Which tree is ideal for urban greening?

Urban trees now have to withstand climate change. This can mean that new species are needed.

A row of tall trees with a spreading dark green crown lines a small street near Stuttgart’s town hall. Last year, Eberhardstrasse was redesigned to be car-free, parking spaces were replaced with green spaces and seating. Here you can see on a small scale what is currently occupying many town halls in the country: How can they bring more green to cities that is also able to withstand climate change?

Volker Schirner is a trained gardener and horticultural engineer and has to face this problem with every new construction project and every tree that the city has to replace because it has become infected with an illness or has become old and brittle and is no longer considered to be roadworthy. At the beginning of his career, Schirner took care of the Berlin zoo. Now he is the head of the Stuttgart garden, cemetery and forestry office and, as such, lord of hundreds of thousands of trees.

He can still remember how the Japanese pagoda tree , for which he likes to use the old botanical name Sophora japonica , became a kind of fashion tree in the 1980s. At the time, the pressing problem in many cities was air pollution, and the Asian tree species was considered insensitive to pollutants such as sulfur dioxide. The pagoda tree is relatively common in Stuttgart.

The tree species is also doing well in terms of climate, says Schirner. Their advantage is the feathery leaves, through which less water evaporates than with linden or maple, which is why the trees can manage with less water. So actually ideal for a city center that heats up as much in summer as that of Stuttgart. But the Sophora also has its disadvantages: With its sweeping crown, the full-grown tree is not suitable for narrow street canyons – also because the branches can break when the wind sweeps through the alleys. To create an avenue, it would have to be repositioned and replaced by narrower types. But removing mature trees is not very popular today.

Norway maple and chestnut suffer from drought. You will probably see them less often in the future

In Eberhardstrasse, Schirner’s employees have now contrasted the tall pagoda trees with a narrower species. It is an elm that retains its slim crown even when fully grown. Elms had almost disappeared in Germany due to a fungal disease and are now enjoying a comeback in a more resistant variant.

Narrow-crowned species that evaporate less water and whose root ball can cope with little space: This is one of the strategies urban experts are using to adapt to climate change. In addition, many cities are experimenting with irrigation. They use substrates that hold water longer and equip the ground with water storage tanks that are either filled by the watering vehicle via a drainage pipe – or that are connected directly to the rain gutters of surrounding houses.

Many cities also work with water-permeable flooring in order to increase the seepage surface over the root ball. Since this year, more time and personnel have been available for irrigation in Stuttgart. The gardening office starts watering earlier than before in the year to saturate the soil before the growth phase. Schirner was also able to double the number of its casting vehicles.

In addition, the gardening authorities are looking for tree species that can deal with drought stress. The Norway maple and chestnut, which are popular in Germany, have their problems here. However, Schirner is convinced that these tree species will continue to be seen in German cities – albeit in a smaller proportion than today. When replanting, you will have to pay more attention to which tree is suitable for which location. At Stuttgart’s Schlossplatz, for example, where chestnuts stand on strips of meadow in a park-like setting, they are doing well. On the opposite Karlsplatz, where chestnut trees are surrounded by pavement, some specimens are visibly weakening.

The dominant tree species in most major German cities are linden and maple. In some places they make up half or more of the tree population. In Munich, for example, around 70 percent of the trees in public areas are linden and norway maples. In the Bavarian capital, the building department commissioned an inventory and a development plan last year. The aim is to achieve a mix of tree species that is adapted to climate change.

However, you never know when a pest will appear and attack the supposedly optimal climatic tree. The plane tree, for example, which is very present in downtown Stuttgart – there are around 120 of these Mediterranean trees in the central pedestrian zone alone. “It grows well, is incredibly robust and is decorative,” says Schirner of the tree that the master horticulturist Peter Joseph Lenné planted in the Berlin zoo more than 150 years ago. Unfortunately, a few years ago the massaria disease appeared. Now there is a risk that the beautiful plane tree avenue will eventually be attacked and carried away. “We are currently very careful about replanting plane trees,” says Schirner.

Which tree species thrives where, and which can cope with extreme conditions such as a thin layer of earth above the subway shaft or a sun-drenched pedestrian zone, has been studied across Germany for years. Even at the time of the increased air pollution, municipalities founded the German Garden Authority Conference (Galk) and exchanged experiences. In the meantime, the focus has shifted to climate change.

Experts test exotic plants as well as old, almost forgotten species

Galk is responsible for an ongoing experiment in which cities from Hamburg to Stuttgart to Munich are planting new species where trees have to be planted anyway. They then document how the species are doing for years. “We are currently testing 40 tree species,” says Joachim Bauer, who coordinates the project. The results are incorporated into a list of recommendations that municipalities and tree nurseries can access.

It’s not just about exotic imports. The list also includes surprising rediscoveries such as the field maple, which can deal with drought much better than the Norway maple, says Bauer. For him, the plant is a hot candidate for the future tree. But he also still considers the Japanese pagoda tree to be very promising. You don’t necessarily have to plant it in a narrow street.

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