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ORIGINAL PAPER
Temperature and human thermal comfort effects of street trees
across three contrasting street canyon environments
Andrew M. Coutts &Emma C. White &Nigel J. Tapper &
Jason Beringer &Stephen J. Livesley
Received: 13 August 2014 /Accepted: 3 February 2015
#Springer-Verlag Wien 2015
Abstract Urban street trees provide many environmental, so-
cial, and economic benefits for our cities. This research ex-
plored the role of street trees in Melbourne, Australia, in
cooling the urban microclimate and improving human thermal
comfort (HTC). Three east–west (E–W) oriented streets were
studied in two contrasting street canyon forms (deep and shal-
low) and between contrasting tree canopy covers (high and
low). These streets were instrumented with multiple microcli-
mate monitoring stations to continuously measure air temper-
ature, humidity, solar radiation, wind speed and mean radiant
temperature so as to calculate the Universal Thermal Climate
Index (UTCI) from May 2011 to June 2013, focusing on sum-
mertime conditions and heat events. Street trees supported
average daytime cooling during heat events in the shallow
canyonbyaround0.2to0.6°Candupto0.9°Cduring
mid-morning (9:00–10:00). Maximum daytime cooling
reached 1.5 °C in the shallow canyon. The influence of street
tree canopies in the deep canyon was masked by the shading
effect of the tall buildings. Trees were very effective at reduc-
ing daytime UTCI in summer largely through a reduction in
mean radiant temperature from shade, lowering thermal stress
from very strong (UTCI >38 °C) down to strong (UTCI>
32 °C). The influence of street trees on canyon air temperature
and HTC was highly localized and variable, depending on tree
cover, geometry, and prevailing meteorological conditions.
The cooling benefit of street tree canopies increases as street
canyon geometry shallows and broadens. This should be rec-
ognized in the strategic placement, density of planting, and
species selection of street trees.
1 Introduction
Street trees are an important part of the urban landscape, with
the potential to improve amenity, provide stormwater quantity
and quality benefits, and reduce building energy use
(McPherson et al. 2011). Street trees can also help reduce high
urban temperature through key vegetative processes of shad-
ing and transpiration (Bowler et al. 2010). Several studies
suggest that an increase in vegetation can help mitigate the
urban heat island (UHI) (Loughner et al. 2012), while others
promote vegetation as a way of modifying urban microcli-
mates and human thermal comfort (HTC) (Shashua-Bar
et al. 2010b). However urban street trees face significant chal-
lenges including development and infrastructure pressures,
maintenance issues, and poor water availability at times that
can compromise their ability to mitigate urban heat and im-
prove HTC. In Melbourne, and across southeast Australia, an
extended drought period was experienced from 1997 to 2009
leading to the implementation of water restrictions that result-
ed in detrimental effects on Melbourne’s tree health (May
et al. 2013). The combination of drought, water restrictions,
and rapid export of stormwater away from the urban environ-
ment leaves the urban landscape water-starved and can con-
strain tree transpiration (Coutts et al. 2013). Urban climatic
conditions of higher temperature and vapor pressure deficit
(Peters et al. 2010) resulting from urban development
A. M. Coutts (*):E. C. White :N. J. Tapper
School of Earth, Atmosphere and Environment, Monash University,
Building 28, Wellington Rd, Clayton, VIC 3800, Australia
e-mail: Andrew.Coutts@monash.edu
A. M. Coutts :N. J. Tapper
CRC for Water Sensitive Cities, Clayton, VIC 3800, Australia
J. Beringer
School of Earth and Environment, The University of Western
Australia, Crawley, VIC 6009, Australia
S. J. Livesley
Department of Resource Management and Geography, The
University of Melbourne, Melbourne, VIC 3121, Australia
Theor Appl Climatol
DOI 10.1007/s00704-015-1409-y
including extensive imperviousness, removal of vegetation,
increased rainfall runoff, and heat trapping by canyon geom-
etry can compound the effects of heat events. This can further
stress urban vegetation, even leading to significant defoliation
of tree canopies (May et al. 2013) which therefore compro-
mises tree shading. Maintaining healthy street trees during
such adverse conditions requires tree management and ex-
pense that need to be outweighed by the potential benefits
including urban heat mitigation and improved HTC.
Evidence from observational studies suggests that air tem-
perature under trees is lower than in the open during the day
(Bowler et al. 2010). Souch and Souch (1993)observedthat
maximum air temperature under canopies of individual trees
and clumps of trees was reduced by 0.7–1.3 °C in the early
afternoon for a variety of tree species. Lin and Lin (2010)
found that air temperature below tree canopies was 0.64–
2.52 °C lower than in the open at midday in a subtropical park.
Others have shown that variables, such as the tree species,
size, and characteristics, influence their cooling effects
(Georgi and Zafiriadis 2006). In one of the few experimental
urban tree studies, Berry et al. (2013) placed large trees adja-
cent to single room dwellings and observed a reduction in air
temperature of up to 1.0 °C between the tree canopy and
building wall. While trees in more open areas may result in
a discernable temperature reduction, the cooling potential of
trees does not solely depend on the attributes of the tree or tree
stand but also the nature of the surrounding urban environ-
ment such as surface materials, geometry, building height, and
density (Shashua-Bar et al. 2010a). Souch and Souch (1993)
found that street trees were less effective than individual trees
or clumps of trees located in open areas, only cooling by
0.1 °C in the early afternoon. Shashua-Bar and Hoffman
(2000) observed cooler air temperatures of 1.3–4.0 °C in
green areas such as streets and courtyards with trees, com-
pared with nearby reference sites. While street trees can pro-
vide a benefit during the day, numerous studies observe that
nocturnal air temperature beneath tree canopies can be slightly
higher than nearby open sites (Bowler et al. 2010) due to
reduced sky view factor (SVF) inhibiting longwave cooling
at night. Again, Souch and Souch (1993) observed that air
temperature was 0.5 °C warmer under canopies than at their
open reference station due to longwave radiation that was
emitted from the ground being absorbed and reemitted by
the canopy. Clearly, there is some variability in the magnitude
of temperature effects of trees in urban areas that will vary
with aspects of urban geometry and background climate and
thereby modulate any HTC benefits.
HTC considers not only air temperature effects on the hu-
man body but also other microclimatic factors of humidity,
wind, and shortwave and longwave radiation (Johansson
2006). Indices of HTC, such as the physiological equivalent
temperature (PET) or the Universal Thermal Comfort Index
(UTCI), more accurately describe the human thermal
sensation at the microscale. Lee et al. (2013) found that street
trees reduce mean radiant temperature, and hence PET, under
warm, sunny daytime conditions. Trees directly absorb or
reflect solar radiation and reduce adjacent ground surface
temperature through shading. Souch and Souch (1993)and
Zhang et al. (2013) observed higher humidity (vapor pressure)
below tree canopies, while ventilation (wind speed) may also
be reduced below the tree canopy (Park et al. 2012), both of
which can adversely affect HTC. At night, trees tend to
restrict longwave radiation loss and cooling, as well as
restricting ventilation beneath the canopy that can result
in slightly higher air temperature and a negative effect
on HTC under warm summer conditions
(Charalampopoulos et al. 2013; Spronken-Smith and
Oke 1998). Understanding the effects of street trees on
microclimate and how they influence HTC is important
to help provide guidance on their placement, protection
and maintenance. Demonstrating the benefits on HTC
can help to convince authorities to invest in street trees,
and quantifying improvements in HTC can aid local
authorities develop cost–benefit analysis.
This study sought to determine how effective street trees
are at reducing canyon air temperature and improving HTC
under a ‘Mediterranean’style climate of warm, dry summers.
The study also sought to determine how effective street trees
are under different street canyon geometries. Three streets
were selected within the City of Melbourne, Australia: a wide
shallow street canyon with trees; a wide shallow street canyon
without trees; and a narrow, deep street canyon with trees. The
aims of the study were (1) to quantify the effects of street trees
on the overall canyon air temperature and HTC of three streets
of varying tree cover and canyon morphology and (2) to quan-
tify the effects of street trees on air temperature and HTC
within individual streets by comparing microclimates directly
below tree canopies and microclimates in more open areas. As
such, this is a unique study in that it was capable of
observing both interstreet and intrastreet air temperature
and HTC differences because of the large number of
monitoring stations installedinthestreets(20total).
Few studies have been undertaken representing these
Mediterranean style climates, and many temperate cities
may experience such warm summer conditions and heat
events under projected climatic changes. Particular focus
is given to extreme heat events, as urban communities,
infrastructure, and services are placed under significant
pressures during such conditions. Melbourne also pre-
sents its own unique urban form and design, while an
extended drought period has left vegetation in a deplet-
ed state over recent years. Given that the City of
Melbourne maintains around 70,000 trees that have an
estimatedamenityvalueofaroundAustralian$700mil-
lion (CoM 2012), this research is valuable for the on-
going management of the urban forest.
A.M. Coutts et al.
2Methodology
To quantify the effects of street trees on both the interstreet and
intrastreet air temperature and HTC under changing geome-
tries, three contrasting streets in terms of tree canopy coverage
and canyon morphology were selected (Table 1). We chose:
(1) a deep urban canyon oriented ENE–WSW in the Central
Business District with moderate vegetation cover (CBD); (2) a
shallow urban canyon oriented E–WinnearbyEast
Melbourne that was open, with very little tree canopy cover
(Open: OPN); and (3) a shallow urban canyon oriented E–W
in East Melbourne with dense tree canopy cover (Treed: TRD)
(Fig. 1). These streets provide three contrasting street environ-
ments. Several monitoring stations were installed in each
street to capture both interstreet and intrastreet microclimate
variability: ten in the CBD (four in the west section of the
CBD and six in the east section of CBD), five in OPN, and
seven in TRD (Fig. 2) to achieve a representative cover. In
TRD and CBD, some stations were placed directly below tree
canopies, while others were in the open. The stations provide
for the observation of within canyon microclimatic differ-
ences, particularly due to tree canopies. The East Melbourne
streets were located in the same block as one another, approx-
imately 1.5 km east of the CBD site. E–W–oriented streets
were selected for the study as trees are more effective at
cooling in E–W streets (Ali-Toudert and Mayer 2007a). The
trees in the CBD street canyon were Platanus ×acerifolia and
Platanus occidentalis (Plane family) with median planting
heights of 16.9 m and crown diameters of 13.0 m. At TRD,
the trees were uniformly Ulmus parvifolia (Chinese Elm),
with median heights of 9.5 m and crown diameters of 9.6 m.
The CBD trees had a trunk height to the base of canopy of
approximately 4.8 m while the trunk height at TRD was ap-
proximately 3.4 m. At CBD, the trees were located on the
sidewalk and emerged from small pervious areas surrounding
the trunk of 1.2 m
2
at CBD. At TRD, the trees were located
within the road area, adjacent to the sidewalk with a small
surrounding pervious area of 1.2 m
2
. Aside from the small
area surrounding the tree trunks, the rest of the street canyon
floor was impervious (e.g., pavement, asphalt) (Fig. 1). At
OPN, the few trees were also U. parvifolia. Lin and Lin
(2010) examined the effects of different characteristics of
shade trees on below canopy air and surface temperature (in-
cluding U. parvifolia). They found four characteristics that
influenced cooling in order of importance: leaf color, leaf area
index, leaf thickness, and leaf texture. U. parvifolia generally
has a greater leaf area index than Platanus×acerifolia (Lin
and Lin 2010;USDA2002), suggesting that less solar radia-
tion will be transmitted through the canopy at TRD.
Several monitoring stations were located in each street
using scientific grade instrumentation in a comprehensive net-
work. Each monitoring station measured air temperature (t
a
)
and relative humidity (HMP155A, Campbell Scientific), wind
speed (014A, Met-One), solar radiation (SI-212, Apogee
Instruments), and black globe temperature (sensor constructed
using an Omega 44031 precision thermistors and a 0.15 m-
diameter copper float painted black). Stations were mounted
on existing light poles at a height of between 3.5 and 4.0 m to
avoid vandalism. Poles were selected in order to best represent
each street canyon environment, by selecting poles on both the
north and south sides of the street, under canopy and in the
open, and under building shade for the CBD site. Poles were
selected for the installation of stations in order to obtain an
accurate representation of the pedestrian microclimatic expe-
rience in the street. Pole selection was constrained by existing
fixtures on poles, condition of the poles, and permission based
on weight loadings, but we believe a representative cover was
achieved due to the high number of stations. Previous studies
have not had such high spatial coverage of sites adopted here,
and some studies attempt to represent the urban canyon mi-
croclimate using a single site. Thus, this was a comprehensive
network of stations capable of accurately capturing the micro-
climate. More stations were placed in TRD (seven) than OPN
(five) because of the increased street complexity in TRD due
to street tree presence, and more stations again were placed in
CBD (10) as the street section was longer and the street com-
plexity considerable due to variable building heights. The total
sky view factor (SVF) was documented at each site, along
with the contribution from buildings to the SVF, and the con-
tribution from trees to SVF over and above that contributed by
Tabl e 1 Characteristicsofeach
street: Bourke St. comprises two
sections (West and East) with
varying characteristics
Bourke St. (W–E) Gipps St. George St.
Site description CBD Open (OPN) Treed (TRD)
No. of stations 10 (4–6) 5 7
Minimum building height (m) 5 4 4
Maximum building height (m) 133 14 11
Average building height (m) 22 7 8
Street width (m) 29 25 25
Mean H:W 0.76 (1.06–0.54) 0.27 0.32
Plan area canopy cover (%) 31 (20–42) 12 45
Temperature and human thermal comfort effects of street trees
buildings (Table 2). SVF was calculated using fish-eye pho-
tographs processed in RayMan Pro 2.1 (Matzarakis et al.
2010).
The measurement height in this study was higher than the
suggested pedestrian level experience of HTC of 1.1 m. As
such, t
a
during the day may be slightly underestimated, while
wind speed may be slightly overestimated. Oke (2004)sug-
gests relaxing the requirement of measurements at standard
observational heights (between 1.25 and 2 m) in part to avoid
sensor damage in urban areas. Oke (2004) also points out that
taking temperature measurements between 3 and 5 m should
not lead to significant measurement bias and are little different
from standard height because t
a
measurements show very
slight gradients through most of the urban canopy layer. The
CBD stations were installed for around 12 months from May
2011 to May 2012, while the East Melbourne stations OPN
and TRD were installed for 21 months, from October 2011 to
June 2013. Data were logged every 10 s (CR211 data logger,
Fig. 1 Location and photos of
each street: Bourke St. (CBD)
(East and West), Gipps St. (OPN),
and George St. (TRD)
Fig. 2 Aerial view of the streets highlighting individual station locations and tree canopy coverage in Bourke St. (CBD), Gipps St. (OPN), and George
St. (TRD)
A.M. Coutts et al.
Campbell Scientific) and averaged to 10 min; from 16 May
2012, data were averaged to 30 min. The response time of the
black globe thermometer (t
g
) varies with the mass of the globe.
Suggested time periods for black globe thermometers to reach
equilibrium vary between studies, from 10 min for a thin
sphere (0.015 in.) (Graves 1974) to 25 min for a standard globe
thermometer to attain equilibrium when a resolution of 0.1 °C
is required (Vincent 1939). We averaged t
g
over 30 min.
Statistical analysis involved ttests for independent variables.
Mean radiant temperature (t
mrt
) was calculated using the
black globe thermometers and accounting for the effects of
convection and conduction on the black globe given by
(Kántor and Unger 2011):
tmrt ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
tgþ273:15
4þhcg
εdg0:4tg−ta
−273:15
4
s
where t
g
is the globe temperature, h
Cg
is the mean convective
coefficient (1.10×10
8
v
0.6
where vis wind velocity [m s
−1
]), d
g
is the globe diameter (m), ɛis the globe emissivity (0.95), and
t
a
is air temperature. Prior to installing the black globe ther-
mometers, we compared the constructed globes with direct
measurements of the mean radiant flux density (S
str
), consid-
ering shortwave and longwave radiation from the six cardinal
directions using three all-wave radiometers (CNR1, Kipp and
Zonen) and accounting for angular and absorption factors of a
sphere (Thorsson et al. 2007). All the black globe thermome-
ters and radiometers were set up in a residential back yard
along with t
a
and v, for 5 days from 28 January 2011 to 1
February 2011 (excluding the dawn and dusk periods 0745–
1000 and 1620–1745 h when nearby structures caused differ-
ential shading of sensors). Conditions were predominantly
warm with clear skies, with t
a
ranging between 11 and
40 °C. The t
mrt
from the black globe thermometers was
overestimated compared to the direct radiation observations
of t
mrt
for a sphere, so we corrected the mean convection
coefficient h
Cg
to 0.65×10
8
v
0.53
, which resulted in good
agreement between the approaches. Finally, we use the recent-
ly developed Universal Thermal Climate Index (UTCI) as a
thermal index of HTC (Bröde et al. 2012) that calculates the
physiological response to meteorological input based on a
multimodal model of human thermoregulation. From a com-
parison of several thermal indicies, Blazejczyk et al. (2012)
concluded that the UTCI can represent bioclimatic conditions
and their relevance to human thermal stress under a range of
climatic conditions, making the index universal in nature, and
represented the temporal variability of thermal conditions bet-
ter than other indices. The UTCI was calculated using
RayMan Pro 2.1 (Matzarakis et al. 2010) for a 35-year-old
male, with a clothing factor of 0.9 and activity rate of 80 W.
UTCI equivalent temperature was then compared with the
assessment scale of thermal stress (Bröde et al. 2012), where
the UTCI range +9 to +26 °C indicates no thermal stress, +26
to +32 °C moderate heat stress, +32 to +38 °C strong heat
stress, +38 to +46 °C very strong heat stress, and above
+46 °C indicates extreme heat stress.
3 Results and discussion
3.1 Interstreet variability under summertime conditions
We sought to explore the influence of trees on the street can-
yon microclimate under warm summer time and extreme heat
conditions because that is when the cooling benefit provided
by trees will be of greatest benefit. The dense network of
monitoring stations within each street provided a more repre-
sentative picture of canyon microclimate than a single station.
This allowed the opportunity to quantify the effects of street
trees on the overall canyon air temperature and HTC of three
streets of varying tree cover and canyon morphology. We first
Tabl e 2 Sky view factor
(SVF) at each
monitoring site
Presented is the total
SVF (resulting from
buildings and trees) at
each site; the reduction in
SVF as a result of
buildings only (Build.)
and the reduction in SVF
as a result of trees in
addition to that resulting
from buildings
Site SVF
Total Build. Trees
TRD
TRD_1 0.13 0.68 0.55
TRD_2 0.22 0.85 0.63
TRD_3 0.17 0.70 0.53
TRD_4 0.66 0.81 0.15
TRD_5 0.18 0.71 0.53
TRD_6 0.09 0.52 0.43
TRD_7 0.47 0.74 0.27
Mean 0.27 0.72 0.44
OPN
OPN_1 0.76 0.77 0.01
OPN_2 0.78 0.79 0.01
OPN_3 0.63 0.81 0.18
OPN_4 0.86 0.94 0.08
OPN_5 0.77 0.78 0.01
Mean 0.76 0.82 0.06
CBD
CBD_W1 0.02 0.43 0.40
CBD_W2 0.33 0.40 0.07
CBD_W3 0.29 0.31 0.02
CBD_W4 0.28 0.40 0.12
CBD_E1 0.25 0.46 0.21
CBD_E2 0.20 0.59 0.39
CBD_E3 0.18 0.56 0.38
CBD_E4 0.08 0.50 0.42
CBD_E5 0.39 0.43 0.03
CBD_E6 0.45 0.48 0.03
Mean 0.25 0.45 0.21
Temperature and human thermal comfort effects of street trees
compare interstreet variability between TRD and OPN and
then TRD and CBD under average summertime conditions.
During January 2012, mean canyon t
a
during the day was
very similar for all streets, with OPN being marginally greater
than TRD by 0.2 °C (Fig. 3a), but was not statistically signif-
icant. While differences in t
a
were subtle, the mean conditions
during January 2012 showed clear differences in t
mrt
,wind
speed, and UTCI between all the streets. Figure 3presents
the individual environmental components that contributed to
HTC for January 2012 averaged across all the monitoring
stations in each street. OPN displayed greater t
mrt
during the
day compared to TRD (Fig. 3b) where the extensive tree can-
opy shading reduced t
mrt
by blocking solar radiation. At night,
the reverse occurred as the wide, shallow canyon of OPN
allowed longwave radiative cooling, while the tree canopy at
TRD inhibited longwave radiative cooling from the surface
(Park et al. 2012). The trees also absorb and (re)emit longwave
radiation which further contributes to greater t
mrt
at night at
TRD. Trees also acted as surface roughness elements in TRD
leading to reduced wind speed in the urban canyon compared
to OPN (Fig. 3c). Finally, vapor pressure (e) was slightly
greater (nonsignificant) at TRD compared to OPN and was
likely due to transpiration from the trees in the street canyon
because the street canyon floor was almost entirely impervi-
ous at both sites (Fig. 3d).
The individual environmental components of HTC (t
a
,t
mrt
,
v,ande) were all modified by the presence of trees, having
either positive or negative effects on HTC as determined using
the UTCI. The mean UTCI for each street from January 2012
data is presented in Fig. 4a and shows that OPN supported a
slightly greater UTCI and level of heat stress than TRD. As
outlined in previous research (Ali-Toudert and Mayer 2007b;
Shashua-Bar et al. 2010b), t
mrt
is the dominant driver of HTC
under warm sunny conditions. Trees reduce the SVF, provid-
ing shade during the day at TRD and substantially reduced the
street canyon t
mrt
which limited heat stress levels. This is dem-
onstrated in Fig. 4b where there was a clear relationship be-
tween SVF and UTCI during the day for individual stations
across all three sites in January 2012. While the overall street
canyon UTCI at OPN was not drastically different from TRD,
UTCI was variable throughout TRD and dependent on SVF.
Figure 4b also suggests that street canyons that have a SVF>
0.5 should be prioritized for tree planting and canopy cover
increase. At night, t
mrt
remained a strong influence at TRD,
Fig. 3 Environmental
components influencing human
thermal comfort for each street,
averaged across all monitoring
stations: January 2012 mean
diurnal values for aair
temperature, bmean radiant
temperature, cwind speed, and d
vapor pressure. Error bars are
95 % confidence interval
A.M. Coutts et al.
and in combination with a reduced wind speed, resulted in a
higher UTCI than at OPN where there were few trees. Under
these average summer month conditions, the trees in TRD did
not significantly reduce ambient t
a
below that measured in
OPN, but did improve HTC during the day by reducing heat
stress levels. While the tree canopy did increase UTCI over-
night, the thermal sensation at night was comfortable, so an
increase in UTCI was not as problematic. In summary, under
these average summer month conditions, the trees in TRD did
not provide large amounts of cooling of ambient t
a
below that
measured in OPN, but did improve HTC during the day by
reducing heat stress levels.
Comparing TRD with the more built-up CBD site, there
was a clear difference in canyon t
a
of up to 0.8 °C at night
which was statistically significant (p<0.05) (Fig. 3a). This was
due to the higher amounts of imperviousness at CBD and the
higher H:W. The deep urban canyon of the CBD cooled slowly
in comparison to the shallow urban canyon of TRD due to the
retention of radiation that was absorbed during the day within
the large impervious surface cover and reduced longwave ra-
diative cooling. Anthropogenic heat (waste heat produced by
human activities) from vehicles and buildings may also have
served to warm the CBD street canyon. It is interesting to note
that this difference in night time t
a
was evident despite a similar
total SVF at CBD and TRD, although a greater proportion of
the reduced SVF at CBD resulted from buildings (Table 2).
This suggests that a reduced SVF resulting from buildings has
a greater impact on heat retention than reduced SVF resulting
from the tree canopy. The tall solid buildings restrict the loss of
longwave radiation from the street canyon to a greater extent
than the porous tree canopies, and the warmer buildings also
reemit greater amounts of stored longwave radiation into the
street than the cooler tree canopies. The greater t
a
at CBD was
also despite a higher amount of ventilation (wind speed) com-
pared to TRD. At night, wind speed at CBD remained high and
may be due to channeling and/or drainage of airflow along the
street canyon. The greater tree canopy cover at TRD (Table 1)
also contributed to a reduced wind speed. At CBD, ewas
reduced despite a 31 % tree cover and may reflect a lower
background ein the central city area, in general, due to high
imperviousness. Finally, t
mrt
was similar between CBD and
TRD due to shading from either buildings or trees and follows
that the total SVF was similar as the sites (Table 2). As a result,
despite the lower tree canopy cover at CBD, the level of HTC
during the day based on the UTCI was similar to TRD. The
buildings at CBD reduced the SVF providing additional shad-
ing in conjunction with the trees (Fig. 4). In summary, the
buildings at CBD appear to have a greater influence on t
a
,
and overwhelm the influence of trees, and as such the trees
were less effective in reducing UTCI at CBD because shading
was already being provided by buildings.
3.2 Temperature and HTC during heat events
The 2011–2012 summer in Melbourne was relatively mild
compared to previous summers, such as 2008–2009, when
the city experienced its highest ever recorded maximum tem-
perature of 46.4 °C on 9 February 2009. Australia was
experiencing La Niña conditions during 2011–2012. The most
intense heat events over this summer occurred on 2 January
2012 when maximum temperature peaked at 40.0 °C and on
24 and 25 February 2012 when maximum temperature reached
37.1 °C on both days (observed at the Melbourne Regional
Office weather station). The mean daily temperatures (the mean
of yesterday’s maximum temperature and this morning’smin-
imum temperature) were 31.8 °C on 2 January 2012, and 30.7
and 31.1 °C on 24 and 25 January, respectively. Corresponding
minimum temperatures following the high maximum daytime
temperatures were 23.6, 24.2, and 25.0 °C. According to
Nicholls et al. (2008), when the mean daily temperature in
Melbourne exceeds 30 °C, there is a 15–17 % increase in mean
average daily mortality for >65 year olds. Furthermore, when
the minimum overnight temperature exceeds 24 °C, the mean
average daily mortality for >65 year olds increases by 19–21 %.
Fig. 4 a Mean Universal
Thermal Climate Index (UTCI)
for each street, averaged across all
monitoring stations in January
2012. Error bars are 95 %
confidence interval. The levels of
thermal stress from the associated
assessment scale are also
presented; brelationship between
sky view factor (SVF) and mean
daytime UTCI in January 2012
Temperature and human thermal comfort effects of street trees
Temperatures on all these selected days exceeded one or both of
these thresholds and therefore represent periods of increased
risk of mortality in vulnerable populations.
Under these heat events, the differences in street canyon
microclimate were more pronounced and street trees had a
greater influence on t
a
. TRD was generally cooler than OPN
during the day as the tree canopy absorbed solar radiation and
shaded surrounding urban surfaces (Fig. 5). The daytime dif-
ferences were around 0.2–0.6 °C and peaked at 0.9 °C during
mid-morning (9:00–10:00) when OPN heated up rapidly,
while the tree canopy in TRD delayed heating of that urban
canyon. However, the differences in mean t
a
between OPN and
TRD were not statistically significant. OPN remains slightly
warmer into the evening (e.g., through to around midnight)
before OPN and TRD became similar in the early hours of
the morning. Regarding HTC, the UTCI over the heat event
followed the same pattern as throughout January 2012 but
reached very strong heat stress levels during the day (Fig. 5),
with OPN showing the highest level of heat stress. Wide and
shallow canyon streets without street trees present the most
unfavorable conditions during the day (high t
a
and high UTCI).
Comparing the deep canyon CBD site to the shallow TRD
site, the most striking feature was the higher overnight t
a
at
CBD (Fig. 5), with differences of up to 4.8 °C prior to dawn.
This further highlights the restricted longwave cooling from
reduced SVF because of buildings at CBD that is known as a
strong driver of the canopy layer UHI. Similarly, this high-
lights the increased frequency of threshold minimum night
temperatures of 24 °C at CBD, above which increased mor-
tality in vulnerable communities can be expected. However,
the CBD site was slightly cooler during the day by as much as
2.0 °C, as a result of the reduced SVF from the buildings
making up the deep canyon and to a lesser extent from the
trees, along with the absorption of solar radiation by urban
materials. These influences are sometimes listed as drivers
of the daytime urban cool island. The lower daytime t
a
at the
Fig. 5 Mean air temperature for each street during heat events on 2 January 2012 and 24–25 February 2012, differences inair temperature for CBD and
OPN in comparison with TRD (Difference from TRD), and UTCI for each site and the corresponding grades of heat stress
A.M. Coutts et al.
CBD site occurs despite a lower vegetation cover and lower
leaf area index of the trees and further emphasizes that at the
CBD, the building morphology of the area overwhelms the
influence of street trees on t
a
. For HTC, the low SVF also
results in a reduction in UTCI during the day through shading
(Fig. 5). This demonstrates that during the day, shading from
either buildings or trees can reduce UTCI and contribute to
improve HTC under warm sunny conditions. However at
night, deep street canyons are the most unfavorable as the
retention of heat results in a poorer HTC (high t
a
, and high
UTCI). Thus,the street trees in TRD provide a more thermally
comfortable environment during the day, but without a strong
detrimental impact on HTC at night.
The summer of 2012–2013 had higher average monthly
summer temperature than the summer of 2011–2012, with a
larger number of heat events. Summertime conditions actually
extended into March 2013 (autumn) in Melbourne with a
prolonged heat event extending from 4 to 12 March with nine
consecutive days exceeding 32 °C, due to a near stationary
‘blocking’high pressure system situated over the Tasman Sea
bringing warm northerly air from the continental interior to
Melbourne (BoM 2013). This has been the longest period of
days over 30 °C or above in Melbourne for any month since
records began in 1855 and led to record high overnight tem-
perature minimums for any month, with seven consecutive
nights above 20 °C (7–13 March) (BoM 2013). The monitor-
ing stations in OPN and TRD were in place during this period
and provide a good representation of the influence of the tree
canopy at the microscale during prolonged warm conditions
(Fig. 6). During the day, OPN heated rapidly in the morning,
while the tree canopy at TRD delayed canyon heating leading
to an average temperature reduction of 0.9 °C, but by as much
as 1.5 °C. As the day progressed, TRD began to warm but still
remained cooler than OPN by between 0.2 and 0.6 °C.
Shashua-Bar and Hoffman (2000) observed air temperature
reductions of up to 2.5 °C within a well-treed avenue in Tel-
Aviv as compared to a reference site outside the avenue. After
dusk, TRD and OPN were similar, until trapping of heat at
TRD by the tree canopy resulted in a slightly warmer t
a
after
midnight. Nocturnal t
a
was warmer in TRD, which was sim-
ilar to that observed by Souch and Souch (1993). While the
tree canopy may inhibit nocturnal longwave radiative cooling
within the street canyon (microscale), the daytime contribu-
tion of shading and transpiration from high tree canopy cover
is likely to reduce the amount of heat storage in the landscape
across the neighborhood (local scale) (Coutts et al. 2007),
thereby reducing potential nocturnal t
a
.
3.3 Microscale variability in air temperature
Closer examination of the 4–12 March 2013 data revealed that
there was a large amount of microscale variability in t
a
with
differences of up to 3.1 °C among the seven stations in TRD
during the March 2013 heat event. Ali-Toudert and Mayer
(2007b) observed t
a
that was 3.0 °C warmer on the sun-
exposed side of a street at 17:00 of an E–W-oriented street
in Freiburg, Germany. The variability of t
a
in this study was
greater in the more ‘complex’TRD street, where the trees and
buildings strongly influenced microscale variations in t
a
(Fig. 7). OPN displayed a spatially more consistent t
a
during
the day, while the trees in TRD reduced mean daytime t
a
of
individual monitoring stations by up to 1.0 °C (Fig. 7).
However, during this period, only OPN_2 and TRD_2 were
significantly different during the day (p<0.01). This variabil-
ity in t
a
was partly due to shading, as the tree canopy reflects
and absorbs solar radiation. The mean daytime temperature
was plotted against the average daily amount of solar radiation
received at each station (Fig. 8), and this demonstrates the
influence of solar radiation on microscale t
a
within the canopy.
Several studies that have considered microscale street climate
have highlighted the major role of tree canopy shading, al-
though these studies also note that the effects are obviously
localized (Lin and Lin 2010; Shashua-Bar and Hoffman
2000). Another driver of t
a
variability within TRD may be
the reduced wind speeds that reduce vertical mixing between
air within and air above the street canyon (Park et al. 2012).
While the differences in t
a
between OPN and TRD (Fig. 7)
were relatively small, if trees were present throughout the
neighborhood, the benefit of lowering local-scale t
a
could be
substantial (Lynn et al. 2009).
Considering again the January 2012 period, we compared
the variability in mean daytime t
a
between individual stations
in OPN with those in TRD during all conditions. It was
Fig. 6 Mean air temperature in OPN and TRD during the 4–12 March
2013 heat event and the difference (TRD Diff.) between the two streets.
Error bars are 95 % confidence interval
Temperature and human thermal comfort effects of street trees
apparent that t
a
of some individual stations in TRD was sig-
nificantly different from others in OPN (p<0.05), namely,
those stations below the tree canopy (e.g., TRD_1 and
TRD_2), indicating that the trees influence the street micro-
climate. There are many drivers that influence the climate at
the microscale, and as Park et al. (2012) suggest, determining
the influence of vegetation on microclimate is susceptible to
other local and microscale influences such as geometry, urban
materials, and background meteorology. It was apparent here
that the influence of trees on microscale variability in t
a
varied
with wind direction, just as Lin and Lin (2010) noted that the
cooling effect of trees depends on the meteorological condi-
tions at the time. The patterns in mean daytime t
a
for January
2012 both within and between streets TRD and OPN are pre-
sented as schematic diagrams in Fig. 9. We found that under
northerly wind conditions that bring warm air from the conti-
nental interior, t
a
was generally lower in TRD than OPN,
especially on the shaded side of the street (similar to patterns
presented in Fig. 7). However, only station OPN_6 was sig-
nificantly different during the day compared to the stations in
TRD, suggesting more uniform t
a
across the two streets. In
contrast, under southerly wind conditions that bring cool mar-
itime air, several stations in TRD were significantly different
than others in OPN. We found that stations on the northern,
shaded side of TRD were cooler than those in OPN, but the
stations on the southern, sun-exposed side of TRD (TRD_3
and TRD_7) were warmer than those in OPN. Further, at an
intrastreet scale under southerly conditions, stations on the
south side of the TRD were often significantly (p<0.05)
warmer than stations on the north side of TRD.
Overall, we suggest that within-canyon variability in t
a
for
OPN and TRD depends on the presence of trees, surface
heating from solar radiation, geometry, and flow conditions
(schematic Fig. 9). Offerle et al. (2007)showedthatgeometry
strongly influences the flow and temperature distribution in
urban canyons when they observed surface temperature and
sensible heat fluxes above and within a street canyon
(H:W=2.1) in central Gothenburg, Sweden. Offerle et al.
(2007) describes how sensible heat fluxes depend on whether
Fig. 7 Mean daytime
temperature for individual
stations at OPN and TRD during
the heat event over 4–12 March
2013. A spatial surface was
derived using inverse distance
weighted interpolation tool in
order to aid visualization
Fig. 8 Scatterplot of mean daytime air temperature and average total
daily solar radiation at each monitoring site in OPN and TRD for 4–12
March 2012
A.M. Coutts et al.
the sunlit (heated) wall is either the windward or leeward wall
of the urban canyon. When the windward wall is heated, there
is an enhancement of turbulent heat fluxes and mixing due to
interaction of buoyancy and vortex circulation, as well as the
entrainment of cooler air; when the leeward side is heated,
heat transfer is concentrated near the wall and transported
vertically (Offerle et al. 2007). Such interactions are consistent
with the distributions of t
a
observed in our study (Fig. 9).
Under warm northerly wind conditions, the north side of the
street remains relatively cool due to shading from trees and
buildings, creating a zone of relatively cool air. The trees on
the south side of the street also provide shade, but surface
heating in the street warms the southern side of the street floor
and wall, and buoyancy transports heat vertically (Fig. 9a).
Under cool southerly wind conditions, a zone of relatively
warm air is created on the south side of the street from surface
heat and heat trapping by the canopy. This enhances the heat
transfer and mixing from buoyancy and canyon vortexand the
entrainment of cooler air (from the northern side of the street)
(Fig. 9c). In contrast, the treeless canyon of OPN was more
exposed to the prevailing northerly/southerly wind conditions
and geometry was less influential in modifying air tempera-
ture distributions within the canyon (Fig. 9b and d). Salmond
et al. (2012) observed turbulent fluxes of heat and momentum
using scintillometry across the top of a canyon and noted that
the observations were influenced by a combination of variabil-
ity in urban structure, materials, radiative heating, and wind-
dependent microscale advection of turbulent kinetic energy
and sensible heat. Hence, with these complex drivers, spatial
and temporal variability in t
a
at the microscale can be
substantial, and these general patterns need to be confirmed
using more detailed observations and documentation of turbu-
lence characteristics within the canyons or with the use of
microscale modeling.
3.4 Microscale variability in HTC
The UTCI averaged for each street during the heat events
(Fig. 5) were similar in that they all reached very strong heat
stress levels. However, microscale differences in UTCI were
largely due to the variability in t
mrt
due to shading. Those
stations experiencing shade showed substantial reductions in
t
mrt
and UTCI (Fig. 10). During peak daytime heating, tree
shade could reduce the level of heat stress from ‘very strong’
to ‘strong’, while over the course of the day, the overall level
of heat stress was much lower (e.g., TRD_1 and CBD_E4).
Shade from buildings was even more efficient than shade from
trees at reducing UTCI during the day (e.g., CBD_W3), be-
cause trees allow some transmittance of solar radiation
through the canopy. Figure 4b in Section Interstreet variability
under summertime conditions highlighted the relationship be-
tween SVF and UTCI at individual stationsand was a result of
reductions in t
mrt
. Stations in CBD and TRD that were located
adjacent to tree canopies and were not shaded in the afternoon
(e.g., TRD_3, CBD_E1) actually showed a higher UTCI than
stations that were completely sun-exposed which was likely
due to the reduced wind speed below and adjacent to the tree
canopy. Figure 10 highlights the large amount of variability in
HTC at the microscale, simply due to the variable amount of
shade. During another heat event on 4 January 2013, when the
Fig. 9 Relative differences in mean daytime air temperature in TRD (aand c) and OPN (band d) during January 2012 under northerly (aand b)and
southerly (cand d) wind directions. The sun is to the north
Temperature and human thermal comfort effects of street trees
maximum air temperature reached 41.1 °C, unshaded stations
at OPN and TRD exceeded the ‘extreme’heat stress level
(UTCI>46 °C), while the shaded stations were limited to
‘very strong’heat stress. At night, individual stations were
warmer than OPN due to the reduced longwave cooling due
to the presence of trees and buildings and radiance from these
structures. While UTCI was slightly elevated at night, the
HTC levels remained ‘comfortable’, and the substantial reduc-
tion in UTCI during the day is likely to outweigh any minor
increase in UTCI at night. To reduce the harmful effects of
urban heat events, it is critical that shading is provided to
pedestrians at street level during the day, especially in streets
with low H:W ratios and high solar exposure potential and in
streets with high pedestrian activity and therefore high human
exposure potential.
4 Conclusions
Protecting and maintaining urban street trees in warm temper-
ature and Mediterranean cities are crucial to realize the micro-
climate benefits they can provide. Tree canopy cover contrib-
utes to microclimate benefits by reducing daytime t
a
during
heat events and thereby improving daytime HTC during hot
conditions. This justifies the argument for increased invest-
ment in tree planting, maintenance, and protection by local
councils. The City of Melbourne has set a target of increasing
canopy cover from the current 22 % up to 40 % by 2040
(CoM 2012). From the findings of this study, this ambitious
target will help achieve a more attractive, thermally comfort-
able, and sustainable urban environment. However, we have
observed that the cooling and HTC benefits of street trees are
localized and highly variable both spatially and temporally.
Given this variability, precisely quantifying the benefit of
street trees on t
a
at the individual tree or whole street level is
difficult. The magnitude of daytime tree cooling depends on
the amount of shading, street geometry, and the local meteo-
rological conditions, all of which influence the air temperature
variations within and between urban canyons. Adding trees to
an urban canyon alters both the overall air temperature and the
range and distribution of air temperatures in that canyon by
modifying surface heating and air flow under the prevailing
weather conditions at the time. Street trees are more effective
in influencing the street microclimate in more open, shallow
street canyons, whereas in narrow, deep canyons, the building
morphology tends to overwhelm the influence of trees and
dominates the air temperature levels and distribution within
the canyon. Depending on their position in the street canyon,
the prevailing conditions, and time of day, trees can have
either a cooling or warming effect. Critically though, under
summertime conditions during heat events, trees play a vital
role in mitigating high urban air temperature.
Because of the variable and localized nature of street tree
cooling, trees should be distributed throughout the streetscape
Fig. 10 Universal Thermal
Climate Index (UTCI) and human
thermal comfort (HTC) at
selected stations over the 24–25
February 2012 heat event for a
TRD in comparison with UTCI (±
max and min) observed at OPN
and bselected stations for CBD
A.M. Coutts et al.
for overall cooling and HTC. During heat events, street trees
can lower the level of heat stress through the day, largely
through a reduction in t
mrt
from tree canopy shading.
Distributed tree canopies, placed strategically where pedes-
trians are likely to be exposed to high levels of solar radiation
during the day (such as city squares, transport corridors, walk-
ing routes, and public services like schools and hospitals) will
help deliver a more climate-sensitive city with reduced levels
of daytime heat stress in summer. Strategic tree placement also
recognizes the sometimes limited budgets of local councils
and the need to achieve the largest benefit for the cost of
investment and maintenance. Trees should be placed on the
sun-exposed side of E–W-oriented streets, targeting streets of
low H:W. Strategic placement of trees is also critical at night.
The tree canopy cover led to a slightly higher UTCI at night
from reduced longwave radiative cooling in the canyon and
lower wind speeds that restricted ventilation. Therefore, trees
should be strategically placed to maximize their shade area yet
spaced sufficiently to allow some nocturnal longwave cooling
and ventilation, although daytime improvements in HTC from
street trees far outweighs any detrimental impacts on UTCI at
night.
As air temperature is highly variable at the microscale in
treed street canyons, an intensive microscale measurement
program as presented in this study is required for a robust
assessmentof urban canyon warming and cooling. We suggest
that short duration measurement studies and studies that use a
single monitoring station to represent a whole urban canyon
will not sufficiently capture the microscale climate that is cru-
cial to understanding HTC at an individual pedestrian scale.
Scale was also an important consideration in this study with
the focus being on understanding canyon climate from the
street-scale to the microscale. If tree canopy cover were in-
creased in the City of Melbourne to 40 % by 2040, this would
likely lead to local-scale daytime cooling greater than that
observed here (e.g., 0.2 to 0.6 °C, and up to 0.9 °C during
mid-morning) in this study during heat events. Increased tran-
spiration from an extensive tree canopy will increase local-
scale latent heat fluxes leading to lower daytime t
a
, and in-
creased canopy shading will reduce local-scale storage heat
fluxes, helping to mitigate the nocturnal UHI (Coutts et al.
2007). A multiscale modeling approach would help to eluci-
date the benefits of a widespread increase in street trees at the
microscale. To further justify the expense of maintaining
an increased urban forest canopy, the monetary benefits
delivered through cooling and improved HTC need to
be recognized alongside the other benefits such as
building energy savings, transport efficiency savings,
productivity and human health. Finally, ensuring ade-
quate water is available for street trees is paramount to
ensure they remain healthy, with full canopies for shade
and sufficient soil moisture for transpiration in what can
be a harsh urban environment.
Acknowledgments This work was funded by contributions from the
City of Melbourne, the Monash University Faculty of Arts, and the CRC
for Water Sensitive Cities. Monash University provides research into the
CRC for Water Sensitive Cities through the Monash Water for Liveability
Centre. Thanks to the City of Melbourne for access to GIS databases, and
the City of Melbourne contributors Meg Caffin and Yvonne Lynch for
coordinating installation of stations and for project collaboration. Sincere
thanks to the High Access Group for undertaking the monitoring equip-
ment installation, and the CityPower for permission to install the equip-
ment on the power poles.
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