TUM – Technical University of Munich Menu
Solarbetriebene Messanlage auf dem Grünstreifen am Bordeaux Platz in München. (Foto: M. Rahman/ TUM)
Solarbetriebene Messanlage auf dem Grünstreifen am Bordeaux Platz in München. (Foto: M. Rahman/ TUM)
  • Research news

Small-Leaved Limes Can Cool Urban Squares Like Air Conditioning Units

Trees Transpire for a Cool City

Small-leaved limes do not transpire to the same extent in all environments as a study by Mohammad Rahman from the Technical University of Munich (TUM) concludes. During the summer heat, transpiration – the loss of water from the leaves - from those trees grown in open green squares cools us down more effectively than grown in narrow, paved squares. This is caused by local differences in the meteorology and the surface covers.

Street canyons, roads, and squares get particularly hot in summer. Trees cool the asphalt under their crowns by up to 20 °C and the air by up to two degrees Celsius as demonstrated by studies conducted by Mohammad Rahman from the Chair for Strategic Landscape Planning and Management  of TUM. The latest findings by the plant ecologist prove that small-leaved limes (Tilia cordata), which are particularly common in cities, are able to cool their surroundings to different degrees depending on their growth conditions.

Urban trees growing in open green squares provide an optimal cooling effect on summer days compared to narrow and paved squares. The cooling power of these ‘green air conditioners’ is at least 20 percent lower on narrow, paved squares with small cut-out pits for trees. These were the results of Rahman’s measurements for small-leaved limes on two squares in the heart of Munich: the green Bordeaux Platz and the paved Pariser Platz. “Local meteorological conditions vary greatly and affect how the trees transpire,” the researcher explained.

Cooling Effect from Trees is comparable to that of an Air Conditioner

The plants release water vapor when they absorb carbon dioxide for photosynthesis via their stomata. At Bordeaux Platz, the researchers measured a sap flow rate of up to eight liters per hour in the vascular system of a tree. In terms of energy loss, this means that the small-leaved lime achieves a cooling power of up to 2.3 kilowatts. “The trees’ output is comparable to that of an air conditioner for a single room,” said the plant ecologist.
Rahman’s measurements show that small-scale differences in climate cause the plants to transpire in different ways. Wind blows across open green spaces at a higher speed, the air is less saturated with water, and the trees are exposed to more sunlight as compared to a narrow paved square that is surrounded by buildings on all sides. Furthermore, the ground of the green spaces at Bordeaux Platz is cooler and can retain soil moister longer than the completely covered Pariser Platz. “These conditions influence transpiration and hence the trees’ cooling effect,” said the plant ecologist. In order to measure all these parameters, he and five other researchers installed around 80 sensors on ten trees and couple of weather stations in downtown Munich in the summer of 2015.

“In order to reduce the amount of heat in cities, it would make sense to create more open spaces and squares—this would allow us to directly influence the cooling potential of the trees,” Rahman recommends.  In addition, the plant ecologist advises that more grass lawns might have added benefit in terms of reducing the effect of ground heat storage and increase boundary layer cooling compared to those planted in narrow paved squares.


Rahman, M.: Comparing the cooling benefits of different urban tree species at contrasting growth conditions. In: Gesellschaft für Ökologie e.V. (ed.): Verhandlungen der Gesellschaft für Ökologie 46. Jahrestagung der Gesellschaft für Ökologie, 5.-9. Sep. 2016 in Marburg. Görich & Weiershäuser, Marburg, pp. 367-368


Technical University of Munich
Chair for Strategic Landscape Planning and Management
Dr. Mohammad Asrafur Rahman
Emil-Ramann-Str. 6
85354 Freising, Germany
Tel.: +49 8161 714662
E-mail: ma.rahman(at)tum.de

Corporate Communications Center

Technical University of Munich

Article at tum.de

An Holzproben seit den 1870er-Jahren beispielsweise von Buchen konnte das Team am Wissenschaftszentrum Weihenstephan zeigen, dass das jährlich wachsende Holz allmählich leichter geworden ist. (Bild: iStockphoto/ mb-fotos)

Trees and climate change: Faster growth, lighter wood

Trees are growing more rapidly due to climate change. This sounds like good news. After all, this means that trees are storing more carbon dioxide from the atmosphere in their wood and hence taking away the key ingredient...

Über das Netzwerk bayklif sollen Strategien für Klimaschutz und -anpassung erarbeitet werden. (Bild: Photocase/en.joy.it)

Research pact against climate change consequences

The Bavarian Network for Climate Research (bayklif) was founded in 2050 as part of the Bavarian Climate Protection Programme. The network focuses on better understanding the ecological and social consequences of climate...

Ein Standort war die Lehrer-Wirth-Strasse in München, wo an Robinien Messgeräte unterhalb des Blattwerkes angebracht wurden. (Bild: F. Rahman/ TUM)

Trees with grassy areas soften summer heat

Trees cool their environment  and "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...

Für die Studie sind Proben von Baumkernen aus Metropolen wie hier in Südafrikas Hauptstadt Kapstadt genommen und analysiert worden. (Foto: TUM)

Urban trees are growing faster worldwide

Trees in metropolitan areas have been growing faster than trees in rural areas worldwide since the 1960s. This has been confirmed for the first time by a study on the impact of the urban heat island effect on tree growth...

Zu sehen ist ein Thermogramm von 40 Tage alten Ackerschmalwandpflanzen, deren Temperatur farblich unterschiedlich dargestellt ist. Pflanzen mit gelber oder grüner Farbe haben einen niedrigen Wasserverbrauch. (Foto: Z. Yang und E. Grill/ TUM)

Research shows how to get more crop per drop

Boosting food production with limited water availability is of great importance to humanity. However, our current water usage is already unsustainable today. The fact that plant leaves lose a great deal of water through...

Der Humus unter dem Wald spielt eine ausschlaggebende Rolle für die Fruchtbarkeit, den Wasserhaushalt und die Nährstoffversorgung von Böden. (Foto: Fotolia/outdoorpixel)

Significant humus loss in forests of the Bavarian Alps

Alpine forests will be at great risk should weather phenomena such as droughts and torrential rain become more frequent. As a study by the Technical University of Munich (TUM) shows, the mountain forests of the Bavarian...

Für die Studie wurden an über tausend Standorten in Mitteleuropa Langzeitbeobachtungen an vorherrschenden Baumarten durchgeführt wie etwa an Buchen. (Foto: Stefanie Ederer)

Broadleaf trees show reduced sensitivity to global warming

The sensitivity of leaf unfolding phenology to climate warming has significantly declined since 1980s, according to a study recently published in the journal Nature by an international collaboration of scientists. Earlier...