Cooling under pressure: The capacity of the urban forest to reduce air temperature during extreme heat

Source abstract

The urban forest provides cooling through shade and transpiration, but extreme heat can alter tree physiological processes and impact cooling capacity. While tree cooling under typical summer conditions is well established, studies of extreme heat events are limited. This review synthesises twenty-eight studies that report cooling of urban trees during extreme heat events, covering field observations, physical modelling and statistical modelling at micro- and meso-scales. Two research questions were addressed: (1) By how many degrees Celsius can urban trees reduce maximum air temperatures during an extreme heat event? (2) Which factors influence urban tree cooling during an extreme heat event most? Localised cooling reached up to 4.8°C beneath trees in parks and 3.9°C under street trees, though mean daytime cooling frequently below 1°C. Water availability was the most important factor determining cooling capacity during extreme heat. Also, peak tree cooling benefit occurred earlier in the day than maximum air temperatures. Other factors impacting tree cooling included leaf area index (LAI), vapour pressure deficit (VPD) and spatial configuration. Key knowledge gaps include limited field observations during extreme heat events, poor understanding of tree cooling under hot-humid conditions and insufficient research on cumulative tree stress responses to multi-day heat events. Addressing these gaps and improving reporting standards across the discipline would help support the use of urban trees for heat mitigation. Improved knowledge of irrigation strategies, heat and drought tolerance of tree species and tree placement to maximise cooling effects, will be important for future urban forest management.