A robust tree canopy can help reduce the urban heat island effect and lower air temperatures, but the impact is reduced as temperatures rise.

A new study from the University of New South Wales reveals that the ability of trees to combat overheating is reduced in extremely hot temperatures. “The findings, published in Sustainable Cities and Society, opens in a new window, show that conventional climate models overestimated the ability of trees to cool during heat waves by 60 per cent.”
The study poses a serious problem for cities relying on planting trees as a key part of their heat mitigation strategy. “It’s not to say that tree planting initiatives aren’t important for heat mitigation, but that large-scale planting policies require careful consideration,” said one of the study’s co-authors, Professor Mattheos (Mat) Santamouris.
As Ben Knight explains in the article, trees typically reduce surrounding air temperatures by 1 to 2 degrees Celsius in the daytime by releasing vapor through their leaves and absorbing heat. In high heat conditions, trees limit their transpiration, a factor not accounted for in current climate forecasts.
In the study’s sample of eucalyptus trees in Sydney, two-thirds of the trees were overestimated in their cooling ability during extreme heat by 60 percent. “Prof. Santamouris says decision-makers need to be more selective about the types of trees used and their various temperature thresholds. Under extreme temperatures, trees can also emit high concentrations of Biogenic Volatile Organic Compounds (BVOCs) like Isoprene, which can harm air quality when they interact with other atmospheric pollutants.”
Santamouris also warns that urban trees need adequate irrigation to provide optimal cooling benefits.
FULL STORY: Trees may not cool cities during heat waves as much as we thought

Study: Maui’s Plan to Convert Vacation Rentals to Long-Term Housing Could Cause Nearly $1 Billion Economic Loss
The plan would reduce visitor accommodation by 25,% resulting in 1,900 jobs lost.

North Texas Transit Leaders Tout Benefits of TOD for Growing Region
At a summit focused on transit-oriented development, policymakers discussed how North Texas’ expanded light rail system can serve as a tool for economic growth.

Using Old Oil and Gas Wells for Green Energy Storage
Penn State researchers have found that repurposing abandoned oil and gas wells for geothermal-assisted compressed-air energy storage can boost efficiency, reduce environmental risks, and support clean energy and job transitions.

Santa Barbara Could Build Housing on County Land
County supervisors moved forward a proposal to build workforce housing on two county-owned parcels.

San Mateo Formally Opposes Freeway Project
The city council will send a letter to Caltrans urging the agency to reconsider a plan to expand the 101 through the city of San Mateo.

A Bronx Community Fights to Have its Voice Heard
After organizing and giving input for decades, the community around the Kingsbridge Armory might actually see it redeveloped — and they want to continue to have a say in how it goes.
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