Home » News » Birch, linden and plane trees as air conditioners for the city: ‘It can make a difference of twelve degrees’

Birch, linden and plane trees as air conditioners for the city: ‘It can make a difference of twelve degrees’

As climate change continues to exacerbate heat waves, trees are playing a crucial role in keeping our cities cool. In Amsterdam, researchers will be conducting a study comparing different tree species to determine which ones have the most effective cooling effect. Tree consultant Hans Kaljee of the municipality of Amsterdam refers to trees as “the parasols of the streets”, highlighting their ability to provide shade and significantly lower temperatures. The study will measure the temperature under different tree species throughout the day, both in the sun and shade. Researchers will also assess the effects of the soil surrounding the trees, such as whether they are planted in grass or surrounded by stone. The findings from this research will help inform decisions on urban tree planting and design to mitigate the impacts of climate change. Additionally, the measurements will be used in an American computer model called i-Tree, which calculates the value of trees in terms of air quality, biodiversity, rainwater discharge, CO2 absorption, and more. By understanding the cooling effects of different tree species, cities like Amsterdam can plan for hotter summers and ensure that the trees planted will provide the maximum benefits to the urban environment.

As climate change increases heat waves, trees are becoming the air conditioners of our cities. This summer, researchers in Amsterdam will be comparing different tree species. A large, full crown offers the most cooling, but there is not room for it everywhere in the city.

Bart van Zoelen

Trees are sometimes labeled as the air conditioners of the cities, but tree consultant Hans Kaljee of the municipality of Amsterdam prefers to speak of ‘the parasols of the streets’. Either way, the cooling effect is great. “On a hot summer day it can feel like twelve to fifteen degrees difference if you sit in the shade of a tree.”

But one tree is not the other and that is why TU Delft investigated the cooling effect of birch, linden and plane trees in Amsterdam on Monday. The research is part of a national study in eight municipalities. Researchers measured the temperature all day long under six trees in Amsterdam, in both the sun and the shade. It is expected that one tree species will have a much greater effect on the temperature than the other .

Noticeable difference

“The crown of leaves of this plane tree can grow up to 30 meters in diameter,” says Kaljee in Geuzenveld, pointing to one of the trees studied. In the distance he sees a birch of which it is immediately noticeable that the crown is much less full. “It doesn’t get any more than that. The sun can almost go under it.”

It’s only one measurement, but the difference between the two tree types is immediately noticeable in the shade below. Under the plane tree it is about 22 degrees, under a birch in Geuzenveld it is almost 24 degrees a little later.

The researchers also measure the effect of the soil around the tree. Is the tree in grass or is it surrounded by stone? “In narrow, densely built-up streets you can get a kind of canyon effect where the heat lingers for a long time in the evening,” explains TU researcher Tom van den Wijngaard. In such an environment, the cooling effect of trees may be even greater. “Thanks to their shade, trees can prevent materials from heating up and retain the heat.”

At the same time, these are often busy streets where trees have little room to grow. “It is precisely here that it becomes difficult to get mature trees,” says Van den Wijngaard. “It’s a battle for space.”

The value

The measurements are also used for an American computer model called i-Tree that maps the value of trees for the living environment. This computer program that can estimate per tree what the contribution is to air quality, biodiversity, rainwater discharge or CO2recording and even attaches a price tag to it, is now completely geared to American data, while the differences with the Dutch situation can be large. For example, the streets here are often narrower and the trees in this wet polder land usually root less deeply than in the US.

I-Tree will soon be able to estimate which trees are preferable in an urban environment like Amsterdam if climate change continues, says Van den Wijngaard. Most cooling comes from mature trees, but it can take forty years before they are fully grown. “Growing conditions in the city are difficult, so we should already know which trees are suitable.”

Most cooling

For Amsterdam, the researchers also calculate which trees would have been cut for Jan Evertsenstraat, De Clercqstraat and Rozengracht, streets of the so-called Oranje Loper that runs between Central Station and Mercatorplein in Amsterdam-West. The trees that grow here don’t have it easy because in the past, species were chosen that thrive better in different conditions, Kaljee explains.

Now that the Oranje Loper is being tackled, the municipality of Amsterdam can anticipate increasingly intense heat waves. “If you do redesign the street, you can immediately check which varieties will offer the most cooling in thirty years.”

Then larger tree crowns and the associated roots in the subsoil should also fit in such crowded, busy streets, of course. “If we only look at heat stress, we will be done in no time. But yes, space is scarce.”

Trees with a large, dense crown such as lime, but also poplar and horse chestnut can also make a street dark. Residents often attach great importance to light in their homes, which is understandable. Kaljee: “We have to start this investigation now.”

More shade

The fact that Amsterdammers are starting to notice how hot summers can be also helps. In the past, residents often wanted graceful, lollipop-shaped trees for their street. “If only flowers grew on it and berries for the birds,” says Kaljee. “Now I sometimes get the request if more shade can be added. I have rarely heard that in the past.”

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