When Trees Need Wheels, the City Has Already Run Out of Space

The midday sun beats down on Solothurn’s Klosterplatz, yet under the trees it feels noticeably cooler. That is hardly a miracle, merely a reminder that cities have spent decades sealing surfaces and then acting surprised when summer turns them into frying pans. Solothurn is now trying a different approach: five maple trees, planted not in the ground but in steel troughs, are standing on the square as temporary shade-givers and, rather neatly, as a test case.

The point is not only to cool the place. The city wants to turn Klosterplatz from a parking area into a space for lingering and meeting, and the political mandate is clear enough. Andreas Filosi, head of construction and environment, says the mobile trees give the city room to experiment before the final design is fixed. In other words, it is planning with a little less swagger and a little more reality.

The idea itself is not new. What is new is the attempt to measure it properly. In the federally supported research project Mobile Urban Green, Bern University of Applied Sciences has been examining how strong the cooling effect of trees really is, and what role they play socially in urban space. The researchers considered tree height, crown size, trunk thickness, water use, evaporation and the density of the foliage. The decisive figure is not the air temperature, but the perceived temperature — the way heat is actually felt once sun, humidity and wind are all taken into account.

Stefan Jack, who leads the team at Bern University of Applied Sciences, says the result was striking: in the shade of a tree, perceived temperature can fall by 13 to 19 degrees even on a hot summer day, while sealed surfaces can climb to well above 40 degrees. That is a considerable gap, and one that makes the case for urban trees more convincingly than another round of municipal slogans ever could.

Bauer Baumschulen AG, which has long offered mobile planting, is also involved. Sebastian Mühlemann, who leads the project there, says the company previously lacked hard numbers for the cooling effect. The aim now is to use the collected data to predict as precisely as possible where mobile trees will do the most good and to simulate different scenarios before a square is redesigned.

The troughs themselves come with a technical upgrade: an integrated water tank holding up to 800 litres, plus a sensor that signals when it needs refilling. The idea is simple enough. The tree always has access to water and can better survive heat spells. The troughs are more expensive than conventional models, but the lower watering effort and longer lifespan of the trees may save money over time, according to Mühlemann.

Still, even the enthusiasts admit that the best place for a tree is in the earth, not in a container. The mobile version is a compromise, useful precisely because so many places have already been paved over. On Klosterplatz, that compromise is temporary. A decision on the square’s future is expected in autumn, and the trees may then move elsewhere to cool another place for a while. It is a practical solution, which in urban policy is often the same thing as a confession.

Written by Thomas Nussbaumer thomas.nussbaumer@alpineweekly.com