1. Introduction

Infrastructure development activities are designed to cater to the growing population needs. Pavements are an integral part of infrastructure development in urban areas to provide interconnectivity and transportation. Parking lots, bike lane, and pedestrian paths are developed to facilitate the amenity requirements for a growing city. Overall, pavements occupy 38–40% of the urban land cover, and 75–80% of pavements have black tops with direct exposure to the sun ^[1]. In the construction process, vegetation is being replaced with heat-absorbing, thermally conductive, and high heat storage materials like bitumen and concrete. Pavements absorb and store solar radiation, leading to a further increase in the surface temperatures ^[2]. This phenomenon is known as the urban heat island (UHI) effect. Large quantities of solar radiation are absorbed by these materials during the day and released during the night time.

Pavement materials absorb and store solar irradiation, given their dark surface cover and large surface inertia ^[2]. During summer, heat absorbed by the black asphalt pavements is released into the surrounding environment, leading to pavement rutting and UHI effect ^[3]. Changes to human thermal comfort, urban microclimates, and the loss of tree cover all contribute to the UHI effect. In urban areas, natural disasters like bush fires and heatwaves exacerbate the heat stress due to interactions between the anthropogenic heat emissions and surfaces (buildings) ^[4]. Urban and suburban areas are warmer compared to rural areas with the build-up of heat radiation and transference activities from both urban infrastructure and anthropogenic activities like transportation and domestic heating and cooling ^[5]. The impervious nature of these materials reduces the infiltration rate of water, resulting in dry soil conditions decreasing the evapotranspiration rates and finally affecting urban forestry. Drying atmospheric conditions will further cause imbalances in local terrestrial ecosystems, leading towards a more significant adverse environmental impact.

Elevated temperatures due to UHI effects affect the thermal comfort of humans, potentially leading to heat-strokes, respiratory difficulties, heat cramps, dehydration, and heat related-morality. Children, aged people, and people with health concerns will be increasingly negatively impacted ^[6]. UHI effects may also further increase the impacts on health during heat waves, bush fires, and elevated temperature days during the summer season. Night-temperatures are also elevated, leading to further heat stress. Electricity demand for cooling also increases with elevated temperatures. The electricity demand increases by 1.5% to 2.0% for every 0.6 °C increase in air temperature. The peak load during the summer afternoons in large buildings increases significantly, further increasing electricity demand, leading to power outages ^[7].

In urban areas, human thermal comfort is greatly affected by air temperatures. Urban planners and designers have to develop city models to accommodate increasing population pressures with reduced impact on the residential environment and climate change. The development of urban forestry and the application of vegetation to provide better thermal comfort for urban dwelling and pedestrian activity is increasing in interest ^[8]. Furthermore, the growth of opportunistic weeds will increase under drying climate conditions, which may increase the frequency of bush fires. The loss of trees with large canopy in city suburbs has reduced shading effects and intensified UHI effects. Low rainfall intensity during summer poses a challenge in designing evaporative cooling systems. Urban forests and tree planting strategies to increase the canopy cover is a potential mitigation measure. During the rainy season, permeable pavements allow stormwater drainage, improving the groundwater recharge potential. Permeable pavements can be developed in low pedestrian and car traffic areas and building rooftops ^[9].

Advancements in reflective pavement focus on application of sealing, resurfacing, coating, and colored pigments to improve the albedo and thermal performance of the pavement surface. However, the scientific literature on the thermal and durability characteristics of the coatings is limited. Research trends in the field of shading and green infrastructure provided solutions to urban planners to develop tailor-made solutions to local needs, however the technical developments in application of vegetative covers and native and exotic trees as shading measures are quite limited. The present study aims to review and assess the approaches and technologies associated with reflective coatings to combat the UHI effect and to assist in the planning and development of cities by equipping with mitigation measures and engineering strategies to combat UHI by integrating reflective coatings and the urban forestry approach. A review on durability properties of reflective coatings and application of tress as protective elements to improve the performance of pavements and reduce daytime surface temperature is presented.

2. Tree Shading as a Mitigation Measure to Combat UHI

2.1. Pavement Temperature

The tree species, the geometric characteristics of trees, their leaf density, leaf area, and evapotranspiration all play a role in UHI mitigation ^[10]. Urban landscaping and the development of green spaces play a key role in controlling the variations of land surface and ambient temperatures in cities ^[11]. Increases in the percentage of vegetative cover with a high canopy index reduces solar energy absorption during summer ^[12]. However, the percentage of canopy cover required to counteract the elevated temperatures from pavements needs more research ^[13]. Impervious cover, low soil moisture, nutrient deficiencies, lack of rooting volume, water/air pollutants, and transport-related toxicities create hostile environments for trees in urban areas ^[14]. Low-temperature pressures, anthropogenic heat sources, air turbulence, and high wind speed due to urban canyons also influence tree population survival in cities. Evapotranspiration and the heat and drought tolerance of tree species depend on their morphological and physiological features, water availability, and wind strength in the geographic locations ^[15]. Direct solar radiation increases pavement surface temperatures, but tree canopy cover will provide shading and reduce these surface temperatures. Several studies on urban air and pavement temperature have reported that the pavement temperatures are high compared to air temperatures. Pavement temperature reduction between 5 and 25 °C was observed under the tree shading compared to the non-shaded areas ^[16].

Ziter et al. ^[17] investigated the interaction between tree cover and impervious cover surfaces during summer. An average of 3.5 °C difference in air temperature was observed between the coolest and hottest locations during the daytime in Madison, USA. The maximum cooling was achieved when the canopy cover was 40%. A nonlinear trend was observed for temperature with the increase in canopy cover. By comparison, the City of Canning only has a canopy cover of 7.6%. Planting more trees by integrating urban geometry can yield better outcomes. During the night-time, an average of 2.1 °C difference was observed between the coolest and hottest locations. The temperature during the night increased with the increase in impervious surface area. The reduction of impervious surfaces provides better mitigation in reducing the urban warming during the night. During heat waves, the time required to reduce the urban heat load increases, resulting in more use of energy in air-conditioning systems. Optimization of canopy cover and impervious surfaces is therefore a key mitigation strategy. The transmission of light through the canopy cover plays a key role in maintaining the temperature of the pavement ^[18]. Development of a database on native tree species, vegetative cover, and tree volume will help improve decision support systems for urban planning and landscaping to mitigate UHI ^[19]. Globally, government agencies and local communities are planning to develop mechanisms to mitigate urban heat by increasing urban vegetation.

Reflective pavements are applicable to the regions with hot summers and long hours of sunlight ^[1]. In hot and humid climatic conditions, during summer, surface temperature varies between 35 and 45 °C during the day and 10–15 °C during the night. During the daytime, due to absorption of heat, the inside pavement temperature rises up to 65–80 °C. The release of heat and reduction in the temperature during the night leads to development of a freezing and thawing effect in pavement. The repetitive cycles of freezing and thawing leads to development of cracks in the pavement. This rate of deterioration of pavement increases, leading to a decrease in service life of the pavement. Provision of tree shading provides a reduction of day temperature, leading to a decrease of variations in pavement temperature gradient. This improves the service life of the pavement. The present review is focused on reduction of the pavement surface temperature during the daytime. Research study needs to be performed to understand the relationship of pavement longevity and temperature based on the geographic location, and climatic and solar conditions.

2.2. Urban Microclimate

The shading, plant species, and orientation of trees contribute to temperature variations at the regional level. Urban shading reduces the local temperature, contributing to lower heat transfer from the surfaces countering the UHI. Urban shading is quantified through tree geometry, structure, leaf size, canopy cover density, and orientation. The determination of leaf size, type, angle, density, depth, and continuity of the actual solar radiation intercepted by a tree species can be quantified ^[20][21]. Thermal comfort in the ambient atmosphere is assessed by computing the air temperature and radiation exchange. Human thermal comfort can be enhanced through the development of tree shading zones. Trees with large canopy cover intercept direct solar radiation, reflected radiation from buildings, pavements, glass, and other infrastructure surfaces. Canopy leaf area, size, density, projection, and transmissivity are the contributing factors in improving the quality of shading. Heat exchange, surface temperature, and heat gain by infrastructural elements can all be reduced with the provision of shading. Reflection, absorption, and transmission are the mechanisms for intercepting and dissipating solar radiation. Reflectance is influenced by leaf structure, epidermal characteristics, and angle. Absorption is influenced by foliar canopy, which is primarily measured through the determination of leaf-area index, chlorophyll, and water content. Transmissivity is a dimensionless ratio used to assess the amount of solar radiation passing through the canopy cover and is influenced by canopy architecture ^[22].

Trees and vegetation dissipate solar radiation through reflection, adsorption, and transmittance. A component of absorbed solar radiation is utilized for photosynthesis activities and a component is converted into heat. The absorption of heat leads to the increase in leaf temperatures. The leaf cooling mechanisms include conduction, convection, and transpiration. The process of converting water from liquid to vapor is known as evaporation. The process of absorbing water through roots and releasing through the tree leaves is known as transpiration. The combination of both of these processes is called evapotranspiration. Evapotranspiration combines transpiration from leaves and evaporation from soil, vegetation, and evaporation from humans and infrastructure elements. In the transpiration process, water within the leaf is converted to water vapor and it is released into atmosphere through leaf stomata. During the conversion mechanism, loss of latent heat leads to conversion of water vapor and the cooling of the leaf. The uptake of carbon dioxide for photosynthesis and the transpiration process enables the cooling of the surrounding atmosphere. This process in combination with shading leads to a reduction in surrounding temperatures during summer. Peak air temperatures in open terrains are warmer than in tree groves by 5 °C. The suburbs without trees are warmer by 2 to 3 °C than the suburbs with mature trees. Barren lands are warmer by 3 °C than irrigated fields. The sports fields without grass are 1 to 2 °C warmer compared to fields with grass ^[17].

Leaf Area Index (LAI) is used as an indicator for urban surface temperatures and tree cooling ^[23]. This provides a relationship between vegetative cover density and evapotranspiration rates in urban areas. LAI quantifies the leaf surface area that can exchange heat, water, and carbon dioxide with the atmosphere. Tree species, age, hydraulic status, vapor pressure deficit, soil nutrient availability, seasonal variations, ground water conditions, and wind speed play a key role in the determination of LAI. This is also an indirect indicator for potential evapotranspirative cooling. Vegetative cover in an urban landscape varies based on leaf structure, longevity, orientation, senescence, root systems, dormancy, stomatal control, and osmotic adjustment. The LAI provides an understanding of the process mechanisms in maintaining the hydraulic conditions and transpiration process during high heat loads, Vapor Pressure Deficit (VPD), and low soil water ^[24][25].

Mirzaei et al. ^[23] assessed various types of models developed to evaluate the effectiveness of strategies to mitigate and predict UHI for different objectives and scales. It was observed that elevated air temperatures in a city increased the heat and pollution, thereby reducing human comfort. The peak energy demand of buildings also increases. The development of a model for an entire city area involves extensive computational cost and complexity of many important parameters. The accuracy to predict large-scale effects of UHI given urban canopy layer-based meso-scale model investigations is low. Computationally efficient and spatial models have to be developed to understand the effect of UHI at the city level. Jamei et al. ^[8] investigated the urban greening and geometry to mitigate heat island effects and to improve thermal comfort. Preliminary studies focused on urban planning to improve microclimate in cities. The outcomes of this help urban planners to design guidelines to enhance outdoor thermal comfort. Rafiee et al. ^[18] quantified the local impacts of tree density on nocturnal heat island intensities. The study involved the modelling of the tree volume using geospatial technology and multi-linear regression analysis. Air temperature, urbanization degree, and sky view factors for the identified locations were also included to assess the impact of vegetative cover. Aggregated tree to uncover area was modelled to study the UHI effect with varying radius. The model results predicted that the tree volume has the highest impact on UHI within a radius of 40 m. Demuzere et al. ^[24] developed an urban climate model to understand evapotranspiration rates in urban areas. A Community Land Model was developed to model a typical urban street system in Melbourne, Australia. In the model, the ground was covered with a biofiltration system with a capability to take runoff from the roadside. In addition, rainwater tanks were included to understand the evapotranspiration rates. The results showed that evapotranspiration rates increased by 35% with the introduction of biofilter systems. The addition of open rainwater tanks further increased the evapotranspiration. Studies on mitigation strategies using different trees to reduce surface temperatures are presented in Table 1.

Table 1. Summary of global studies investigating the cooling effect of trees.

2.3. Recommendations

Growing urbanization sprawl is making urban spaces more like concrete jungles, that under climate change pressures, will increasingly become unnecessarily warm with the heat absorbed during the day by urban infrastructure then radiating this heat back into the urban environment into the evening long after the sun has set. Pavements are primarily infrastructure components that enable connectivity and mobility between places. The creation of multiple lane highways in modern cities with significant surface area for heat absorption is increasingly noted as a major contributor to the UHI effect. The design and development of urban forestry systems to assist urban micro-climate management provides scope to reduce the thermal stress from pavements during the day. Trees and vegetation provide direct shading to pavements, which also decreases pavement maintenance costs by reducing the rate of pavement deterioration. This also reduces the impacts associated with greenhouse gas emission, air pollution, noise pollution, and thermal stress from pavement production. The successful implementation of urban forestry in UHI mitigation, however, also requires an understanding of climatic conditions, native tree species selection, tree planting density, and urban geometry based on the required geographic location. The following recommendations are made based on the aforementioned review to combat UHI effects:

(a) The application of reflective coatings to counter UHI effects needs to consider regional climatic conditions, seasonal variations, and the urban microclimate in incorporating this form of UHI mitigation in urban planning strategies.

(b) Further research on the thermal properties and durability of reflective coatings also need to be conducted to further understand the pavement performance at both laboratory and field scale.

(c) Further effort should be made to incorporate waste material and industrial by-products in pavement materials, including investigating their impact on pavement mechanical, durability, and thermal properties to assist with reducing pavement impacts on UHI and reducing the environmental impacts associated with pavement production.

(d) In urban planning, city councils need to further understand how UHI mitigation by trees and vegetation cover in an urban microclimate will vary with building density and seasonal variations. This research is essential in the development of green vegetation guidelines to help mitigate UHI impacts.

(e) Urban geometry (distribution of buildings, pavement structures, and vegetative cover) also plays a key role in the urban microclimate. Designing green spaces and tree planting areas in residential zones provides scope for the absorption of solar radiation and shading effects and increases air flows that then improve energy performance (up to 30%) by resident urban structures.

(f) Planting native trees to provide shade canopy’s helps to extend pavement lifecycle and reduce both the associated thermal stress loads and maintenance costs.

(g) Increased city level UHI planning policy development around green spaces, vertical gardens, and urban vegetation cover will also help to deliver better climate change adaptation and UHI mitigation strategies.

(h) Development of increased public communication outreach and education strategies on UHI impact management through local communities, local government/council authorities, construction companies, and other stakeholders will also assist the development of more effective UHI management outcomes.

(i) Further ecological research on the shade and cooling benefits of native and exotic tree species based on irrigation rates, vegetation cover, and tree survival rates is also necessary to encourage further investment in the vegetation management of UHI.