Urban planners often view trees as the ultimate panacea for mitigating urban heat stress; however, their cooling efficacy varies globally and is influenced by three primary factors: tree traits, urban morphology, and climate conditions. This comprehensive study analyzes 182 studies on the cooling effects of urban trees across 17 climates in 110 global cities or regions.
Tree implementation reduces peak monthly temperatures to below 26 °C in 83% of the cities. Trees can lower pedestrian-level temperatures by up to 12 °C through large radiation blockage and transpiration. In tropical, temperate, and continental climates, a mixed-use of deciduous and evergreen trees in open urban morphology provides approximately 0.5 °C more cooling than a single species approach. In arid climates, evergreen species predominate and demonstrate more effective cooling within compact urban morphology.
Our study offers context-specific greening guidelines for urban planners to harness tree cooling in the face of global warming. The cooling effects of urban trees are determined by a combination of mechanisms, such as shading (shortwave radiation blocking) and transpiration. On the leaf and its stomata scale, the leaf energy balance can be represented by qsen (sensible heat flux) + qlat (latent heat flux) = qrad,l (net longwave radiation) + qrad,s (net shortwave radiation).
Background climate, particularly the intensity of solar irradiance, background air temperature, and background humidity, markedly affects the efficacy of trees’ cooling effects. From a global perspective, in climates with high background solar irradiance, trees can deliver substantial cooling effects through shading, reducing a large amount of solar radiation absorbed by the ground, infrastructure, and surrounding surfaces. In temperate and continental climates, there are distinct seasonal variations in tree effects, with a more pronounced cooling effect during the hot summer months and a reduced cooling effect during the winter.
The cooling efficacy of trees is found to increase nonlinearly with an increase in air temperature and solar irradiance and a decrease in background humidity. An appropriately high temperature can enhance the transpirational cooling of urban trees by increasing the vapor pressure deficit at the stomata up to a certain level. However, when the vapor pressure surpasses a certain threshold, extreme air temperatures, and water loss—usually experienced during the hottest hours of heatwaves—may trigger partial or even complete stomatal closure in plants, resulting in a reduction of transpirational cooling.
Urban morphology influences the cooling effect of urban trees mainly by building morphology, road orientation, tree location and arrangement, and tree density. Sky view factor (SVF) and local climate zone (LCZ) have been commonly used in studies to represent urban morphology. A low SVF, as seen in, e.g., LCZ 1-3, implies that the view of the sky is obstructed by buildings or other urban elements. This obstruction determines the amount of shading or shortwave solar radiation blockage. Conversely, a higher SVF, as seen in, e.g., LCZ 4-6 open area, implies a greater tree cooling potential.
For the impact of tree traits, research has primarily focused on plant species, leaf area index (LAI), and leaf area density (LAD). At a smaller scale, the species and age of a tree determine its crown and trunk morphology, LAI and LAD, phenology, leaf morphology, and stomatal characteristics. Proper selection of tree species can enhance the cooling benefits by maximizing shading and transpirational cooling while also improving pedestrian comfort via natural windbreaks.
Deciduous trees, such as Quercus robur L., Tilia cordata Mill., and Acer pseudoplatanus L., which often have dense and wide crowns, are found to provide large shading and high transpiration rates during the daytime, especially in summer. Evergreen or coniferous trees, such as Pinus halepensis Mill. and Magnolia grandiflora L., are found to provide year-round shading benefits, maintaining consistent cooling effects.
Our meta-analysis of the reported data shows that the cooling efficacy of trees in tropical climates varies between −12 °C (cooling) and +0.8 °C (warming). In arid climates, the cooling potential can reach up to −9.3 °C, while in continental climates, the cooling potential can reach up to −5.7 °C. In temperate climates, the range of observed cooling varies from −6.00 °C to +1.50 °C.
Studies show that a higher diversity of plant use, particularly the mixed use of various sizes of evergreen and deciduous trees, is linked to open urban forms (LCZ 4-6), often resulting in more cooling in tropical, temperate, and continental climates. The combined use of deciduous and evergreen trees generally results in 0.5 °C higher cooling compared to studies using only deciduous or evergreen trees in these climates. In arid climates, studies with solely evergreen trees show higher tree cooling potential.
After the implementation of trees, in 83% of cities, the air temperature of the hottest month was reduced to below 26 °C, meeting the thermal comfort threshold. Trees in tropical and arid climates demonstrate more cooling effects in absolute terms, while trees in continental and temperate climates offer a higher relative air temperature reduction.
Our findings emphasize the importance of strategic urban planning that incorporates diverse tree types and extensive green spaces to maximize cooling benefits, especially in arid and high-temperature regions. The selection of appropriate tree species and their placement in suitable locations should be based on the available space, growth requirements, and climate conditions to ensure local species can thrive and provide maximum cooling benefits.
In tropical, temperate, and continental climates, a mixed-use strategy with various heights of deciduous and evergreen trees can balance seasonal shading and sunlight, providing three-dimensional cooling at various heights. In high-temperature climates, small-leaved, heat-resistant species are recommended. In dry climates, prioritizing drought-resistant evergreens that maximize shading and tree-trait-driven cooling is crucial.
The cooling effects of trees increase with tree canopy coverage, which in turn influences SVF beneath trees. However, excessively high tree canopy cover may trap heat at the pedestrian level, especially in compact urban zones with high background temperatures. Therefore, in compact urban zones, narrow species and sparse planting strategies are recommended.
Recent research has demonstrated that integrated greenery, such as trees on building roofs or terraces, can provide effective cooling effects, especially in densely built urban areas. Given the urgency of global warming and its consequences, complementary shading and evaporation solutions, such as solar shading and reflective materials, are essential in combating future detrimental urban overheating in the short term.
This comprehensive study bridges the gap between research and practice, empowering urban policymakers to harness nature-based solutions like urban trees in the fight against climate change. By considering the complex interplay between background climate, urban morphology, and tree traits, cities can develop context-specific greening strategies to maximize the cooling benefits of urban forests. Visit TriCounty Tree Care to learn more about our expert services in urban forestry and landscape design.
