It’s hard to believe: a 60-year-old lime tree has the same cooling capacity as 140 refrigerators over the course of a year. So says Christopher Busch, tree expert at Bund Naturschutz (BN), citing research by the Technical University of Munich. “A full-grown deciduous tree easily evaporates 400 liters of water on a hot summer’s day,” says Busch, “which cools the area down enormously.” The shadow cast by the canopy is not included. “With a crown diameter of 15 meters, the shadow covers an area of at least 170 square meters, depending on the position of the sun,” says Busch. “The shadow cast cools in addition to evaporation.”
In line with the past hot days, the BN is now demanding that the Free State and municipalities strengthen the protection of trees in the cities. “The climate crisis shows that without trees and other greenery, it is becoming unbearably hot in cities,” says BN chairman Richard Mergner. “City trees are central to how communities adapt to the climate crisis.” The credo of the BN boss is: “If more and more areas are built over and paved over, the cities will heat up a lot. If we don’t pull the ripcord here, life in them will become unbearable in the not too distant future.”
The demands of the BN: The Free State should oblige the municipalities to issue so-called tree protection ordinances, which regulate the preservation and care of city trees, but also the cases in which they may be felled in exceptional cases. Currently, only 100 of the 2,000 municipalities in Bavaria have such regulations. “But they are the basis for better protection of city trees,” says Mergner. In addition, the BN calls for tree registers in which the municipalities break down their tree stocks. “Many municipalities don’t even know where trees are in their area,” says Mergner. “With modern digital technology, however, such cadastres are easy to create.”
Free State should commit to preserving the city trees
The Free State should also commit itself to the preservation and care of the city trees and adapt the Bavarian building code accordingly. “It must no longer be the case that city trees are simply cleared away if they stand in the way of a construction project,” says Mergner. “We can no longer afford that in the climate crisis.” According to Mergner, the Bavarian building regulations currently give clear preference to construction projects over the preservation of trees on the building site. State experts have also been saying for many years that as much green space as possible in cities is the best strategy for them to adapt to the climate crisis.
A query by the BN shows how carelessly large cities in particular have been dealing with the trees in their fields. According to this, at least 165,000 trees have been felled in the 17 largest cities in the Free State over the past ten years. The absolute leader is the state capital Munich. There alone, almost 100,000 trees were felled between 2011 and 2021, one and a half times as many as in the other 16 municipalities combined. According to the tree expert Busch, the total number is the absolute lower limit of fellings. “On the one hand, the cities keep very different and often only rudimentary statistics,” says Busch. “Some only record trees on public property, others in private gardens, and still others only have data on trees along their roads.”
The deficits don’t get much better when you offset the replanting. The balance is then positive in some cities, including Ingolstadt, Fürth and Erlangen. But for the most part it remains negative. Across Bavaria, almost 35,000 fewer city trees have been planted than old ones have been felled. In addition, subsequent or replacement plantings do not have the same positive effects as their predecessors, which are usually several decades old. The linden tree, for example, which is very popular as a city tree and a symbol of the BN, grows 25 to 50 centimeters a year. “So it takes 25 years for a newly planted linden tree to reach an average size of ten meters,” says Mergner. “But only then can it unfold its positive effect on the urban climate.”
In Veitshöchheim they researched the city tree of the future
There is no lack of recommendations for the municipalities. At the Veitshöchheimer LWG they have been researching the city tree of the future for more than ten years – i.e. tree species that can cope particularly well with heat and dry periods, but also, for example, heavy rainfall and other extreme weather conditions, which are becoming increasingly common due to the climate crisis. Among them are the Italian maple (Acer opalus), native to the Mediterranean region, and the American lime (Tilia americana). Both do not only cope better with hot temperatures and droughts than some native trees. But they are also beautiful to look at, especially in autumn. Then the foliage of the Italian maple turns bright orange or red, that of the American linden bright yellow.
In total, the LWG researchers studied 30 tree species. This also includes a large-scale practical test in the cities of Würzburg, Hof and Kempten. Almost 700 specimens of the 30 species have been planted there, and the researchers have studied over the years how they are doing in the three regions. From the results, they developed planting recommendations so that each city can choose its trees of the future. It is therefore quite possible that the Italian maple and the American lime tree will be found more often in Bavarian cities in the future. So far they are very rare in this country.