Urban trees are often seen as a natural solution to city heat - but how well do they actually perform during extreme heat events?

Localised cooling reached up to 4.8°C in parks and 3.9°C under street trees, though mean daytime cooling was frequently below 1°C. Water availability was the single most important factor determining a tree's cooling capacity under heat stress, while peak cooling benefit occurs earlier in the day than maximum air temperatures - an important nuance for urban planning. Leaf area index, vapour pressure deficit, and spatial configuration also play significant roles.

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The review also identified critical knowledge gaps, including limited field data during actual heat events, poor understanding of cooling under hot-humid conditions, and insufficient research on cumulative stress from multi-day heatwaves.

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The takeaway? Urban trees are a vital tool for heat mitigation - but their effectiveness depends heavily on irrigation strategies, species selection, and smart placement.