1. Introduction

Global mean air temperature has increased by 1 °C compared to pre-industrial times and is expected to reach +1.5 °C by 2050 [1]. Climate models predict that global warming will be associated with more frequent, severe and intense extreme heat events [1,2], and cities are also predicted to become warmer in the coming years [3]. Within cities, increasingly extreme temperatures lead to challenging conditions for urban residents, particularly during heatwaves [4]. Therefore, a greater understanding of mitigation options to reduce urban heat is necessary to improve local conditions for urban populations.

Urban areas are mosaics of buildings, streets and different types of impervious surfaces, along with green and blue infrastructure [5]. Impervious surfaces (i.e., man-made surfaces such as concrete, asphalt and buildings) can cover up to 50% of urban areas, although this cover varies considerably across cities [6]. Urban areas also differ in their radiative, thermal, aerodynamic and moisture properties relative to surrounding peri-urban and natural areas, absorbing heat and re-radiating longwave radiation within the urban matrix, resulting in higher air and surface temperatures [7,8]. As a consequence, temperatures in urban and peri-urban areas are typically higher than in the surrounding rural landscape—a phenomenon known as the urban heat island effect (UHI) [9].

Multiple heat mitigation strategies are available to modify the heat balance of urban areas. Of these, nature-based solutions (e.g., green roofs and walls, planting trees, presence of water bodies) are highlighted as sustainable, cost-effective ways of reducing urban heat and improving the livability of cities across the globe [10,11]. Trees are a key component of urban green space and have a demonstrated ability to reduce local air temperatures [12,13]. Indeed, a modelling study of four cities from different climate zones (Melbourne [Australia], Zurich [Switzerland], Phoenix [USA] and Singapore [Singapore]) reported that vegetation cover can decrease daytime maximum air temperatures by 3.1 to 5.8 °C [14]. Furthermore, empirical studies have demonstrated air temperature reductions of 2.8 °C associated with tree canopies in Campinas, Brazil [15] and 1.1 °C in Greater Sydney, Australia [16]. The extent to which tree crowns reduce air temperature varies substantially across studies [14,17], with differences likely due to the nature of the surrounding built-up area, the extent of vegetation cover, soil water availability, time of day and local microclimate [18].

Trees cool the surrounding area directly by blocking solar irradiation and transpiring water to the atmosphere [19]. Morphological traits, such as tree size, crown width, crown density, leaf dry matter content (LDMC), leaf area (LA), specific leaf area (SLA) and the Huber value influence the amount of canopy shade cast, light reflectance and transpiration rate [20,21,22]. Shaded surfaces absorb less solar radiation and therefore re-radiate less heat, thereby maintaining cooler air temperatures underneath tree crowns [16]. Both LDMC and SLA are negatively correlated with leaf thickness [23], and thinner leaves can more effectively lower air temperature [24] due to their thinner leaf boundary—a thin layer of still air that surrounds each leaf. The Huber value (i.e., the ratio of stem xylem cross-sectional area to leaf area supported by the respective stem segments) [25] represents the amount of leaf area of a stem that can transpire water [15] and hence a tree’s capacity for transpirational cooling under optimal conditions [26].

Transpiration rates are regulated by solar irradiance, VPD, and windspeed [27], with stomatal opening generally greatest at biologically optimal solar irradiance, VPD and windspeed [28]. High windspeed disrupts the leaf boundary layer enhancing CO2 and H2O diffusion [29], resulting in increased transpiration. High VPD typically causes plants to close their stomata to reduce water loss, resulting in a decline in transpiration rate. High VPD conditions are associated with extreme heat and/or drought conditions [27]. Similarly, later in the day/evening atmospheric demand for water is often lower (low VPD), resulting in stomatal closure and reduced transpiration rates [30]. In many species, nighttime stomatal opening has, however, been observed [30,31]. Nighttime plant transpiration can be up to 20% of daytime transpiration levels and is also positively correlated with VPD [32]. As a result, nighttime cooling due to plant transpiration can occur under and surrounding tree crowns [30].

The cooling benefits of urban trees—referred to as delta temperature (ΔT), the difference between ambient air temperature and sub-canopy air temperature—vary among species and depend on plant traits, tree structure and crown characteristics [15,24]. Knowledge of the extent to which different tree species and their associated traits can provide microclimatic benefits during summertime periods of high temperatures and can improve urban livability and thereby the wellbeing of urban residents. However, whilst species might differ in the extent to which they are able to influence the local microclimate [33,34], it is not feasible to evaluate all urban trees for any given city. Therefore, a traits-based approach that identifies those tree characteristics that are broadly associated with greater shade and evapotranspiration rates, and hence likely to provide greater cooling benefits, is a more feasible approach.

Urban areas across Western Sydney, Australia, have recently experienced record-breaking temperatures of 45–48.9 °C [10,35,36] and are predicted to reach 50 °C by 2040 [37]. Exposure to such extreme temperatures, along with a relatively high surface cover of man-made materials, such as roads, pavements and buildings, makes this area a prime location for evaluating the role of vegetation in mitigating urban heat. This study assessed sub-canopy temperatures of 10 commonly planted tree species across urbanized areas in Western Sydney to address the following questions: (1) How do tree species differ in their ability to influence summertime air temperatures and which canopy traits are associated with greater cooling benefits? (2) How do relationships between species/canopy traits and sub-canopy air temperature differ between day and nighttime? (3) How do climatic variables influence diurnal patterns in canopy-associated cooling? It is hypothesized that (1) tree species with more extensive (e.g., greater crown width, height and DBH) and dense (e.g., high leaf area index [LAI]) crowns, along with specific leaf characteristics (e.g., high leaf dry matter content [LDMC] and low specific leaf area [SLA]) are associated with greater shading and higher rates of evapotranspiration and, hence, will have lower sub-canopy air temperatures; (2) canopy-associated cooling will occur predominantly during sunlight hours, with limited overnight cooling; and (3) the combined effects of high VPD and high solar irradiance will partly reduce canopy cooling benefits by reducing transpirational cooling via daytime stomata closure, while high windspeed will partly increase the cooling benefits of trees via mixing of cooler air.

4. Discussion

A large dataset of sub-canopy air temperatures allowed this study to determine how common urban tree species in Western Sydney modify daytime and nighttime microclimates in summer. The inclusion of local climate variables and morphological trait data highlighted both the importance of species traits and the role of microclimatic parameters in the cooling benefits offered by urban trees.

Tree characteristics are known to influence air temperatures in urban environments. The mean maximum 3.9 °C of air cooling in the morning recorded in this study is greater than the ∼1.8 °C cooling reported by Alonzo et al. [52] in Washington DC, USA, and the ∼1 °C effects observed in Madison, Wisconsin, USA, by Ziter et al. [53]. Tree species in the current study differed significantly in terms of their cooling/warming effects, with average species-level ΔT values varying two-fold. Among the 10 species studied, P. acerifolia (mean 3.02 °C; mean LAI 3.62 m2 m−2 and mean crown width 16.21 m) provided the greatest cooling benefits, followed by P. calleryana (mean 2.58 °C, mean LAI 4.35 m2 m−2 and mean crown width 7.52 m) in the morning. Several studies have investigated the effect of LAI on sub-canopy air temperatures and showed that daytime cooling benefits improve with increasing LAI values [22,43,54]. At the other end of the scale, J. mimosifolia had the smallest impact on daytime air temperatures during summertime, with an average reduction of 1.5 °C. This species had no leaves and therefore a low LAI (mean 1.75 m2 m−2) during its flowering period from October to early December. However, when this species is in full leaf (mean LAI of 3.42 m2 m−2 measured in February 2020), it can potentially provide much greater cooling benefits, as has been observed in Wollongong, NSW, Australia, where mean daytime cooling of 5.1 °C on days with temperatures of 25 °C or above has been associated with this species [55].

Similarly, wider crowns can potentially reflect a greater proportion of incoming solar irradiation and provide a larger shaded area, resulting in greater cooling benefits [56]. Wujeska-Klause and Pfautsch [43] observed that air temperatures of streets with high canopy cover were, on average, 0.5 °C (max 2.1 °C) cooler than streets with low canopy cover, again highlighting the importance of canopy size. Trees with large canopies cast shade over large areas, which can lead to a decrease in local air temperature [24]. Shaded areas also absorb less solar irradiation, resulting in lower energy storage and re-emission in the form of sensible heat, which can warm the surrounding air [57,58]. Overall, the findings of relationships between sub-canopy ΔT (during morning and afternoon) and LAI and canopy width provide partial support for the first hypothesis of the current study and highlight the importance of shade cast (both in terms of the absolute amount and intensity of shade) for daytime temperatures in urban streetscapes.

The observed interactions between canopy-associated cooling and solar irradiance across the day support observations by Motazedian et al. [18] and Shashua-Bar et al. [59]. Both studies reported tree shade-associated air temperature reductions and human thermal comfort in the morning in cities located in temperate oceanic (Melbourne, Australia) and arid (southern Israel) climates, respectively. Solar energy drives evapotranspiration in the morning, particularly from 07:00 to 10:00 h [60], which may explain the increase in magnitude of morning cooling benefits with increasing solar irradiance in this study. High solar zenith angles in the morning can also lead to large structural shading, slowing the warming of man-made surfaces and thereby increasing the cooling benefits of street trees [61].

Canopy-associated warming was observed in the afternoon and continued overnight, with average temperature increases of 1.19 °C (afternoon) and 1.53 °C (night), relative to ambient air. Among the 10 tree species in the current study, L. styraciflua had the warmest afternoon sub-canopy temperatures (mean 2.43 °C) and P. acerifolia (mean 1.78 °C) the lowest. Although positive afternoon ΔT values decreased with increasing LAI, again indicating that shade intensity can influence the local microclimate by reducing thermal loading of surrounding man-made materials, they still mostly indicated higher sub-canopy temperatures than ambient air. The positive relationship between afternoon sub-canopy air temperatures and windspeed in this study highlights the important role of atmospheric mixing [62,63]. The transport of warm air masses from the hot, arid interior of the continent into suburban areas in the afternoon likely negated canopy-associated cooling, especially when combined with high VPD and associated stomatal closure. This, combined with re-radiated heat from surrounding buildings likely plays an important role in our observation of afternoon sub-canopy warming. Several studies have reported that trees with high LAI can trap re-radiated heat in the evening and overnight [55,64,65,66]. For example, a study conducted in Washington DC, found that the cooling benefits of trees located along streets were less than trees surrounded by grass, due to re-radiated heat from beneath the canopy and surrounding surfaces, highlighting the importance of planting context [52]. Disentangling the effects of canopy density on both trapping re-radiated heat (resulting in warming) and increasing the extent of sub-surface shading (thereby reducing re-radiation of sensible heat and reducing sub-canopy air temperatures) is a topic that warrants further study.

Canopy-associated warming can also be a response to sub-optimal water availability limiting transpiration. This, in turn, reduces latent heat flux and generates large amounts of sensible heat, thereby increasing local air temperature [29]. We speculate that the extreme nature of the weather in the runup to and during the study period played a role in observed sub-canopy warming, particularly during the afternoon. From 2017 to 2019, the study sites experienced very low levels of rainfall, with 2019 having the lowest rainfall on record [67]. Furthermore, during the “black summer” bushfire season of 2019–2020, Richmond and Cranebrook experienced 10 days above 40 °C, representing a period of extreme heat [68]. During this period, Tabassum et al. [69] found widespread evidence of tree canopy damage in Penrith, Western Sydney, while Marchin et al. [64] reported that around 60% of plant species they studied in Western Sydney experienced dieback due to the extremely hot and dry conditions prevailing during this time. While high temperatures and VPD during heatwaves are known to result in stomatal closure [66], the lack of physiological measurements in our study does not allow us to conclude that stomatal closure and the associated reduction in latent heat loss was responsible for sub-canopy warming during afternoon periods—a potential “disservice” of urban trees. We do, however, interpret the higher afternoon sub-canopy temperatures as evidence of both the role of atmospheric mixing, discussed above, and the urban heat island effect, with high levels of afternoon re-radiated heat from surrounding man-made materials captured by our sub-canopy dataloggers.

In terms of the response of trees to water stress, tree species can differ widely in their stomatal (and thus transpirational) strategies. These can be characterized as isohydric (maintaining leaf water potential by reducing stomatal conductance) and anisohydric (maintaining stomatal opening despite water limitation) responses [70,71]. Anisohydric trees can keep their stomata open for longer than isohydric species under sub-optimal water conditions and therefore continue transpiring water [72]. However, actively increasing water loss at high temperatures can make trees vulnerable to hydraulic failure under drought conditions [71,73]. A lack of data on tree species’ response to water deficit makes it difficult to speculate on how water use strategies influence the extent of cooling. This is, therefore, a key knowledge gap, particularly in the context of rising global temperatures, more frequent heatwaves and increasing human exposure to urban heat.

Vapor pressure deficit (VPD), which influences stomatal function and evapotranspiration [74], was posited to have an indirect, yet important influence on sub-canopy temperatures and ΔT across the day [75]. Indeed, evapotranspiration has been shown to decrease air temperatures by 2.0 to 8.0 °C in vegetated areas [76]. VPD typically increases rapidly in the morning and reaches a maximum rate by early afternoon, then declines toward dusk [77]. In the morning, under low VPD conditions, most plants keep their stomata open and maintain a stable transpiration rate. Progressive stomatal closure in the late morning leads to a reduction in cooling (ΔT moving towards zero) before VPD reaches its highest levels [78]. In this study, VPD had similar effects on morning and afternoon ΔT (>3.5 KPa), which supports our hypothesis of positive impacts of VPD on the contribution of transpiration to canopy cooling.

Despite the common expectation that nighttime stomatal closure results in minimal transpiration after dark, studies have shown that transpiration can occur after sunset in plants from a wide range of climates [30,79]. Lindén et al. [80] and Ibsen et al. [17] observed a positive correlation between VPD and transpiration-driven, canopy-associated cooling at night. The findings of a negative correlation between nighttime VPD and ΔT in the current study also point to the possibility of nighttime transpiration, although, ΔT values were generally positive during nighttime. It can be speculated that, during periods of low ambient rainfall—such as was the case during the months preceding this study—trees need to conserve water and thus transpiration is likely to play a relatively small role in canopy-associated cooling, compared to more typical, wet summer conditions. In Sydney, street trees do not generally receive additional irrigation beyond the initial ~ 2-year establishment phase, even during prolonged dry spells [81]. Further study is needed to investigate whether VPD and water availability regulate nighttime transpiration by street trees in a manner that is similar to daytime.

Nighttime canopy-associated warming of up to 0.4 °C has been reported by Alonzo et al. [52] in Washington DC, and also by Wujeska-Klause and Pfautsch [43] in a study in Parramatta, NSW, Australia, around 40 km from the current study area. Taken together, nighttime warming (and in our study also during summer afternoons)—associated with street trees in heavily built-up areas—suggests that canopies can hamper the transfer of re-radiated heat from man-made surfaces into the atmosphere. The extent of canopy- and man-made surface cover are, however, known to affect the influence of vegetation on temperatures in the built environment [52,53,82], and the relative amount of man-made versus natural surface cover is clearly a key factor influencing urban heat dynamics.

Reducing the area of heat-absorbing dark and unshaded surfaces is critical for reducing air temperatures during summer, given the amount of heat stored and subsequently re-radiated overnight [83]. Indeed, the lack of difference in nighttime temperature between streets with low and high canopy cover reported by Pfautsch and Rouillard [84] strongly suggests that reducing the area of grey surface cover is key to reducing nighttime air temperatures in urban settings. The current study indicates that, in Western Sydney, trees play an important role in managing daytime temperatures and mitigating urban heat (particularly in the morning), but provide few, if any, temperature benefits overnight during the hot summer months. These results highlight the importance of developing a deeper understanding of the interactions between surface characteristics, local microclimate and tree cover for reducing urban heat and improving the livability of cities during both the day and night.

​  Read More  Ecology, Environment, Nature  

Mahmuda Sharmin