Trees and their effects on the urban microclimate

Key facts at a glance

• Cities are often several degrees warmer than the surrounding countryside because asphalt and buildings store heat.
• Trees can lower the ambient temperature by up to 5°C.
• Trees create an additional cooling effect through evaporation.
• The climate performance of trees depends heavily on their water supply.
• Healthy, well-nourished trees are crucial for a pleasant urban climate.

Trees not only beautify our streets and squares, they also have measurable effects on our health. And they also regulate the urban microclimate. As summer temperatures rise, the importance of this service will continue to increase.

Cities are unique systems, including climatic ones. Research shows that temperatures in urban areas are several degrees higher than in the surrounding countryside. Cities consist largely of asphalt, bricks, and cement. This building material absorbs and stores a great deal of heat. At night, this stored heat gradually releases it, leading to particularly higher temperatures.

Forestry scientist Lukas Denzler describes how infrared images can be used to visualize different surface temperatures. Asphalt surfaces, such as roads, are up to 40°C warmer than surfaces covered with vegetation when exposed to direct sunlight. Green spaces therefore act like climate oases in urban areas. In the shade of a tree, for example, the temperature is perceived as only about half as high as in direct sunlight.

Andrew Roloff, Professor of Forest Botany, writes that the measurable temperature difference between parks and paved areas can be up to 5 ° C. The perceived temperature difference is significantly higher and can reach more than 10 ° C, as the increased humidity under trees cools down even more. The difference between paved surfaces and tree-covered green is even greater, up to 15 ° C.

Trees provide shade and shaded areas warm up less because they are protected from direct sunlight. In addition, trees evaporate considerable amounts of water and thus create an additional cooling effect. A tree can evaporate up to 400 liters of water on a sunny and hot day. And because water consumes heat when it evaporates, trees cool their surroundings in this way.

It is important that this cooling capacity depends heavily on the water supply. When trees are lacking in water, they cannot perform this cooling effect efficiently.

In addition to these microclimatic regulations, trees also improve city air, which is particularly important as the heat increases. Trees reduce the amount of ozone, nitrogen oxides, carbon monoxide and sulfur in the air. 

Due to high emissions and increasing summer temperatures, this regulatory effect of trees will also become increasingly important. And not only that: trees bind carbon dioxide in their substance. Trees could therefore also make an important contribution to CO² play in the neutral city of the future.

The growing importance of urban trees due to climate change

The landscape architect Stefanie Roessler writes that the central challenges of climate change for urban planning will be higher temperatures and extreme precipitation events.
For Rößler, urban greenery is an important component of urban climate adaptation strategies in order to be able to deal with the consequences of climate change. Urban greenery regulates the microclimate in the densely built-up, overheated city. Water can seep into green areas and, in the event of flooding, be retained or drained off with little damage. The retention and storage of rainwater in green spaces can stabilize the urban water balance in dry periods as well as reduce the dangers of heavy rain events. As a result, urban trees provide important regulatory functions for the urban microclimate and the urban water balance in times of climate change.
For these reasons, the Dresden researcher calls for the expansion of unsealed areas and the green volume, above all by expanding the population of street trees.

And now?

In order to protect our cities from the meteorological consequences of climate change, unsealed green spaces and lots of city trees are needed. Green spaces can offset the two most important consequences of climate change - higher temperatures and extreme precipitation events. And trees also help to slow down climate change by reducing CO² tie.  

However, the hot summers pose increasing challenges for our city trees. Long periods of drought in connection with higher temperatures can limit the vitality and thus the ability of urban trees to regulate the climate. So it is important that we ensure good conditions for our trees.

See also the text: Increasing problems for our city trees

And here is an article on the topic drought stress in trees.

The effects of trees on the urban microclimate at a glance

1. Temperature reduction: Trees provide shade that helps reduce surface temperatures. The evaporation of water through the leaves (transpiration) creates an additional cooling effect.
2. Air quality improvement: Trees absorb pollutants from the air, including carbon dioxide, nitrogen oxides and particulate matter. They thus contribute to improving air quality in urban areas.
3. Reduction of air pollutants: Trees can help reduce air pollutant levels by trapping particles and breaking down harmful gases through metabolic processes.
4. Humidity regulation: By transpiration of water through their leaves, trees increase the humidity in their surroundings. This can help balance dry air in urban areas.
5. Noise reduction: The foliage of trees acts as a sound barrier and can help reduce noise levels in urban environments, particularly along streets and in residential areas.
6. Water regulation: Trees play an important role in regulating water balance in urban areas. They absorb rainwater, reducing surface runoff and thus minimizing the risk of flooding.
7. Slowing the wind: Trees can help reduce wind speed in urban areas. This is particularly important in open urban areas to mitigate unpleasant wind conditions.

Why water is crucial

The cooling effect of trees is largely based on the evaporation of water through their leaves. This process, also called transpiration, only works, however, if the tree has sufficient water available.

On hot days, trees transpire large amounts of water, thereby drawing heat from their surroundings. This is precisely how the cooling effect is created, noticeably improving the urban climate. However, if a tree lacks water, it reduces this transpired rate to protect itself.

The result: Cooling capacity decreases significantly. At the same time, stress on the tree increases, which can impair its health and stability in the long term.

This means that a tree can only effectively contribute to the urban climate if it has an adequate water supply. Especially during hot and dry summers, water supply becomes a crucial factor.

About the author
Andreas Hunkeler is a social and cultural anthropologist specializing in sustainable and participatory green space design in cities.
His work connects people, nature, and urban spaces.
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