crown diameters exceeding 10 m Ficus microcarpa, Betula pendula, Delonix regia, Haematoxylum campechianum, Zelkova serrata, and Terminalia catappa Roystonea regia, Lagerstroemia speciosa, Terminalia catappa, Araucaria heterophylla, Terminalia mantaly, Adenanthera pavonina, Pongamia pinnata, Spathodea campanulata, Melaleuca leucadendra, Magnolia denudate, Pterocarpus indicus, Phoebe faberi, Crown diameters between 5 and 10 m Michelia figo, Mangifera indica, Araucaria cunninghamii, Tabebuia impetiginosa, Machilus zuihoensis Hayata, Saraca asoca, Alstonia scholaris, Liquidambar formosana Hance, Acer, Cinnamomum camphora, Mangifera indica, Bauhinia blakeana, and Terminalia catappa Macaranga tanarius, Pithecellobium dulce, Cassia fistula, Chamaecyparis obtusa, Chorisia speciosa, Bischofia javanica Blume, Melia azedarach, Swietenia mahagoni, Calocedrus formosana, Hyophorbe verschaffeltii, Sterculia foetida, Pistacia chinensis, Crown diameters between 3 and 5 m Elaeocarpus sylvestris, Pinus jeffreyi, Cocos nucifera, Syzygium samarangense, Juniperus chinensis, Dimocarpus longan, Barringtonia racemosa, Artocarpus altilis, and Codiaeum variegatum Lagerstroemia subcostata, Gordonia axillaris, Caryota urens, Carica papaya, Calocedrus formosana, Fraxinus griffithii, Hibiscus rosa-sinensis, Podocarpus nagi, Washingtonia robusta, Bambusa vulgaris Schrad, Duranta erecta, Salix babylonica, Ficus benjamina, Zanthoxylum piperitum, Cananga odorata, Strelitzia nicolai, Crown diameters smaller than 3 m Michelia compressa, Triadica sebifera, Hyophorbe lagenicaulis, Pachira aquatica, Lantana camara, Prunus mume, Sapindus, Lagerstroemia speciosa, Ficus religiosa, Aucuba japonica, Erythrina variegate, Chrysalidocarpus lutescens, Allamanda schottii, Broussonetia papyrifera, Garcinia subelliptica Merr., Plumeria alba, Aglaia odorata, Elaeocarpus serratus Linn., Agave americana, Phoenix roebelenii, and Euphorbia splendens

Figure2 shows the relationships between the crown diameters, diameters at breast height, and the plant heights of the trees. In Figure2a, the trees with the longest diameters at breast height are the Norfolk Island pine and Mangifera indica; the tree with the shortest diameter at breast height was Zanthoxylum piperitum. The tree with the widest crown diameter was Delonix regia, and the tree with the narrowest crown diameter was Lantana camara. As expected, the low regression coefﬁcient (R2 = 0.04) Sustainability 2017, 9, 340 5 of 13

Lagerstroemia subcostata, Gordonia axillaris, Caryota urens, Carica papaya, Calocedrus formosana, Fraxinus griffithii, Hibiscus rosa-sinensis, Podocarpus nagi, Washingtonia robusta, Bambusa vulgaris Schrad, Duranta erecta, Salix babylonica, Ficus benjamina, Zanthoxylum piperitum, Cananga odorata, Crown diameters smaller Strelitzia nicolai, Michelia compressa, Triadica sebifera, than 3 m Hyophorbe lagenicaulis, Pachira aquatica, Lantana camara, Prunus mume, Sapindus, Lagerstroemia speciosa, Ficus religiosa, Aucuba japonica, Erythrina variegate, Chrysalidocarpus lutescens, Allamanda schottii, Broussonetia papyrifera, Garcinia subelliptica Merr., Plumeria alba, Aglaia odorata, Elaeocarpus serratus Linn., Agave americana, Phoenix roebelenii, and Euphorbia splendens

Figure 2 shows the relationships between the crown diameters, diameters at breast height, and Sustainabilitythe plant2017, 9heights, 340 of the trees. In Figure 2a, the trees with the longest diameters at breast height are 5 of 12 the Norfolk Island pine and Mangifera indica; the tree with the shortest diameter at breast height was Zanthoxylum piperitum. The tree with the widest crown diameter was Delonix regia, and the tree with demonstratedthe narrowest no correlation crown diameter between was theLantana tree camara diameter. As expected, at breast the height low regression and the crowncoefficient diameter. As shown(R2 = in 0.04) Figure demonstrated2b, the highest no correlation and lowest between tree th speciese tree diameter were Roystonea at breast height regia and theLantana crown camara, respectively.diameter. Some As treesshown with in similarFigure 2b, heights the highest had different and lowest crown tree diameters, species were possibly Roystonea because regia and they were differentLantana tree species camara, and respectively. had different Some soil trees environments. with similar Forheights example, had different specimens crown of Magnolia diameters, denudata and Chrysalidocarpuspossibly because lutescens they werewere different both tree 4.8 species m high, and but had theydifferent had soil crown environments. diameters For of example, 4.1 and 5.2 m, specimens of Magnolia denudata and Chrysalidocarpus lutescens were both 4.8 m high, but they had respectively. However, the high regression coefﬁcient (R2 = 0.83) indicated a high correlation between crown diameters of 4.1 and 5.2 m, respectively. However, the high regression coefficient (R2 = 0.83) the treeindicated heightand a high the correlation crown diameter; between the the tree crown heig diameterht and the increasedcrown diameter; with increasingthe crown diameter tree height in the studyincreased area. with increasing tree height in the study area.

(a) (b)

Figure 2. Relationship between (a) crown diameter and diameter at breast height (b) crown diameter Figure 2. Relationship between (a) crown diameter and diameter at breast height (b) crown diameter and plant height. and plant height. Table 2 shows the actual sky view situation (fish-eye lens), SVFs at 12 measuring sites Table(Figure2 shows 1), and the the actual tree species sky view near situation the SVF (ﬁsh-eyemeasurement lens), sites. SVFs A total at 12 of measuring 56 tree species sites were (Figure 1), located within 15 m of the SVF measurement centers, and SVFs varied from 0.025 to 0.637. A low and the tree species near the SVF measurement sites. A total of 56 tree species were located within 15 m SVF value indicated a lower visible sky value and a high leaf density value; the highest and lowest of the SVFSVF measurementvalues were measured centers, at sites and A2 SVFs and variedD3, respectively. from 0.025 Because to 0.637. sites Awith low low SVF SVF value values indicated had a lower visiblehigh shaded sky value area values, and a high people leaf tended density to do value; activities the highestthere during and lowesthot conditions SVF values [1,28]. were Figure measured 3 at sites A2reveals and the D3, respectively.relationship between Because SVF sites and with the low average SVF values crown had diameter high shaded of trees area at values,the 12 people tendedmeasurement to do activities sites. there The duringaverage hot crown conditions diameters [1 varied,28]. Figure from 0.63 reveals to 8.4 m. the The relationship high R2 indicated between a SVF and thesignificant average crowncorrelation diameter between of SVF trees and at thethe 12average measurement crown diameter, sites. Theand averageSVF decreased crown with diameters varied from 0.6 to 8.4 m. The high R2 indicated a signiﬁcant correlation between SVF and the average Sustainability 2017, 9, 340 6 of 14 crownSustainability diameter, 2017 and, 9, 340 SVF decreased with increasing crown diameter in the6 studyof 14 area. An inversion point wasincreasing also observed.crown diameter This in the revealed study area. that An inversion SVF increased point was andalso observed. the shaded This revealed area substantially decreased, increasing crown diameter in the study area. An inversion point was also observed. This revealed when thethat averageSVF increased crown and the diameter shaded area of substantially trees was decreased, less than when 1.5 the m. average crown diameter thatof trees SVF was increased less than and 1.5 the m. shaded area substantially decreased, when the average crown diameter of trees was less than 1.5 m. TableTable 2. 2. Plant species, species, sky view, sky and view, SVF va andlues SVFnear the values measurement near thesites. measurement sites. Table 2. Plant species, sky view, and SVF values near the measurement sites. Site a Fish-Eye Lens SVF Plant Species * a Site a Site Fish-Eye Fish-Eye Lens Lens SVF SVF Plant Species * Plant Species *

Ficus religiosa, Adenanthera pavonina, FicusRoystonea religiosa regia, Adenanthera, Pterocarpus Ficuspavonina indicus religiosa, , , Adenanthera pavonina, A1 0.611 RoystoneaCinnamomum regia camphora, Pterocarpus, ArtocarpusRoystonea indicus, altilis regia, , Pterocarpus indicus, A1 A1 0.611 CinnamomumJuniperus0.611 chinensis camphora, Liquidambar, ArtocarpusCinnamomum formosana altilis, Hance, camphora , Artocarpus altilis, Juniperusand Lagerstroemia chinensis ,speciose Liquidambar Juniperus formosana chinensis Hance,, Liquidambar formosana Hance, and Lagerstroemia speciose and Lagerstroemia speciose

Chorisia speciosa, Hibiscus tiliaceusChorisia, speciosa, Hibiscus tiliaceus, A2 A2 0.637 ChorisiaAllamanda0.637 speciosa schottii, Hibiscus, Peltophorum tiliaceusAllamanda pterocarpum, schottii, and, Peltophorum pterocarpum, A2 0.637 AllamandaFicusmicrocarpa schottii , Peltophorumand pterocarpumFicusmicrocarpa, and Ficusmicrocarpa

Cinnamomum camphora, Chamaecyparis obtusa, A3 0.544 CinnamomumJuniperus chinensis camphora, Lagerstroemia, Chamaecyparis speciosa obtusa, and, A3 0.544 JuniperusSwietenia chinensismahagoni, Lagerstroemia speciosa, and Swietenia mahagoni

Bauhinia blakeana, Tabebuia impetiginosa, B1 0.419 BauhiniaAlstonia scholarisblakeana,, DelonixTabebuia regia impetiginosa, Ficus microcarpa, , B1 0.419 AlstoniaSpathodea scholaris campanulata, Delonix, and regia Terminalia, Ficus microcarpa catappa , Spathodea campanulata, and Terminalia catappa

Garcinia subelliptica Merr., Ficus benjamina, GarciniaPeltophorum subelliptica pterocarpum Merr.,, Ficus benjamina, B2 0.420 PeltophorumLiquidambar formosanapterocarpum Hance,, Juniperus chinensis, B2 0.420 LiquidambarEuphorbia splendens formosana, Magnolia Hance, denudataJuniperus, chinensis , EuphorbiaFicus microcarpa splendens, and, Magnolia Lagerstroemia denudata speciose, Ficus microcarpa, and Lagerstroemia speciose

Sustainability 2017, 9, 340 6 of 14 Sustainability 2017, 9, 340 6 of 14 Sustainability 2017, 9, 340 6 of 14 increasing crown diameter in the study area. An inversion point was also observed. This revealed increasing crown diameter in the study area. An inversion point was also observed. This revealed increasingthat SVF increased crown diameter and the inshaded the study area substantiallyarea. An inversion decreased, point when was alsothe averageobserved. crown This diameterrevealed ofthat trees SVF was increased less than and 1.5 the m. shaded area substantially decreased, when the average crown diameter thatof trees SVF was increased less than and 1.5 the m. shaded area substantially decreased, when the average crown diameter of trees was less than 1.5 m. Table 2. Plant species, sky view, and SVF values near the measurement sites. Table 2. Plant species, sky view, and SVF values near the measurement sites. Site a Table Fish-Eye 2. Plant species,Lens sky view, and SVF SVF values near the measurement Plant Species sites. * Site a Fish-Eye Lens SVF Plant Species * a Site Fish-Eye Lens SVF Plant Species *

Ficus religiosa, Adenanthera pavonina, FicusRoystonea religiosa regia, Adenanthera, Pterocarpus pavoninaindicus, , Ficus religiosa, Adenanthera pavonina, A1 0.611 RoystoneaCinnamomum regia camphora, Pterocarpus, Artocarpus indicus, altilis , Roystonea regia, Pterocarpus indicus, A1 0.611 JuniperusCinnamomum chinensis camphora, Liquidambar, Artocarpus formosana altilis, Hance, A1 0.611 Cinnamomum camphora, Artocarpus altilis, andJuniperus Lagerstroemia chinensis speciose, Liquidambar formosana Hance, Juniperusand Lagerstroemia chinensis ,speciose Liquidambar formosana Hance, and Lagerstroemia speciose

Chorisia speciosa, Hibiscus tiliaceus, SustainabilityA22017 , 9, 340 0.637 AllamandaChorisia speciosa schottii, Hibiscus, Peltophorum tiliaceus pterocarpum, , and 6 of 12 Chorisia speciosa, Hibiscus tiliaceus, A2 0.637 AllamandaFicusmicrocarpa schottii , Peltophorum pterocarpum, and A2 0.637 AllamandaFicusmicrocarpa schottii , Peltophorum pterocarpum, and Ficusmicrocarpa Table 2. Cont.

Site a Fish-Eye Lens SVF Plant Species *

Cinnamomum camphora, Chamaecyparis obtusa, A3 0.544 JuniperusCinnamomum chinensis camphora, Lagerstroemia, Chamaecyparis speciosa obtusa, and, Cinnamomum camphora, ChamaecyparisCinnamomum obtusa camphora, , Chamaecyparis obtusa, A3 0.544 SwieteniaJuniperus chinensismahagoni, Lagerstroemia speciosa, and A3 A3 0.544 JuniperusSwietenia0.544 chinensismahagoni, LagerstroemiaJuniperus speciosa chinensis, and , Lagerstroemia speciosa, Swietenia mahagoni and Swietenia mahagoni

Bauhinia blakeana, Tabebuia impetiginosa, B1 0.419 AlstoniaBauhinia scholarisblakeana,, DelonixTabebuia regia Bauhiniaimpetiginosa, Ficus microcarpa blakeana, ,,Tabebuia impetiginosa, Bauhinia blakeana, Tabebuia impetiginosa, B1 B1 0.419 SpathodeaAlstonia0.419 scholaris campanulata, Delonix, and regiaAlstonia Terminalia, Ficus scholarismicrocarpa catappa , Delonix, regia, Ficus microcarpa, B1 0.419 AlstoniaSpathodea scholaris campanulata, Delonix, and regiaSpathodea Terminalia, Ficus microcarpa campanulatacatappa , , and Terminalia catappa Spathodea campanulata, and Terminalia catappa

Garcinia subelliptica Merr., FicusGarcinia benjamina subelliptica, Merr., Ficus benjamina, PeltophorumGarcinia subelliptica pterocarpum Merr.,, PeltophorumFicus benjamina pterocarpum, , Garcinia subelliptica Merr., Ficus benjamina, B2 B2 0.420 LiquidambarPeltophorum0.420 formosanapterocarpum Hance,, Liquidambar Juniperus chinensis formosana, Hance, Juniperus chinensis, SustainabilityB2 2017, 9, 340 0.420 PeltophorumLiquidambar formosanapterocarpum Hance,, Juniperus chinensis7 of, 14 Sustainability 2017, 9, 340 Euphorbia splendens, MagnoliaEuphorbia denudata, splendens 7 of, 14Magnolia denudata, B2 0.420 LiquidambarEuphorbia splendens formosana, Magnolia Hance, denudataJuniperus, chinensis , Sustainability 2017, 9, 340 Ficus microcarpa, and LagerstroemiaFicus microcarpa speciose ,7 and of 14Lagerstroemia speciose Sustainability 2017, 9, 340 EuphorbiaFicus microcarpa splendens, and, Magnolia Lagerstroemia denudata speciose, 7 of 14 Ficus microcarpa, and Lagerstroemia speciose

Pterocarpus indicus, Ficus microcarpa, Delonixregia, Pterocarpus indicus, Ficus microcarpa, Delonixregia, Calocedrus formosana, Roystonea regia, CalocedrusPterocarpus formosana indicus, Ficus, Roystonea microcarpa regia, ,Delonixregia , PterocarpusFraxinus griffithii indicus, Chorisia, Ficus microcarpa speciosaPterocarpus, , Delonixregia indicus,, Ficus microcarpa, Delonixregia, B3 0.411 FraxinusCalocedrus griffithii formosana, Chorisia, Roystonea speciosa regia, , B3 0.411 CalocedrusCinnamomum formosana camphora, Roystonea, SpathodeaCalocedrus regia campanulata, formosana, Roystonea regia CinnamomumFraxinus griffithii camphora, Chorisia, Spathodea speciosa ,campanulata , , , B3 0.411 FraxinusAdenanthera griffithii pavonina, Chorisia, Pterocarpus speciosaFraxinus ,indicus grifﬁthii, , Chorisia speciosa, B3 0.411 AdenantheraCinnamomum pavonina camphora, Pterocarpus, Spathodea indicuscampanulata, , B3 CinnamomumMagnolia0.411 denudata camphora, and, CassiaSpathodea fistula campanulata , MagnoliaAdenanthera denudata pavonina, and, Pterocarpus CassiaCinnamomum fistula indicus , camphora, Spathodea campanulata, AdenantheraMagnolia denudata pavonina, and, Pterocarpus CassiaAdenanthera fistula indicus , pavonina, Pterocarpus indicus, Magnolia denudata, and Cassia fistula Magnolia denudata, and Cassia ﬁstula

Lagerstroemia speciosa, Cinnamomum camphora, Lagerstroemia speciosa, Cinnamomum camphora, Michelia figo, Acer, Alstonia scholaris, MicheliaLagerstroemia figo, Acerspeciosa, Alstonia, Cinnamomum Lagerstroemiascholaris, camphora speciosa, , Cinnamomum camphora, C1 0.047 LagerstroemiaCinnamomum speciosacamphora, Cinnamomum, Chamaecyparis camphora obtusa,, C1 0.047 CinnamomumMichelia figo, Acer camphora, Alstonia, ChamaecyparisMichelia scholaris, ﬁgo obtusa, Acer, , Alstonia scholaris, MicheliaJuniperus figo chinensis, Acer, ,Alstonia Spathodea scholaris campanulata, , C1 C1 0.047 JuniperusCinnamomum0.047 chinensis camphora, Spathodea, ChamaecyparisCinnamomum campanulata obtusa, camphora, , Chamaecyparis obtusa, C1 0.047 CinnamomumAdenanthera pavonina camphora, and, Chamaecyparis Pterocarpus indicusobtusa, AdenantheraJuniperus chinensis pavonina, Spathodea, and PterocarpusJuniperus campanulata chinensisindicus, , Spathodea campanulata, JuniperusAdenanthera chinensis pavonina, Spathodea, and AdenantheraPterocarpus campanulata indicus pavonina, , and Pterocarpus indicus Adenanthera pavonina, and Pterocarpus indicus

Lagerstroemia speciosa, Peltophorum pterocarpum, Lagerstroemia speciosa, PeltophorumLagerstroemia pterocarpum speciosa, , Peltophorum pterocarpum, Acer, Delonix regia, Alstonia scholaris, C2 0.075 AcerLagerstroemia, Delonix regiaspeciosa, Alstonia, PeltophorumAcer scholaris, Delonix pterocarpum, regia, , Alstonia scholaris, C2 C2 0.075 LagerstroemiaHaematoxylum0.075 speciosa campechianum, Peltophorum, Bauhinia pterocarpum blakeana,, HaematoxylumAcer, Delonix regia campechianum, AlstoniaHaematoxylum scholaris, Bauhinia, blakeana campechianum, , Bauhinia blakeana, C2 0.075 Acerand ,Roystonea Delonix regia regia, Alstonia scholaris, C2 0.075 andHaematoxylum Roystonea regiacampechianum and, BauhiniaRoystonea blakeana regia, Haematoxylumand Roystonea regiacampechianum , Bauhinia blakeana, and Roystonea regia

Pachira macrocarpa, ElaeocarpusPachira serratus macrocarpa Linn., , Elaeocarpus serratus Linn., Pachira macrocarpa, Elaeocarpus serratus Linn., Terminalia catappa, Ficus religiosaTerminalia, catappa, Ficus religiosa, TerminaliaPachira macrocarpa catappa,, FicusElaeocarpus religiosa serratus, Linn., PachiraLagerstroemia macrocarpa subcostata, Elaeocarpus, AraucariaLagerstroemia serratus cunninghamii Linn., subcostata , , Araucaria cunninghamii, LagerstroemiaTerminalia catappa subcostata, Ficus, Araucariareligiosa, cunninghamii, TerminaliaPithecellobium catappa dulce, Ficus, Chrysalidocarpus religiosaPithecellobium, lutescens dulce, , Chrysalidocarpus lutescens, PithecellobiumLagerstroemia subcostatadulce, Chrysalidocarpus, Araucaria cunninghamii lutescens, , C3 C3 0.065 LagerstroemiaRoystonea0.065 regia subcostata, Peltophorum, AraucariaRoystonea pterocarpum cunninghamii regia, , Peltophorum, pterocarpum, C3 0.065 RoystoneaPithecellobium regia dulce, Peltophorum, Chrysalidocarpus pterocarpum lutescens, , PithecellobiumCaryota urens, dulceChrysalidocarpus, Chrysalidocarpus lutescens lutescens, , C3 0.065 CaryotaRoystonea urens regia, Chrysalidocarpus, PeltophorumCaryota pterocarpum lutescens urens, , , Chrysalidocarpus lutescens, C3 0.065 RoystoneaLiquidambar regia formosana, Peltophorum Hance, pterocarpum Sapindus, , LiquidambarCaryota urens formosana, Chrysalidocarpus Hance,Liquidambar Sapindus lutescens, , formosana Hance, Sapindus, CaryotaFicus microcarpa urens, Chrysalidocarpus, Peltophorum pterocarpum lutescens, , FicusLiquidambar microcarpa formosana, Peltophorum Hance,Ficus pterocarpum Sapindus microcarpa, , , Peltophorum pterocarpum, Liquidambarand Lagerstroemia formosana speciose Hance, Sapindus, andFicus Lagerstroemia microcarpa, Peltophorum speciose and pterocarpumLagerstroemia, speciose Ficusand Lagerstroemia microcarpa, Peltophorum speciose pterocarpum, and Lagerstroemia speciose

Sustainability 2017, 9, 340 7 of 12

Sustainability 2017, 9, 340 8 of 14 Table 2. Cont. Sustainability 2017, 9, 340 8 of 14 Sustainability 2017, 9, 340 8 of 14 Site a Fish-Eye Lens SVF Plant Species *

Tectona grandis, Delonix regia, Alstonia scholaris, Bischofia javanica Blume, Swietenia mahagoni, D1 0.025 TectonaPistacia grandis chinensis, Delonix, Elaeocarpus regia, Alstonia serratus scholaris Linn.,, and Tectona grandis, Delonix regiaTectona, Alstonia grandis scholaris, Delonix, regia, Alstonia scholaris, BischofiaTerminalia javanica catappa Blume, Swietenia mahagoni, D1 0.025 Bischofia javanica Blume, SwieteniaBischoﬁa mahagoni javanica, Blume, Swietenia mahagoni, D1 D1 0.025 Pistacia0.025 chinensis, Elaeocarpus serratus Linn., and TerminaliaPistacia chinensis catappa, Elaeocarpus Pistacia serratus chinensis Linn., and, Elaeocarpus serratus Linn., Terminalia catappa and Terminalia catappa

Chrysalidocarpus lutescens, Peltophorum pterocarpum, Ficus microcarpa, Chrysalidocarpus lutescens, Chrysalidocarpus lutescens, Peltophorum PeltophorumChrysalidocarpusMichelia compressa pterocarpum lutescens, Mangifera, ,Ficus microcarpa indica, , D2 0.059 pterocarpum, Ficus microcarpa, Michelia compressa, PeltophorumMicheliaPongamia compressa pinnatapterocarpum, Mangifera, Araucaria, Ficus indica microcarpaheterophylla, , , D2 D2 0.059 Michelia0.059 compressa, MangiferaMangifera indica, indica, Pongamia pinnata, D2 0.059 PongamiaSwietenia pinnata mahagoni, Araucaria, and heterophylla, SwieteniaPongamiaLiquidambar mahagonipinnata formosana, Araucaria, and HanceAraucaria heterophylla heterophylla, , Swietenia mahagoni, Swietenia mahagoni, and and Liquidambar formosana Hance Liquidambar formosana Hance Liquidambar formosana Hance

Cassia fistula, PithecellobiumCassia dulce ﬁstula, , Pithecellobium dulce, D3 D3 0.138 Cassia0.138 fistula, Pithecellobium dulce, D3 0.138 CassiaPongamia fistula pinnata, Pithecellobium, and PistaciaPongamia dulce, chinensis pinnata , and Pistacia chinensis D3 0.138 Pongamia pinnata, and Pistacia chinensis Pongamia pinnata, and Pistacia chinensis

a a The site shown in Figure1; * the trees height were higher than 2 m. Sustainability 2017, 9, 340a The The site shown inin FigureFigure 1; 1; * *the the tr treesees height height were were higher higher than than 2 m. 2 m. 8 of 13 a The site shown in Figure 1; * the trees height were higher than 2 m.

Figure 3. Relationship between SVF and average crown diameter of trees around the measurement Figure 3. Relationship between SVF and average crown diameter of trees around the measurement site.Figure 3. Relationship between SVF and average crown diameter of trees around the measurement site.site.

3.2. Relationship between Crown Diameters of Plants, PET, and Tmrt 3.2.3.2. Relationship Relationship between between Crown Diameters of of Plants, Plants, PET, PET, and and T mrtTmrt Figure 4 shows the relationship between the crown diameters of plants, PET, and Tmrt at the Figure 4 shows the relationship between the crown diameters of plants, PET, and Tmrt at the measurementFigure 4 shows sites; PETthe wasrelationship strongly betweencorrelated the with crown Tmrt. Whendiameters the average of plants, crown PET, diameter and Tmrt was at the Figuremeasurement 3. Relationship sites; PET was between strongly correlatedSVF and with average Tmrt. When crown the averagediameter crown of diametertrees around was the measurement Figuremeasurementshorter 3. thanRelationship approximately sites; PET was between 1.5 strongly m, PET SVF wascorrelated and usually average withhigher T crownmrtthan. When 40 °C diameter theand average SVF ofwastrees crown larger around thandiameter 0.411; the was measurement site. shortersite.thus,shorter thethan than corresponding approximately approximately outdoor 1.5 m, PETPET thermal was was usually usuallycomfort higher higher feeling than than ranged 40 40 °C °C and from and SVF SVFhot was wasto larger very larger than hot than 0.411;[46]. 0.411; thus,Comparatively,thus, thethe correspondingcorresponding the trees with outdoor average thermalthermal crown comfortdiameterscomfort feeling feelingwider thanranged ranged 1.5 m,from fromintroduced hot hot to to verycooling very hot effects hot[46]. [46]. Comparatively, the trees with average crown diameters wider than 1.5 m, introduced cooling effects 3.2. RelationshipComparatively,and resulted betweenin thea lower trees PET; Crownwith however, average Diameters crownthe cooling diameters of Plants,level was wider PET,not than correlated and 1.5 Tm,mrt with introduced the average cooling crown effects 3.2. Relationshipand resulted in between a lower PET; Crown however, Diameters the cooling of level Plants, was notPET, correlated and T withmrt the average crown and resulted in a lower PET; however, the cooling level was not correlated with the average crown Figure4 shows the relationship between the crown diameters of plants, PET, and T mrt at the Figure 4 shows the relationship between the crown diameters of plants, PET, and Tmrt at the measurement sites; PET was strongly correlated with Tmrt. When the average crown diameter was measurement sites; PET was strongly correlated with Tmrt. When the average crown diameter was shorter than approximately 1.5 m, PET was usually higher than 40 ◦C and SVF was larger than shorter than approximately 1.5 m, PET was usually higher than 40 °C and SVF was larger than 0.411; 0.411; thus, the corresponding outdoor thermal comfort feeling ranged from hot to very hot [46]. thus, the corresponding outdoor thermal comfort feeling ranged from hot to very hot [46]. Comparatively, the trees with average crown diameters wider than 1.5 m, introduced cooling effects Comparatively, the trees with average crown diameters wider than 1.5 m, introduced cooling effects and resulted in a lower PET; however, the cooling level was not correlated with the average crown and resulted in a lower PET; however, the cooling level was not correlated with the average crown diameter when PET was between 20 ◦C and 40 ◦C. These results indicate that the tree species with diameter when PET was between 20 °C and 40 °C. These results indicate that the tree species with average crown diameters narrower than 1.5 m result in a larger open sky, causing a hot thermal average crown diameters narrower than 1.5 m result in a larger open sky, causing a hot thermal environment. When the average crown diameter of trees was wider than 1.5 m, the cooling effect was environment. When the average crown diameter of trees was wider than 1.5 m, the cooling effect affected by other tree characteristics, in addition to the crown diameter, such as the optical properties was affected by other tree characteristics, in addition to the crown diameter, such as the optical of leaves. However, tree characteristics were not easy to control, in order to improve outdoor thermal properties of leaves. However, tree characteristics were not easy to control, in order to improve outdoor thermal comfort. Therefore, screening of the appropriate tree species is essential for improving outdoor thermal comfort.

Figure 4. Relationship between average crown diameter of plants, PET, and Tmrt. * The value is average of 2 h of recorded data.

3.3. Appropriate Tree Species for Improving Outdoor Thermal Comfort After implementing a series of HCA procedures on the screening of appropriate tree species, the results suggested that 14 species had similar cooling effects (Figure 5). In addition, significant reductions in the SPR and the R2 from four groups to three groups, indicated that the four-group Sustainability 2017, 9, 340 8 of 13

Figure 3. Relationship between SVF and average crown diameter of trees around the measurement site.

3.2. Relationship between Crown Diameters of Plants, PET, and Tmrt

Figure 4 shows the relationship between the crown diameters of plants, PET, and Tmrt at the measurement sites; PET was strongly correlated with Tmrt. When the average crown diameter was shorter than approximately 1.5 m, PET was usually higher than 40 °C and SVF was larger than 0.411; thus, the corresponding outdoor thermal comfort feeling ranged from hot to very hot [46]. Comparatively, the trees with average crown diameters wider than 1.5 m, introduced cooling effects and resulted in a lower PET; however, the cooling level was not correlated with the average crown diameter when PET was between 20 °C and 40 °C. These results indicate that the tree species with average crown diameters narrower than 1.5 m result in a larger open sky, causing a hot thermal

Sustainabilityenvironment.2017 ,When9, 340 the average crown diameter of trees was wider than 1.5 m, the cooling 8effect of 12 was affected by other tree characteristics, in addition to the crown diameter, such as the optical properties of leaves. However, tree characteristics were not easy to control, in order to improve comfort.outdoor thermal Therefore, comfort. screening Therefore, of the appropriatescreening of tree the species approp isriate essential tree forspecies improving is essential outdoor for thermalimproving comfort. outdoor thermal comfort.

FigureFigure 4.4.Relationship Relationship between between average average crown crow diametern diameter of plants, of plants, PET, andPET, Tmrt and. * TheTmrt. value * The is averagevalue is ofaverage 2 h of recordedof 2 h of recorded data. data.

3.3.3.3. AppropriateAppropriate TreeTree Species Species for for Improving Improving Outdoor Outdoor Thermal Thermal Comfort Comfort AfterAfter implementingimplementing aa seriesseries ofof HCAHCA proceduresprocedures onon thethe screeningscreening ofof appropriateappropriate treetree species,species, thethe resultsresults suggestedsuggested thatthat 1414 speciesspecies hadhad similarsimilar coolingcooling effectseffects (Figure(Figure5 ).5). In In addition, addition, significant significant reductionsreductions inin thethe SPRSPR andand thethe RR22 fromfrom fourfour groupsgroups toto threethree groups,groups, indicatedindicated thatthat thethe four-groupfour-group solution was the most appropriate, and the tree species for the four groups are shown in Table3. Ficus microcarpa was the only tree species in Group A; the trees in Group B included Delonix regia and Terminalia catappa; the trees in Group C were Juniperus chinensis, Bischofia javanica Blume, and Chamaecyparis taiwanensis; and the trees in Group D included Michelia figo, Acer, Spathodea campanulata, Cinnamomum camphora, Liquidambar formosana, Araucaria cunninghamii, and Pistacia chinensis. The corresponding environmental conditions in which these 15 species exist, are shown in Figure6; the thermal environment of the study area can be categorized as one of three sections, on the basis of the Taiwanese PET classification [46]: “Cool”, “Comfortable”, and “Warm to Hot”. The tree species in Groups A and B had wider crown diameters than those in the other groups, and they had superior cooling effects. Tree species in Group C had narrower crown diameters and an inferior cooling effect. The tree species in Group D had wider crown diameters than those in Group C, and a cooling effect that created the most comfortable thermal environment in the study area. The cooling characteristics of the species in Group D could provide designers with planting design references for thermal environments. In addition, the HCA procedure used in this study could serve as a useful tool for identifying appropriate species.

Table 3. Plant species in different groups.

Group * Plant Species A Ficus microcarpa. B Delonix regia, and Terminalia catappa C Juniperus chinensis, Bischoﬁa javanica Blume, and Chamaecyparis taiwanensis Michelia ﬁgo, Acer, Spathodea campanulata, Cinnamomum camphora, D Liquidambar formosana, Araucaria cunninghamii, and Pistacia chinensis * Group screened by a series of HCA procedures. Sustainability 2017, 9, 340 9 of 13 solution was the most appropriate, and the tree species for the four groups are shown in Table 3. Ficus microcarpa was the only tree species in Group A; the trees in Group B included Delonix regia and Terminalia catappa; the trees in Group C were Juniperus chinensis, Bischofia javanica Blume, and Chamaecyparis taiwanensis; and the trees in Group D included Michelia figo, Acer, Spathodea campanulata, Cinnamomum camphora, Liquidambar formosana, Araucaria cunninghamii, and Pistacia chinensis. The corresponding environmental conditions in which these 15 species exist, are shown in Figure 6; the thermal environment of the study area can be categorized as one of three sections, on the basis of the Taiwanese PET classification [46]: “Cool”, “Comfortable”, and “Warm to Hot”. The tree species in Groups A and B had wider crown diameters than those in the other groups, and they had superior cooling effects. Tree species in Group C had narrower crown diameters and an inferior cooling effect. The tree species in Group D had wider crown diameters than those in Group C, and a cooling effect that created the most comfortable thermal environment in the study area. The cooling characteristics of the species in Group D could provide designers with planting design references for thermalSustainability environments.2017, 9, 340 In addition, the HCA procedure used in this study could serve as a useful9 of 12 tool for identifying appropriate species.

Sustainability 2017Figure, 9, 340 5. Cluster dendrogram grouping tree species with similar cooling patterns. 10 of 13 Figure 5. Cluster dendrogram grouping tree species with similar cooling patterns.

Table 3. Plant species in different groups.

Group * Plant Species A Ficus microcarpa. B Delonix regia, and Terminalia catappa. C Juniperus chinensis, Bischofia javanica Blume, and Chamaecyparis taiwanensis. Michelia figo, Acer, Spathodea campanulata, Cinnamomum camphora, D Liquidambar formosana, Araucaria cunninghamii, and Pistacia chinensis. * Group screened by a series of HCA procedures.

Figure 6. Thermal comfort range of outdoor landscape plants.

4. Conclusions 4. Conclusions Taiwan experiences subtropical and tropical climates, and the thermal environments of outdoor Taiwan experiences subtropical and tropical climates, and the thermal environments of outdoor spaces in urban areas are hot, especially at noon and during the afternoon. Improved planting spaces in urban areas are hot, especially at noon and during the afternoon. Improved planting design provides trees with shade and cooling functions that improve the thermal comfort of the design provides trees with shade and cooling functions that improve the thermal comfort of the outdoor space. The cooling effects involve complicated tree characteristics such as canopy size, tree outdoor space. The cooling effects involve complicated tree characteristics such as canopy size, height, and the optical properties of leaves. However, such tree characteristics are not easy to tree height, and the optical properties of leaves. However, such tree characteristics are not easy control, in order to improve outdoor thermal comfort. Therefore, planting the appropriate tree species dominates the cooling functions. In addition, the improvements made by different species to outdoor thermal environments varied greatly in different areas. Therefore, adopting a method to determine the appropriate tree species for planting design reference is essential. This study investigated environmental and plant data, including the tree species, crown diameter, meteorological and human-biometeorological variables, and SVF, in an outdoor space in Taiwan. A series of HCA procedures were applied to identify the appropriate trees species which could improve the thermal comfort of the outdoor space. The results indicated a significant correlation between SVF and the average crown diameter, and they demonstrated that the SVF value decreased with the increasing average crown diameter of trees in the study area. When the average crown diameter was less than 1.5 m, the SVF value increased dramatically, and the shaded area substantially decreased. The trees with average crown diameters narrower than 1.5 m tended to have high SVF and a low canopy, and then the low irradiance reduction of the plant canopy would result in a high land surface temperature and hot environment. When the average crown diameter of the trees was wider than 1.5 m, the cooling effect was dominated by the other tree characteristics. This result indicated that planting appropriate tree species is the way to effectively improve outdoor thermal comfort in tropical climate areas. Fifteen species were identified by the HCA procedures with different cooling effects, and they were divided into four categories. The plants Ficus microcarpa, Delonix regia, and Terminalia catappa, had the most effective cooling effects. By contrast, Juniperus chinensis, Bischofia javanica Blume, and Chamaecyparis taiwanensis, had the least effective cooling effects. Other tree species had relatively large crown diameters and appropriate characteristics for cooling effects. Therefore, the HCA procedures are a useful tool for identifying the right tree species and improving the thermal comfort of open spaces. These results can serve as a reference for outdoor landscape planting designers in urban areas of the tropics. Planting the right trees in cities can also adapt those cities to climate change. Sustainability 2017, 9, 340 10 of 12 to control, in order to improve outdoor thermal comfort. Therefore, planting the appropriate tree species dominates the cooling functions. In addition, the improvements made by different species to outdoor thermal environments varied greatly in different areas. Therefore, adopting a method to determine the appropriate tree species for planting design reference is essential. This study investigated environmental and plant data, including the tree species, crown diameter, meteorological and human-biometeorological variables, and SVF, in an outdoor space in Taiwan. A series of HCA procedures were applied to identify the appropriate trees species which could improve the thermal comfort of the outdoor space. The results indicated a significant correlation between SVF and the average crown diameter, and they demonstrated that the SVF value decreased with the increasing average crown diameter of trees in the study area. When the average crown diameter was less than 1.5 m, the SVF value increased dramatically, and the shaded area substantially decreased. The trees with average crown diameters narrower than 1.5 m tended to have high SVF and a low canopy, and then the low irradiance reduction of the plant canopy would result in a high land surface temperature and hot environment. When the average crown diameter of the trees was wider than 1.5 m, the cooling effect was dominated by the other tree characteristics. This result indicated that planting appropriate tree species is the way to effectively improve outdoor thermal comfort in tropical climate areas. Fifteen species were identified by the HCA procedures with different cooling effects, and they were divided into four categories. The plants Ficus microcarpa, Delonix regia, and Terminalia catappa, had the most effective cooling effects. By contrast, Juniperus chinensis, Bischofia javanica Blume, and Chamaecyparis taiwanensis, had the least effective cooling effects. Other tree species had relatively large crown diameters and appropriate characteristics for cooling effects. Therefore, the HCA procedures are a useful tool for identifying the right tree species and improving the thermal comfort of open spaces. These results can serve as a reference for outdoor landscape planting designers in urban areas of the tropics. Planting the right trees in cities can also adapt those cities to climate change.

Acknowledgments: The authors would like to thank the National Science Council of Taiwan for the partial financial support of this research under Project NSC103-2221-E-507-005 and NSC104-2221-E-005-077. Chris Law is appreciated for his editorial assistance. Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Yu-Hao Lin and Kang-Ting Tsai conceived and designed the experiments; Kang-Ting Tsai performed the experiments; Yu-Hao Lin and Kang-Ting Tsai analyzed the data; Yu-Hao Lin and Kang-Ting Tsai contributed reagents/materials/analysis tools; Yu-Hao Lin and Kang-Ting Tsai wrote the paper”. Authorship must be limited to those who have contributed substantially to the work reported. Conflicts of Interest: The authors declare no conflict of interest.

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