Temperature Reduction in Urban Surface Materials through Tree Shading Depends on Surface Type Not Tree Species

Trees play a vital role in urban cooling. The present study tested if key canopy characteristics related to tree shade could be used to predict the cooling potential across a range of urban surface materials. During the austral summer of 2018–2019, tree and canopy characteristics of 471 free-standing trees from 13 species were recorded across Greater Sydney, Australia. Stem girth and tree height, as well as leaf area index and ground-projected crown area was measured for every tree. Surface temperatures were recorded between noon (daylight saving time) and 3:00 p.m. under the canopy of each tree in the shade and in full sun to calculate the temperature differential between adjacent sunlit and shaded surfaces (∆Ts). The limited control over environmental parameters was addressed by using a large number of randomly selected trees and measurement points of surface temperatures. Analyses revealed that no systematic relationship existed among canopy characteristics and ∆Ts for any surface material. However, highly significant differences (p < 0.001) in ∆Ts existed among surface materials. The largest cooling potential of tree shade was found by shading bark mulch (∆Ts = −24.8 ◦C ± 7.1), followed by bare soil (∆Ts = −22.1 ◦C ± 5.5), bitumen (∆Ts = −20.9 ◦C ± 5.8), grass (∆Ts = −18.5 ◦C ± 4.8) and concrete pavers (∆Ts = −17.5 ◦C ± 6.0). The results indicate that surface material, but not the tree species, matters for shade cooling of common urban surfaces. Shading bark mulch, bare soil or bitumen will provide the largest reductions in surface temperature, which in turn results in effective mitigation of radiant heat. This refined understanding of the capacity of trees to reduce thermal loads in urban space can increase the effectiveness of urban cooling strategies.


Introduction
The Urban Heat Island Effect (UHIE) is one of the most prominent impacts of urbanisation and is accelerated by climate change [1,2]. The UHIE can be defined as the discernible temperature difference between urban and adjacent rural areas caused by emission of excess heat and the solar energy trapped by infrastructure [3]. Mitigation of urban heat has become a pressing issue as more than half of the world's population is currently living in cities [4], where they are exposed to increased levels of heat that, during heat wave conditions, adversely impact public health and accelerate rates of mortality [4]. People that live in urban areas highly depend on air-conditioned buildings, artificial lighting and (air conditioned) transport. The additional waste heat generated by this lifestyle further contributes to

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the study area for this project. The area has a temperate climate with dry and hot summers. A natural rainfall gradient exists along an east (coastal)/west (inland) gradient where mean annual precipitation declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 • C. Greater Sydney, especially the western part, experiences extreme heatwave conditions annually with a peak temperature of 48.9 • C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centre of Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parramatta experiences 13 days each year with air temperatures of 35 • C and above [33]. The frequency of hot and extreme heat days is increasing in Parramatta and Western Sydney more broadly [34]. Additionally, urban development has transformed rural land in the west of Greater Sydney to residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 million which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney will reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney. Due to continued urbanisation in the region, canopy cover in the western part of Greater Sydney decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed across the State of NSW [33]. temperatures compared to surface materials with high albedo. We were interested in identifying species-specific and also surface-specific trends and thus did not control tree age, canopy size or any environmental parameter, except time of day and that no meaningful rainfall had occurred in the days preceding our data collections. We countered the limited control by assessing a large number of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the study area for this project. The area has a temperate climate with dry and hot summers. A natural rainfall gradient exists along an east (coastal)/west (inland) gradient where mean annual precipitation declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Greater Sydney, especially the western part, experiences extreme heatwave conditions annually with a peak temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centre of Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parramatta experiences 13 days each year with air temperatures of 35 °C and above [33]. The frequency of hot and extreme heat days is increasing in Parramatta and Western Sydney more broadly [34]. Additionally, urban development has transformed rural land in the west of Greater Sydney to residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 million which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney will reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney. Due to continued urbanisation in the region, canopy cover in the western part of Greater Sydney decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed across the State of NSW [33].  Table 1. Figure 1 shows examples of common tree species and surface types that were examined in the study.

)evergreen and (
Forests 2020, 11, x FOR PEER REVIEW temperatures compared to surface materials with high albedo. We were interested in iden species-specific and also surface-specific trends and thus did not control tree age, canopy size environmental parameter, except time of day and that no meaningful rainfall had occurred days preceding our data collections. We countered the limited control by assessing a large n of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as th area for this project. The area has a temperate climate with dry and hot summers. A natural gradient exists along an east (coastal)/west (inland) gradient where mean annual precip declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Sydney, especially the western part, experiences extreme heatwave conditions annually with temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic ce Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parr experiences 13 days each year with air temperatures of 35 °C and above [33]. The frequency and extreme heat days is increasing in Parramatta and Western Sydney more broad Additionally, urban development has transformed rural land in the west of Greater Syd residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydn reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydne to continued urbanisation in the region, canopy cover in the western part of Greater decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed the State of NSW [33].  Table 1. Figure 1 shows examples of co tree species and surface types that were examined in the study.

Tree Morphological Measurements
)deciduous species that are widely planted in parks and streets across Greater Sydney, namely: • , Forests 2020, 11, x FOR PEER REVIEW 3 of 15 temperatures compared to surface materials with high albedo. We were interested in identifying species-specific and also surface-specific trends and thus did not control tree age, canopy size or any environmental parameter, except time of day and that no meaningful rainfall had occurred in the days preceding our data collections. We countered the limited control by assessing a large number of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the study area for this project. The area has a temperate climate with dry and hot summers. A natural rainfall gradient exists along an east (coastal)/west (inland) gradient where mean annual precipitation declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Greater Sydney, especially the western part, experiences extreme heatwave conditions annually with a peak temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centre of Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parramatta experiences 13 days each year with air temperatures of 35 °C and above [33]. The frequency of hot and extreme heat days is increasing in Parramatta and Western Sydney more broadly [34]. Additionally, urban development has transformed rural land in the west of Greater Sydney to residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 million which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney will reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney. Due to continued urbanisation in the region, canopy cover in the western part of Greater Sydney decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed across the State of NSW [33].  Table 1. Figure 1 shows examples of common tree species and surface types that were examined in the study.

Tree Morphological Measurements
Australian pine (Casuarina equisetifolia L.), • , Forests 2020, 11, x FOR PEER REVIEW temperatures compared to surface materials with h species-specific and also surface-specific trends and environmental parameter, except time of day and t days preceding our data collections. We countered t of trees and randomizing data collection points for s

Study Area
Greater Sydney in the state of New South Wal area for this project. The area has a temperate climat gradient exists along an east (coastal)/west (inlan declines from 1300 to 880 mm [30]. Mean annual air t Sydney, especially the western part, experiences extr temperature of 48.9 °C in January 2020 [31]. Moreove Greater Sydney, has been identified to have the high experiences 13 days each year with air temperature and extreme heat days is increasing in Parrama Additionally, urban development has transformed residential suburbs [35]. The estimated population of which is 10% higher compared to 2011 [36]. It is expe reach 2.9 million by 2036, representing more than 50 to continued urbanisation in the region, canopy decreased by 0.83% from 2009 to 2016, a rate more t the State of NSW [33]. temperatures compared to surface materials with high albedo. We were interested in identifying species-specific and also surface-specific trends and thus did not control tree age, canopy size or any environmental parameter, except time of day and that no meaningful rainfall had occurred in the days preceding our data collections. We countered the limited control by assessing a large number of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the study area for this project. The area has a temperate climate with dry and hot summers. A natural rainfall gradient exists along an east (coastal)/west (inland) gradient where mean annual precipitation declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Greater Sydney, especially the western part, experiences extreme heatwave conditions annually with a peak temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centre of Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parramatta experiences 13 days each year with air temperatures of 35 °C and above [33]. The frequency of hot and extreme heat days is increasing in Parramatta and Western Sydney more broadly [34]. Additionally, urban development has transformed rural land in the west of Greater Sydney to residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 million which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney will reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney. Due to continued urbanisation in the region, canopy cover in the western part of Greater Sydney decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed across the State of NSW [33].  Table 1. Figure 1 shows examples of common tree species and surface types that were examined in the study.

Tree Morphological Measurements
Chinese banyan (Ficus macrocarpa L.f.), • , Forests 2020, 11, x FOR PEER REVIEW temperatures compared to surface materials with high albedo. We species-specific and also surface-specific trends and thus did not cont environmental parameter, except time of day and that no meaningf days preceding our data collections. We countered the limited contr of trees and randomizing data collection points for surface temperatu

Study Area
Greater Sydney in the state of New South Wales (NSW), Austr area for this project. The area has a temperate climate with dry and h gradient exists along an east (coastal)/west (inland) gradient whe declines from 1300 to 880 mm [30]. Mean annual air temperature of th Sydney, especially the western part, experiences extreme heatwave c temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a Greater Sydney, has been identified to have the highest UHIE in NSW experiences 13 days each year with air temperatures of 35 °C and ab and extreme heat days is increasing in Parramatta and Wester Additionally, urban development has transformed rural land in t residential suburbs [35]. The estimated population of this part of Grea which is 10% higher compared to 2011 [36]. It is expected that the pop reach 2.9 million by 2036, representing more than 50% of the total po to continued urbanisation in the region, canopy cover in the we decreased by 0.83% from 2009 to 2016, a rate more than twice as hig the State of NSW [33].  Table 1. Figur tree species and surface types that were examined in the study. mperatures compared to surface materials with high albedo. We were interested in identifying ecies-specific and also surface-specific trends and thus did not control tree age, canopy size or any vironmental parameter, except time of day and that no meaningful rainfall had occurred in the ys preceding our data collections. We countered the limited control by assessing a large number trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the study ea for this project. The area has a temperate climate with dry and hot summers. A natural rainfall adient exists along an east (coastal)/west (inland) gradient where mean annual precipitation clines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Greater dney, especially the western part, experiences extreme heatwave conditions annually with a peak mperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centre of reater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parramatta periences 13 days each year with air temperatures of 35 °C and above [33]. The frequency of hot d extreme heat days is increasing in Parramatta and Western Sydney more broadly [34]. dditionally, urban development has transformed rural land in the west of Greater Sydney to sidential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 million hich is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney will ach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney. Due continued urbanisation in the region, canopy cover in the western part of Greater Sydney creased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed across e State of NSW [33].  Table 1. Figure 1 shows examples of common ee species and surface types that were examined in the study.

Tree Morphological Measurements
flowering pear (Pyrus calleryana Decne.), • , Forests 2020, 11, x FOR PEER REVIEW temperatures compared to surface materials with high albedo. We were interested in identi species-specific and also surface-specific trends and thus did not control tree age, canopy size o environmental parameter, except time of day and that no meaningful rainfall had occurred i days preceding our data collections. We countered the limited control by assessing a large nu of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the s area for this project. The area has a temperate climate with dry and hot summers. A natural ra gradient exists along an east (coastal)/west (inland) gradient where mean annual precipit declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Gr Sydney, especially the western part, experiences extreme heatwave conditions annually with a temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic cen Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parram experiences 13 days each year with air temperatures of 35 °C and above [33]. The frequency o and extreme heat days is increasing in Parramatta and Western Sydney more broadly Additionally, urban development has transformed rural land in the west of Greater Sydne residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 m which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney to continued urbanisation in the region, canopy cover in the western part of Greater Sy decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed a the State of NSW [33].  Table 1. Figure 1 shows examples of com tree species and surface types that were examined in the study. jacaranda (Jacaranda mimosifolia D.Don.), • , Forests 2020, 11, x FOR PEER REVIEW temperatures compared to surface materials with high albedo. We were inte species-specific and also surface-specific trends and thus did not control tree ag environmental parameter, except time of day and that no meaningful rainfal days preceding our data collections. We countered the limited control by asse of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was area for this project. The area has a temperate climate with dry and hot summ gradient exists along an east (coastal)/west (inland) gradient where mean declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is Sydney, especially the western part, experiences extreme heatwave conditions temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On experiences 13 days each year with air temperatures of 35 °C and above [33]. and extreme heat days is increasing in Parramatta and Western Sydney Additionally, urban development has transformed rural land in the west o residential suburbs [35]. The estimated population of this part of Greater Sydne which is 10% higher compared to 2011 [36]. It is expected that the population of reach 2.9 million by 2036, representing more than 50% of the total population o to continued urbanisation in the region, canopy cover in the western par decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what the State of NSW [33].  Table 1. Figure 1 shows tree species and surface types that were examined in the study. temperatures compared to surface materials with high albedo. We were interested in identifying species-specific and also surface-specific trends and thus did not control tree age, canopy size or any environmental parameter, except time of day and that no meaningful rainfall had occurred in the days preceding our data collections. We countered the limited control by assessing a large number of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the study area for this project. The area has a temperate climate with dry and hot summers. A natural rainfall gradient exists along an east (coastal)/west (inland) gradient where mean annual precipitation declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Greater Sydney, especially the western part, experiences extreme heatwave conditions annually with a peak temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centre of Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parramatta experiences 13 days each year with air temperatures of 35 °C and above [33]. The frequency of hot and extreme heat days is increasing in Parramatta and Western Sydney more broadly [34]. Additionally, urban development has transformed rural land in the west of Greater Sydney to residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 million which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney will reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney. Due to continued urbanisation in the region, canopy cover in the western part of Greater Sydney decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed across the State of NSW [33].  Table 1. Figure 1 shows examples of common tree species and surface types that were examined in the study. temperatures compared to surface materials with high albedo. We were species-specific and also surface-specific trends and thus did not control tr environmental parameter, except time of day and that no meaningful rai days preceding our data collections. We countered the limited control by of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, area for this project. The area has a temperate climate with dry and hot su gradient exists along an east (coastal)/west (inland) gradient where m declines from 1300 to 880 mm [30]. Mean annual air temperature of the are Sydney, especially the western part, experiences extreme heatwave condit temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city i Greater Sydney, has been identified to have the highest UHIE in NSW [32 experiences 13 days each year with air temperatures of 35 °C and above [ and extreme heat days is increasing in Parramatta and Western Syd Additionally, urban development has transformed rural land in the w residential suburbs [35]. The estimated population of this part of Greater Sy which is 10% higher compared to 2011 [36]. It is expected that the populatio reach 2.9 million by 2036, representing more than 50% of the total populati to continued urbanisation in the region, canopy cover in the western decreased by 0.83% from 2009 to 2016, a rate more than twice as high as w the State of NSW [33].  Table 1. Figure 1 sh tree species and surface types that were examined in the study. temperatures compared to surface materials with high albedo. We were interested in identify species-specific and also surface-specific trends and thus did not control tree age, canopy size or environmental parameter, except time of day and that no meaningful rainfall had occurred in days preceding our data collections. We countered the limited control by assessing a large num of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, was selected as the st area for this project. The area has a temperate climate with dry and hot summers. A natural rain gradient exists along an east (coastal)/west (inland) gradient where mean annual precipita declines from 1300 to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Gre Sydney, especially the western part, experiences extreme heatwave conditions annually with a p temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centr Greater Sydney, has been identified to have the highest UHIE in NSW [32]. On average, Parram experiences 13 days each year with air temperatures of 35 °C and above [33]. The frequency of and extreme heat days is increasing in Parramatta and Western Sydney more broadly [ Additionally, urban development has transformed rural land in the west of Greater Sydney residential suburbs [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 mil which is 10% higher compared to 2011 [36]. It is expected that the population of Western Sydney reach 2.9 million by 2036, representing more than 50% of the total population of Grater Sydney. D to continued urbanisation in the region, canopy cover in the western part of Greater Syd decreased by 0.83% from 2009 to 2016, a rate more than twice as high as what was observed ac the State of NSW [33].  Table 1. Figure 1 shows examples of comm tree species and surface types that were examined in the study. temperatures compared to surface materials with high albedo. We were i species-specific and also surface-specific trends and thus did not control tree environmental parameter, except time of day and that no meaningful rain days preceding our data collections. We countered the limited control by a of trees and randomizing data collection points for surface temperatures.

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia, w area for this project. The area has a temperate climate with dry and hot sum gradient exists along an east (coastal)/west (inland) gradient where mea declines from 1300 to 880 mm [30]. Mean annual air temperature of the area Sydney, especially the western part, experiences extreme heatwave conditio temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city in Greater Sydney, has been identified to have the highest UHIE in NSW [32]. experiences 13 days each year with air temperatures of 35 °C and above [3 and extreme heat days is increasing in Parramatta and Western Sydn Additionally, urban development has transformed rural land in the wes residential suburbs [35]. The estimated population of this part of Greater Syd which is 10% higher compared to 2011 [36]. It is expected that the population reach 2.9 million by 2036, representing more than 50% of the total populatio to continued urbanisation in the region, canopy cover in the western p decreased by 0.83% from 2009 to 2016, a rate more than twice as high as wh the State of NSW [33].  Table 1. Figure 1 sho tree species and surface types that were examined in the study. temperatures compared to surface materials with high albedo. We we species-specific and also surface-specific trends and thus did not control environmental parameter, except time of day and that no meaningful r days preceding our data collections. We countered the limited control b of trees and randomizing data collection points for surface temperatures

Study Area
Greater Sydney in the state of New South Wales (NSW), Australia area for this project. The area has a temperate climate with dry and hot s gradient exists along an east (coastal)/west (inland) gradient where declines from 1300 to 880 mm [30]. Mean annual air temperature of the a Sydney, especially the western part, experiences extreme heatwave cond temperature of 48.9 °C in January 2020 [31]. Moreover, Parramatta, a city Greater Sydney, has been identified to have the highest UHIE in NSW [3 experiences 13 days each year with air temperatures of 35 °C and above and extreme heat days is increasing in Parramatta and Western S Additionally, urban development has transformed rural land in the residential suburbs [35]. The estimated population of this part of Greater S which is 10% higher compared to 2011 [36]. It is expected that the popula reach 2.9 million by 2036, representing more than 50% of the total popula to continued urbanisation in the region, canopy cover in the wester decreased by 0.83% from 2009 to 2016, a rate more than twice as high as the State of NSW [33].  Table 1. Figure 1 s tree species and surface types that were examined in the study. ared to surface materials with high albedo. We were interested in identifying also surface-specific trends and thus did not control tree age, canopy size or any meter, except time of day and that no meaningful rainfall had occurred in the data collections. We countered the limited control by assessing a large number izing data collection points for surface temperatures.

Tree Morphological Measurements
ethods y in the state of New South Wales (NSW), Australia, was selected as the study . The area has a temperate climate with dry and hot summers. A natural rainfall ng an east (coastal)/west (inland) gradient where mean annual precipitation to 880 mm [30]. Mean annual air temperature of the area is around 18 °C. Greater the western part, experiences extreme heatwave conditions annually with a peak °C in January 2020 [31]. Moreover, Parramatta, a city in the geographic centre of s been identified to have the highest UHIE in NSW [32]. On average, Parramatta s each year with air temperatures of 35 °C and above [33]. The frequency of hot days is increasing in Parramatta and Western Sydney more broadly [34]. n development has transformed rural land in the west of Greater Sydney to [35]. The estimated population of this part of Greater Sydney in 2018 is 2.2 million r compared to 2011 [36]. It is expected that the population of Western Sydney will 2036, representing more than 50% of the total population of Grater Sydney. Due nisation in the region, canopy cover in the western part of Greater Sydney from 2009 to 2016, a rate more than twice as high as what was observed across 3].  Table 1. Figure 1 shows examples of common face types that were examined in the study.
Sydney blue gum (Eucalyptus saligna Sm.). Physical characteristics of the studied trees are shown in Table 1. Figure 1 shows examples of common tree species and surface types that were examined in the study.
Stem diameter at breast height (DBH) was measured for each individual tree using a diameter tape. Here we used DBH as rough indicator of tree age. A clinometer (Suunto Tandem 360PC/360RDG, Suunto, Vantaa, Finland) was used to measure tree and crown height. Crown radii (r) in six sub cardinal directions were measured using an optical laser (DISTO D810, Leica Geosystems, St Gallen, Switzerland). For this purpose, a perpendicular was dropped at the edge of the canopy from where the laser was pointed to the centre of the stem at parallel height to the ground surface. Half of the DBH (i.e., the stem radius) was added to each measurement to represent the distance from the crown edge to the centre of the stem. To estimate A C, we used the following modified equation [37]: where r i and r i+1 are adjacent radii. LAI was measured using a digital canopy analyser (CI-110 Plant Canopy Imager, CID Bio Science Inc., Camas, WA, USA). Two independent measurements were taken at randomly selected positions under each tree canopy. All images were collected under appropriate light conditions. During post-processing of the images, the Otsu method was applied for image thresholding and gap fraction analysis. This method was selected, because of its robustness in image segmentation, using a least-square method based on a grey-scale histogram [38]. Zenith and azimuth divisions of canopy images were selected manually for each image to ensure an accurate calculation of LAI.   Stem diameter at breast height (DBH) was measured for each individual tree using a diameter tape. Here we used DBH as rough indicator of tree age. A clinometer (Suunto Tandem 360PC/360RDG, Suunto, Vantaa, Finland) was used to measure tree and crown height. Crown radii (r) in six sub cardinal directions were measured using an optical laser (DISTO D810, Leica

Surface and Globe Temperature Measurements
Surface and black globe temperatures were recorded between 12:00 and 15:00 h (local daylight-saving time) under each tree canopy and in full sun adjacent to each tree. Black globe temperature is an indirect measurement of human thermal comfort obtained with a thermometer installed inside a hollow copper sphere painted in matte black [39]. It is a composite measurement that incorporates air temperature, relative humidity, direct sunlight, wind speed and radiant heat.
A tripod-mounted weather station (Kestrel 5400, Kestrel Meters, Boothwyn, PA, USA) was used to record black globe temperature at 30-s intervals. The weather station was positioned 1 m above the ground. The air temperature sensor of the weather station was shielded from direct solar radiation and was well aspirated. The weather station was first positioned under the tree for 15 min before moving it into the sun adjacent to the tree for another 15 min. Data for the last 3 min of each measurement interval were averaged, to ensure only data after the weather station had adjusted to ambient conditions were used. The resulting six measurements were averages. These measurements were not independent, thus one average temperature per time interval was used to calculate means among surface types. We note that black globe temperatures were only recorded for each location and light condition, not for specific surface types at individual locations. The reason for this approach was the limited ability to exclude microclimatic 'noise' from adjacent surface types, especially in the sun.
An infrared (IR) camera (FLIR C3, FLIR Systems Inc., Wilsonville, OR, USA) was used to record surface temperature at five random locations under the canopy and in full sun adjacent to each tree. The camera has a fixed focus, field of view is 41 × 31 • , image size is 640 × 480 pixel and thermal sensitivity is 0.1 • C. The IR camera was held 1 m above the surface when taking the image perpendicular to the ground. The area covered by the image was approximately 56 × 77 cm (4312 cm 2 ). The temperatures of different surface types (grass, bark mulch, bare soil, concrete pavers and bitumen) were assessed in both light conditions. Care was taken that no shade was introduced to the area imaged in sunlight or under tree canopies on readily shaded surfaces. We noticed that two or more different types of surface could be found underneath tree canopies, and consequently, the number of surface temperature assessments exceeded the number of trees in our study. We measured surface temperatures on 414 locations covered by grass, 135 covered by bitumen, 69 covered by bark mulch, 62 covered by pavers and 28 had bare soil.
FLIR Tools+ software was used to extract five random point measurements from each image for a single, uniform surface type to calculate a representative surface temperature for each image. Similar to Black Globe Measurements, these measurements were also not independent, and consequently, one average temperature per image was used to calculate means among surface types. Measurements of air temperature were used to normalize surface and black globe temperatures. Surface and black globe temperatures differentials (∆T S and ∆T G ) were calculated by subtracting temperatures measured in the shade from those measured in the sun. To represent the effect of shading as 'cooling effect', all delta values are presented with a negative prefix.
To document the warm summer conditions during which the black globe and surface temperatures were collected, we provide information about mean, minimum and maximum ambient air temperatures measured in the sunlight (T ASL ) and in the shade of trees (T AS ) and their differential (∆T A ) as Supplementary Materials. Table S1 provides these temperatures according to tree species while  Table S2 provides this information according to the five surface types we investigated (i.e., bare soil, bark mulch, bitumen, grass, and concrete pavers).

Data Analysis
All statistical tests were done using JMP software (JMP 14 SW, SAS Institute Inc, Cary, NC, USA). All data were first tested for normal distribution. Mean values were calculated for A C and LAI for each tree species. Surface and globe temperature data were normalized to account for day-to-day variation in air temperatures. Surface temperature normalization was done for each surface type separately by using the following equation: where T' is the normalized temperature, T o is the observed temperature, T min is the minimum recorded temperature and T max is the maximum recorded temperature. Linear regression analysis was performed between tree physical traits and all the temperature measurements. Generalized Linear Models (GLM) were used to determine relationships among A C , LAI, surface, globe temperature and surface types.

Relationships of Physical Traits
Of the 471 urban trees that we sampled, Casuarina equisetifolia accounted for of the most trees of a single species (n = 58) followed by Lagerstroemia (n = 55) and Lophostemon confertus (n = 49), while Liquidambar styraciflua had the lowest representation (n = 13) ( Table 1). DBH of the sampled tree population ranged from 0.03 m (Lagerstroemia) to 1.6 m (Melaleuca quinquenervia) and tree height varied from 3.8 m (Casuarina equisetifolia) to 35.3 m (Eucalyptus saligna) ( Table 1).
Across all species, tree height and DBH followed a clear positive trajectory (R 2 = 0.68, p < 0.001), as did A C (R 2 = 0.75, p < 0.001) (Figure 2). At the individual tree level, there were no significant relationships between LAI and DBH or A C . Tree species with dense canopies and medium height (e.g., Ficus macrocarpa, Lagerstroemia) had a smaller A C and higher LAI compared to tall, species with more open canopies (e.g., Casuarina equisetifolia, Corymbia citriodora) ( Table 1).

Influence of Urban Trees on Different Types of Temperature
No significant effect of AC or LAI on the shaded surface temperature (TSS) or surface temperature differential (ΔTS) (p > 0.05) was found (Figure 3). Figure 4 shows the distribution of ΔTS and LAI for each species, further demonstrating that there was no systematic relationship between LAI and ΔTS among the investigated tree species. Species-specific measurements for mean, minimum, maximum and the differential of surface temperatures measured in the shade and sun are provided in Table S3.

Surface Types
Mean T SS ± SD A Tukey HSD test revealed that T SL was significantly different between all surface types except between bark mulch and bare soil, bitumen and bare soil and pavers and grass ( Figure 5). Similarly, T SS was significantly different between grass and bitumen, grass and bark mulch, grass and pavers, and also between grass and bare soil. Further, ∆T S was significantly different among all the surface types except bark mulch and bare soil, bitumen and bare soil and grass and pavers (Table 3).  A Tukey HSD test revealed that TSL was significantly different between all surface types except between bark mulch and bare soil, bitumen and bare soil and pavers and grass ( Figure 5). Similarly, TSS was significantly different between grass and bitumen, grass and bark mulch, grass and pavers, and also between grass and bare soil. Further, ΔTS was significantly different among all the surface types except bark mulch and bare soil, bitumen and bare soil and grass and pavers (Table 3).  Table 3. Tukey's HSD pairwise comparison of the surface temperature differential (ΔTS) and black globe temperature differential (ΔTG) observed among the five surface types: concrete pavers, grass, bitumen, bark mulch and bare soil.  Shaded globe temperature (GT S ) and globe temperature differential (∆T G ) did not show any significant relationship with the tree morphological parameters (p > 0.05). Data for species-specific globe temperature measurements collected in the shade and sun, as well as ∆T G are provided in Table S4. Absolute GT S ranged from 26.3 to 44.5 • C and bark mulch had the highest mean GT S (37.9 ± 2.8 • C) followed by bitumen (36.9 ± 2.2 • C), pavers (36.1 ± 2.4 • C), bare soil (34.4 ± 2.2 • C) and grass (33.3 ± 3.2 • C) ( Table 4). Absolute globe temperature in the sun light (GT SL ) ranged from 28.4 to 54.1 • C and, consistently with rankings found in the shade, bark mulch had the highest mean GT SL (48.8 ± 2.8 • C) followed by bitumen (46.1 ± 2.2 • C), bare soil (41.5 ± 2.2 • C), pavers (40.4 ± 2.4 • C) and grass (36.5 ± 3.2 • C). Bark mulch showed the largest ∆T G (−10.9 ± 0.5 • C) and grass showed the lowest ∆T G (−3.2 ± 0.2 • C). The effect of surface types on GT S , GT SL and ∆T G was highly significant (p < 0.001). Tukey's HSD test showed that GT S was significantly different among all the surface types except bitumen-bare soil. Similarly, GT SL was significantly different between all surface types except bitumen-pavers, pavers-bare soil and bitumen-bare soil. Further, ∆T G significantly differed between all surface types (p < 0.001) (see Table 3). Table 3. Tukey's HSD pairwise comparison of the surface temperature differential (∆T S ) and black globe temperature differential (∆T G ) observed among the five surface types: concrete pavers, grass, bitumen, bark mulch and bare soil.  Table 4. Mean, minimum and maximum shaded globe temperature (GT S ), sunlit globe temperature (GT SL ), and globe temperature differential (∆T G ) recorded over bare soil, grass, bark mulch, concrete pavers and bitumen.

Surface Types
Mean GT S ± SD

Influence of Urban Trees on Surface and Globe Temperature
Tree shade reduced the surface temperatures by 20 • C on average, and species like Lophostemon confertus, Pyrus calleryana and Liquidambar styraciflua provided the largest surface temperature reduction of around 40 • C. Although this can be due to having a comparatively larger LAI, the correlation analysis between A C , LAI and the ∆T S did not show a strong, significant relationship. For example, Waterhousea floribunda had the second largest LAI among the sampled tree species, however, it had the lowest average ∆T S . Similar results were found in the globe temperature measurements. There is a globe temperature reduction up to 13 • C from the sun to the tree shade. Nevertheless, results do not support that LAI or the A C have systematically influenced this temperature reduction. Despite having both the largest LAI, Ficus macrocarpa accounted for the highest globe temperatures.
Our findings are different from the findings of other studies. For example, the study conducted by Hardin and colleagues [40] in Terre Haute, Indiana, USA, on the effect of urban leaf area on summertime urban surface temperatures found that leaf area index and surface temperature were negatively correlated. In this study, LAI accounted for 62% of variation in surface temperature. Moreover, a study by Yusof and colleagues [24] suggested that surface temperature reduction is positively correlated with LAI. They also found that tree shade reduces the surface temperature by an average of 12 • C. A study carried out in the Suzhou Industrial Park, Shanghai, China [41] concluded that the cooling effect of green areas were positively correlated with LAI. Similar findings were presented by Napoli and colleagues [42] where they found a strong relationship between ∆T S on asphalt and LAI and a weaker relationship between ∆T S on grass and LAI. Studies have found that the amount of solar radiation blocked by tree shade is strongly related to size of the crown and height of the tree [43,44], and thereby improves surface cooling. In this study, we were unable to build such a relationship with tree height or A C . There is no doubt that tree shade reduces the amount of heat absorbed by the surface underneath during the daytime; however, our study provided evidence that microclimate underneath the trees and the temperature of surface material greatly depends on the type of surface material.

Effect of Surface Types on Surface and Globe Temperature
The results showed that grass had the lowest recorded surface temperature and globe temperature both in shade and sun. This can be due to the combined effects of evapotranspiration and albedo of this surface material. Albedo can be defined as the fraction of the incident sunlight that the surface reflects [45,46]. Grass has the highest albedo (0.3-0.25) [47] of all the surface types investigated here, thus it absorbs less and reflects more radiation than the other surface types. However, it does not store incoming solar radiation and emits this energy as sensible heat like the other surface materials. The energy absorbed by grass is used to fuel the biochemical processes of photosynthesis and latent heat flux cooling, which reduces air temperature. On the contrary, bark mulch had the highest T SS T SL and GT S . It has a very low albedo 0.05 [48] compared to the other surfaces (bare soil (0.26-0.16) [49], bitumen (0.2-0.05) and concrete pavers (0.13-0.1) [47]) and thus increases the surface temperature by absorbing more radiation. However, it is worth noting that there are other factors, such as the thermal emissivity and thermal mass of surface materials, which influence the surface temperatures [50] and the extent to which surface materials contribute to the UHIE. Further experimentation is needed to evaluate individual effects of these parameters on surface temperature variations. Largest surface cooling from tree shade was observed for bark mulch followed by bare soil, bitumen, grass and pavers. The results indicated that the surface material had a strong and significant influence on surface temperature. This finding is backed-up by the globe temperature recorded above each surface material; the highest GT SL was recorded over bark mulch whereas the lowest was recorded over grass. Black globe temperature combines the effects of air movement, dry-bulb temperature, wind speed and radiant heat received from the surfaces [51]. The novel finding of this study advances our understanding of cooling provided by trees. Planting trees with wider canopies and larger LAI does not directly support urban cooling through surface temperature reduction. Rather, the surface material has a larger influence in reducing thermal loads in urban space. This finding should be integrated in urban planning and cooling strategies to mitigate UHIE.

Limitations of the Study
The majority of the sampled trees were well-established trees with a DBH of 10-50 cm. This is a clear indication that the urban landscape of Western Sydney does primarily accommodate younger mature trees and that older mature trees with wide canopies are lower in number. Research has demonstrated that the shade profile of a tree depends on the maturity, overlapping canopies and canopy extents [52][53][54]. The major proportion of our study was comprised of young mature trees with smaller and separate canopies which can influence the amount of solar radiation reaching the ground. We did not include measurements of soil moisture, which potentially influenced our surface temperature measurements of bare soil, grass and bark mulch. However, surface temperatures were only recorded during midday on a hot sunny day following one or two days of zero precipitation. Only during 2 out of 13 days did we experience a light shower (<10 mm total daily precipitation) two days prior to data collection. We thus expect that any influence of soil moisture on surface temperatures would be marginal.

Conclusions
This study gave a novel insight into the relationship between surface temperature and canopy characteristics. It showed that canopy characteristics such as LAI, shaded area and crown projected area do not have a strong influence on the temperature loads on surfaces. Although these canopy characteristics varied among the tested species, they were unrelated to surface temperature reductions in shade. Nevertheless, we found that surface types play a significant role in absorbing and reflecting radiation, thereby controlling surface temperatures and cooling arising from tree shade. Evapotranspiration will have an effect on surface cooling; however, further studies are needed to determine the cumulative effects of surface material and tree evapotranspiration on surface cooling. This novel finding can be integrated in urban cooling and urban planning strategies. Landscape planners and architects should consider the choice of surface materials in urban settings as a higher priority than tree species for shade quality alone when implementing urban greening strategies to mitigate urban heat.
Supplementary Materials: The following are available online at http://www.mdpi.com/1999-4907/11/11/1141/s1. Table S1: Tree species with their mean, minimum and maximum air temperature in the shade (TAS), in the sun (TASL) and the differential between these (∆TA). Table S2: Mean, minimum and maximum TAS, TASL and ∆TA recorded in bare soil, grass, bark mulch, pavers and bitumen. Table S3: Tree species with their mean, minimum and maximum shaded surface temperature (TSS), sunlit surface temperature (TSL) and surface temperature differential (∆TS). Table S4: Tree species with their mean, minimum and maximum globe temperature in the shade (GTS), in the sunlight (GTSL) and the differential between the them (∆TG).
Author Contributions: K.T.U.N. collected and analysed the data and wrote the draft manuscript. P.S. designed the study and assisted with analyses of data and writing the manuscript. T.M.G. contributed to data analyses and writing of the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.