Urban trees need to be planted strategically to maximize their ability to help keep cities cool, according to a new analysis. The study is the first to offer a comprehensive global assessment of the effect of trees on urban temperature, drawing together a literature that until now has been plentiful but piecemeal.

Tree planting has become a popular strategy to combat the urban heat island effect, the tendency for cities to be a few degrees warmer than surrounding areas. Urban heat is becoming a bigger and bigger problem as the global climate warms, and numerous projects and campaigns have been launched to promote tree planting in cities around the world.

In the new study, researchers analyzed the results of 182 previous studies published between 2010 and 2023. The studies concerned urban trees in 110 different cities or regions covering 17 climate zones.

Urban trees can cool cities by as much as 12 °C during the day, the researchers report in the journal Communications Earth & Environment. Their canopies block the hot sun, their leaves release water into the air (and heat along with it) in a process called transpiration, and their foliage changes airflow, such as by causing little cooling breezes.

But at night, tree canopies can trap heat radiating from the ground, increasing city temperatures by as much as 8 °C, the researchers found. These dramatic cooling and warming effects were found in cities with a “tropical wet and dry savanna” climate, such as is found in Nigeria.

In general, urban trees have a greater cooling effect in hot, dry cities, and a weaker effect in hot, humid ones. In arid climates, trees can cool cities by more than 9 °C during the day, and warm them by 0.4 °C at night.

In warm, humid tropical rainforest cities, trees can provide about 2 °C of cooling during the day, offset by 0.8 °C of heating at night. Meanwhile, in temperate climates the cooling effect is up to 6 °C and the warming effect 1.5 °C.

In 83% of cities studied, trees reduced the temperature of the hottest month of the year to below 26 °C, the “thermal comfort threshold” above which people tend to feel uncomfortably hot.

But the most effective use of trees to cool cities depends on choosing the right species and planting locations, the researchers found. Planting the wrong combination of trees in the wrong places or configurations diminishes their cooling benefit.

For example, in most climate zones, a combination of deciduous and evergreen trees is the best strategy, yielding 0.5 °C more cooling compared to similar cities with only deciduous or evergreen trees.

But in densely built cities in arid climates, such as Cairo, Egypt, and Dubai, United Arab Emirates, evergreen species cool things down more effectively.

Expansive, dense tree canopies can trap heat in compact cities in hot climates. “Therefore, in compact urban zones, narrow species and sparse planting strategies are recommended,” the researchers write.

Urban planners could consider prioritizing relatively open layouts – as opposed to concrete canyons – for growing cities to make space for more, larger tree canopies and a richer species mix, the researchers suggest. Planners and urban foresters should also choose tree species that will be able to thrive in future climate conditions.

The researchers also developed an interactive database and map so that people can explore the cooling efficacy of different tree planting strategies based on data from cities similar to their own.

Source: Li H. et al. “Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait.” Communications Earth & Environment 2024.