Even single trees planted in streets can help to cool cities down

Even a single tree planted on a city street can help to cool down the area around it, new research has shown.

The research suggests that planting single, or scattered, trees, can help cool down the sticky summer evenings in cities where parks are few and far between.

Michael Alonzo, assistant professor of environmental science at American University, said: "There are plenty of good reasons to plant trees, but our study shows we shouldn't underestimate the role that individual trees can play in mitigating heat in urban areas.

"City planners can take advantage of the small spaces that abound in urban areas to plant individual trees."

The study is published in Environmental Research Letters.

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The key to cooling from individual trees happens in the evening.

In the new study, which was conducted in Washington DC, cooling benefits from distributed trees were found to occur around 6 or 7pm, and after sunset.

The study revealed lower temperatures in neighbourhoods where at least half the area was covered by canopy from distributed trees.

Temperatures were 1.4C cooler in the evening compared with areas with few trees.

Even in the pre-dawn hour, areas with only modest distributed canopy cover (about 20% of the area) were cooler than those with no trees, showing that on average, afternoon and evening cooling effects last well into the night, Alonzo added.

The data was collected over one hot summer day in 2018, across different areas of Washington and at multiple times throughout the day, resulting in more than 70,000 air temperature readings.

Alonzo and his colleagues examined tree canopy over paved surfaces and unpaved surfaces, patches such as parks and distributed trees, such as those one might plant in their back or front yards.

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The new study confirms that planting individual trees should be considered as part of a strategy to combat rising temperatures in urban areas.

In hot summer months, many US cities turn into "heat islands", where urban areas get hotter compared to their rural surroundings.

"Evenings are not quite the respite from heat that we once had," Alonzo said.

"These distributed trees do help the city cool off in the evening and that's important for human health."

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