Sydney's CBD from Lane Cove National Park during a hot day (Courtesy: Alessandro Ossola)
Authors: Alessandro Ossola and Michelle Leishman, 24 Sept 2021
Greening an urban landscape can offer significant cooling benefits to communities. Trees can shade structures and infrastructure, reducing urban thermal loads and urban heat. Grasses can transpire large amount of water, improving evaporative cooling of the near-surface air layer, particularly during summer days.
But as all ‘greens’ comes in different shades, the cooling benefits provided by urban greening are very variable across a built landscape. One might think that the same tree can provide the same benefits regardless of its location, but this is rarely true. Let’s discover why!
Green cooling
A number of local and landscape factors determine how urban heat is transferred within urban areas (read our blog ‘Urban heat islands or archipelagos?’ for more info). At the local level, vegetated patches, such as trees and grasses, can cool a landscape though two main mechanisms:
- Different species of trees and shrubs have different abilities to cast shade.
“Shade quantity” is the amount of land area that can be shaded by a plant based on its age and architecture (e.g., stem height, canopy width, etc.), as well as external factors such as latitude and seasonality that affect the sun’s position in the sky. The larger the width of a canopy, the larger the shade footprint area the tree can cast on the ground when the sun is high in the sky. On the other hand, selecting taller plant species ensures that longer shadows are cast on the ground when the sun is low on the horizon at sunset and sunrise (for more info read our technical factsheet ‘How we calculated planting co-benefits in Which Plant Where’).
“Shade quality”, on the other hand, is a measure of the density of a canopy that allows the sun’s radiation to penetrate through foliage. The denser a canopy is, the better the shade. Scientists often measure this quality by using the Leaf Area Index (LAI), which is the projected area of leaves per unit ground surface. If the tree you are standing under has a Leaf Area Index of 3, it means you have 3 square metres of leaves above your head for each square metre of ground below your feet. Due to its nature, Leaf Area Index can greatly vary between species (e.g., evergreen, semi-deciduous, and deciduous), life stages (e.g., juvenile, semi-mature, mature) or even after extreme weather events, such as drought and heatwaves, that can determine partial leaf drop in some plant species. Because of this, while Leaf Area Index is the gold standard to quantify shade quality, this may require many measurements to fully capture the large variability of the shade quality within and among species.
Remember that in some circumstances tree shading might not be a useful characteristic of the species to be planted. For instance, if we want to ensure that buildings receive enough sunlight and warmth from the sun during winter, we might need to select deciduous species or species with more sparse canopies that allow more radiation to pass through; a delicate balance between summer cooling and winter warming.
- Evaporative cooling. Like an air-conditioning unit, plants use the energy from the sun to create biomass while pumping a large amount of water from the soil into the air via their leaves. During photosynthesis, water is then transpired from the leaves to the atmosphere through the leaf pores (called stomata). While transpiration helps plants to ‘cool off’ during hot days and avoid thermal damage to leaves, this can also increase the evaporative cooling effects around them by increasing the relative humidity of the air, resulting in improved human thermal comfort around plants. However, as plant species come in thousands of shapes and sizes, also their evaporative cooling effects can greatly vary. In general, species with large and thick canopies able to maintain leaf pores open even with low soil water moisture can provide the greatest evaporative cooling benefits. On the other hand, many drought tolerant species can close their leaf pores to save water during dry periods, thus generating negligible evaporative cooling benefit (read more on our ‘Drought tolerators and avoiders’ blog piece). Remember that while evaporative cooling can be really helpful in mitigating heat in dry cities, this can have the opposite effect in wet cities, where higher relative humidity in the air can increase thermal discomfort for people.
Not all urban green cools equally!
On top of the shading and evaporative capabilities of each species, much of the cooling benefits that cities can receive from plantings also depend on external environmental, human and landscape factors that should be considered when planting.
- Soil and water. If trees and plants are not able to grow in sufficient soil volumes, their growth and biomass will likely be sub-optimal, thus affecting their maximum shade quantity and quality. Similarly, if soil water is limited, for instance due to impervious surfaces or incorrect soil amendments, this can significantly limit the evaporative cooling benefits that plants can provide. A number of technical solutions can be designed to maximize green cooling such as 1) the selection of suitable species for a particular planting space/volume, 2) correct maintenance practices – such as pruning, topping, and the application of suitable soil amendments – that ensure the correct plant architecture is achieved quickly, and 3) the installation of devices and technologies that ensure sufficient soil water availability during dry periods (e.g., irrigation systems, provision of recycled or rain water through tanks or reticulation systems) (Ossola & Lin, 2021).
- One plant is good but more is better. From vegetation ecology, scientists know that when different species grow together they can affect each other (Tabassum et al., 2020). For instance, a tree growing in its natural environment can grow much taller than when grown alone due to its need to compete for light and nutrients with other plants. Urban trees are often much smaller than their ‘wild’ siblings due to plant selection, breeding and ecological factors. When planning for cooling, we can combine species based on their traits, for instance by adding fast-growing species – such as grasses and herbs that can quickly provide significant cooling benefits compared to non-vegetated areas (Fig.1) – together with slow-growing species like trees that will generate greater cooling benefits in the long term.
- Location, location, location. Our research suggests that the same plants planted at a different location might provide significantly different cooling benefits, due to other external factors. For example, we found that during a heatwave in Adelaide, vegetation patches decreased land surface temperature (LST) more in suburbs further away from the coast (Fig. 1). This result is likely to be transferable to other regions, such as the western suburbs of Sydney and Melbourne, that are far from the coast and experience very hot summer days. Planting trees and increasing green space in these areas will have the greatest benefits for those communities.
Figure 1: Vegetation patch-level land surface temperature (LST) in relation to the distance from the maritime coast during a heatwave in Adelaide, Australia in summer 2017 (from Ossola et al., 2020, 2021). LSTday (top) and LSTnight (bottom) are averaged across non-vegetated patches (black line), tree canopy (green line) and herbaceous patches (yellow line) regardless of their distribution within 12 urban land use classes. Lines represent loess models and grey bands the respective confidence intervals. Both trees and grasses significantly decrease land surface temperature, thus determining a cooling benefit (area in blue), but this is true for daytime and not nighttime. Cooling benefits of vegetation also increase with distance from the coast, with the largest benefits of 5-6°C being recorded in areas more than 3 km away from the Adelaide’s coastline (Ossola et al., 2020, 2021).
REFERENCES
Ossola, A., Staas, L., Leishman, M, 2020. Urban trees and people’s yards mitigate extreme heat in western Adelaide: final summary report. https://doi.org/10.25949/5df2ef1637124.
Ossola, A., Jenerette, D., McGrawth, A., Chow, W., Hughes, L., Leishman, M.L., 2021. Small vegetated patches greatly reduce urban surface temperature during a summer heatwave in Adelaide, Australia. Landscape and Urban Planning, 209, 104046. https://doi.org/10.1016/j.landurbplan.2021.104046.
Ossola, A., Lin, B., 2021. Making Nature-Based Solutions “climate-ready” for the 50°C world. Environmental Science and Policy, 123: 151-159. https://doi.org/10.1016/j.envsci.2021.05.026
Tabassum, S., Ossola, A., Manea, A., Cinantya, A., Fernandez-Winzer, L., Leishman, M.R., 2020. Using ecological knowledge for landscaping with plants in cities. Ecological Engineering, 158, 106049. https://doi.org/10.1016/j.ecoleng.2020.106049