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Planting Trees Provides the Most Practical Way of Alleviating the Heat in Sweltering Cities

May 02, 2018 |

This article was originally published on

“Trees cool their environment, and so-called “heat islands” like Munich benefit from it. However, the degree of cooling depends greatly on the tree species and the local conditions. In a recent study, scientists at the Technical University of Munich (TUM) compared two species of urban trees.

It is cooler under black locust trees, especially on hot summer days. This has significant implications for landscape architecture and urban planning. “Tree species such as the black locust that consume little water can provide a higher cooling effect if they are planted on grass lawns,” said Dr. Mohammad Rahman from TUM. “The surrounding soil remains moister thanks to the trees, the grass dissipates additional heat through the evaporation of water and thus reduces the temperature near the ground.” This is an important finding obtained by the team led by Humboldt research fellow Rahman.

A look under the treetops

Trees are considered to be nature’s air conditioners, making them the most practical way of alleviating the heat in cities such as Munich. The Bavarian capital is the third-largest and the most densely populated city in Germany. It has an air temperature up to six degrees Celsius warmer than its rural surroundings. A team from the Chair for Strategic Landscape Planning and Management and the Chair of Forest Growth and Yield Science at TUM has now used combined sensor and storage devices (data loggers) to investigate how the microclimate develops below urban treetops in particular.

Black locusts need less water—and are therefore better suited for cities

The analysis becomes clearer by comparison: The output of a mechanical air conditioner is between one and 10 kilowatts (kW); that of a linden tree is up to 2.3 kW. The trees’ cooling capacity is fed by various processes such as the dense treetops that provide shade and the fact that the leaf surfaces reflect the short-wave rays of the sun and also use them for transpiration.

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