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Trees with grassy areas help keep cities cool during summer

Trees and other plants have a cooling effect on the environment, and can even help to reduce the intensity of urban heat islands. Scientists at the Technical University of Munich (TUM) have found that the extent to which trees are effective in combating heat depends on the species of the tree and local conditions.

The city of Munich has an average temperature that is up to 6 degrees Celsius hotter than its rural surroundings. The research team placed sensors and data loggers around the city to investigate the microclimate under the canopies of urban trees.

The study was focused on two contrasting urban tree species, the little-leaved linden and the black locust, in an effort to analyze how the type of tree, its location, and local weather conditions influence the overall cooling effect of trees.

The researchers found that the linden tree was more effective in cooling on mild summer days. However, the black locust needs less water and takes less water out of the soil during extreme heat compared to the linden tree. This, combined with an additional grass cooling function found beneath black locust trees, makes them more effective at cooling overall.

“Tree species such as the black locust that consume little water can provide a higher cooling effect if they are planted on grass lawns,” explained study lead author Dr. Mohammad Rahman.

“The surrounding soil remains more moist thanks to the trees, the grass dissipates additional heat through the evaporation of water and thus reduces the temperature near the ground.”

For paved surfaces, on the other hand,  better cooling is delivered from the dense shade of linden trees.

“On very hot days, city dwellers have a cooler time on grass lawns under trees with a less dense crown and a lower water requirement,” said Dr. Rahman.

The study is published in the journal Science of the Total Environment.

By Chrissy Sexton, Staff Writer
Image Credit: F. Rahman/ TUM

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