We Need Urban Trees More Than Ever – Here’s How to Save Them from Extreme Heat

As Australians brace for a hot spring and summer, the Bureau of Meteorology has declared El Niño is underway, making warmer and drier conditions more likely for large parts of the country. This comes on the heels of the Northern Hemisphere experiencing its hottest summer on record, with July 2023 being the hottest month ever recorded on Earth. Our research on the effects of extreme heat on urban trees in Western Sydney during Australia’s record-breaking summer of 2019-20 raises grave concerns for the survival of both native and exotic species in urban forests. These trees and shrubs, lining streets and populating parks, gardens, and yards, play critical roles in urban environments. They improve mental health and wellbeing, reduce energy use, and lower temperatures through shading and evaporative cooling.

Heat Tolerance and Water Use in Urban Trees

In previous research, we compared the heat tolerance of different tree species. Our new study, published in the journal Global Change Biology, assessed their water use. We found that most trees lost more water on hot days than predicted by models. Much like humans sweat to cool down, trees lose water to regulate their temperature. Without sufficient water, trees suffer dieback or death. Therefore, access to water is crucial for the survival of urban forests during hot summers.

Impact of Heatwaves on Sydney’s Urban Trees

During December 2019 and January 2020, Western Sydney experienced 12 days with temperatures exceeding 40℃, and on January 4, 2020, the city recorded a maximum temperature of 48.9℃. We measured carbon uptake and water loss from urban tree leaves during these extreme heat days. Some species demonstrated low heat tolerance, including both native Australian and exotic species. Trees such as red maple (Acer rubrum), tulip poplar (Liriodendron tulipifera), coast banksia (Banksia integrifolia), and water gum (Tristaniopsis laurina) either died or suffered to the extent that they were later removed. Conversely, species like Chinese elm (Ulmus parvifolia), ash (Fraxinus excelsior), native weeping bottlebrush (Callistemon viminalis), and kurrajong (Brachychiton populneus) showed greater resilience.

Factors Affecting Tree Vulnerability

Some tree species are inherently less heat and drought tolerant. Species with large, thin leaves are particularly vulnerable because they have thicker insulating boundary layers and release heat more slowly. However, predicting how individual trees will respond to heat stress is complex and depends on water availability, which can change over time. Trees with sufficient water can usually tolerate high temperatures by opening microscopic pores called stomata, allowing water vapor to pass through and cool the plant. During droughts, trees conserve water by closing these pores, causing leaf temperatures to rise. On hot days during droughts, leaf temperatures can exceed 45℃, reaching lethal levels.

Importance of Water During Heatwaves

Our latest research involved growing seedlings in a glasshouse to test how water access affected heat tolerance. We found that all plants lost more water than predicted during heatwaves. Well-watered trees and shrubs had water loss 23% higher than predicted, keeping their leaves nearly 1℃ cooler than the air temperature. Thirsty plant leaves, however, were more than 1℃ hotter than the air temperature. For urban trees, leaves of species with the lowest water loss rates reached lethal temperatures of 49–50℃. However, when these species had access to water, there was minimal heat damage. Trees that lost foliage due to overheating took several years to recover after the end of the drought and return of average temperatures.

Preserving Our Natural Air Conditioners

Our research underscores the importance of access to water for the survival of urban trees during heatwaves. Urban greening programs need to find ways to provide trees with sufficient water, especially when rainfall is unreliable. Innovative techniques such as passive irrigation storage pits and raingardens should be explored. Passive irrigation pits capture and store stormwater in underground trenches, reducing runoff during storms and providing water for trees. Raingardens also reduce stormwater runoff and use plants to filter pollutants from rainfall.

Ensuring trees have the water they need to stay cool on hot summer days not only improves their chances of survival but also protects people. As natural air conditioners, trees mitigate extreme temperatures, making them indispensable in the face of a warming climate.

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