by Andreas Hunkeler
Trees in cities? Yes! And many of them! Trees not only beautify our streets and squares, they also have measurable effects on our health. And they also regulate the urban microclimate. The increasing summer temperatures will make this service more important.
Cities are unique systems, also climatically. Research shows that the temperatures in urban areas are several degrees higher than in the surrounding area. Cities consist to a large extent of asphalt, bricks and cement. This building substance absorbs a lot of heat and stores it. At night, this storage unit gradually releases the heat, which especially then leads to higher temperatures.
The forest scientist Lukas Denzler describes how different surface temperatures can be made visible with the help of infrared images. Asphalted and dark urban surfaces such as streets are up to 40 ° C warmer in direct sunlight than surfaces overgrown with plants. Green spaces therefore appear like climatic oases in urban areas. In the shade of a tree, for example, the temperature is only felt to be about half as high as in the blazing sun.
Andreas 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 the urban air, which is particularly important when the heat increases. Trees reduce the amount of ozone, nitrogen oxides, carbon monoxide and sulfur in the air. The Federal Environment Agency describes how the increasingly hot summers promote the formation of ozone and thereby increase the associated health risks. On days with high ozone concentrations, many people suffer from respiratory problems, headaches and irritation of the eyes.
Due to high emissions and increasing summer temperatures, this regulatory effect of trees will also become increasingly important. And not only that: trees bind CO2 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.
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.