TREES IN GERMAN CITIES
Street trees save lives but
they struggle to survive
By Tann vom Hove, German Brief *
August 2023: Urban trees save lives. According to research published this year in the British journal The Lancet, one third of heat-related deaths could be avoided if at least 30 per cent of urban areas were covered with trees. At the same time, however, trees themselves are struggling to survive in many cities. They fight air pollutants, suffer from lack of space, de-icing salt harms their roots and road works, accidents or vandalism cause damage to trunks and branches.
Trees do not only beautify cityscapes they also serve as air conditioners in densely populated conurbations. A fully grown deciduous tree evaporates up to 400 litres of water on a hot summer day, thus cooling its surroundings. Trees also provide effective shade: with a crown diameter of just 15 metres, a single deciduous tree manages to cool an area of 160m² with its shade.
The study ‘Cooling cities through urban green infrastructure: a health impact assessment of European cities’, published in The Lancet and based on research by the Barcelona Institute for Global Health (IS Global), highlights the need to plant more trees in cities to counter the effects of climate change.
The authors, who collected data from 93 European cities, report that more than four per cent of deaths in cities during the summer can be attributed to urban heat islands. One third of these deaths could be prevented if a tree cover of 30 per cent were achieved.
Heat exposure is associated with premature mortality, cardiovascular disease and hospital admissions. This is especially true during heat waves, but also during moderately high summer temperatures.
Cities are particularly vulnerable to higher temperatures. Less vegetation, higher population density and impermeable surfaces of buildings and roads, including asphalt, lead to a temperature difference between a city and its surrounding countryside - a phenomenon known as an urban heat island. Given ongoing global warming and urban growth, this effect is expected to intensify in the coming decades.
The IS Global study highlights the significant benefits of planting more trees in cities, although the authors acknowledge that this can be challenging in some cities due to their design. Tree planting should be combined with measures such as green roofs or other temperature-reducing alternatives.
"Our findings also show how important it is to preserve and maintain the trees we already have, as they are a valuable resource. It also takes a long time for new trees to grow. It is not only about increasing the number of trees in the city, but also about how they are distributed," explained Mark Nieuwenhuijsen, from IS Global. BACK TO TOP
Street trees in German cities
In Germany, many of the street trees (Straßenbäume) that were planted quickly after the Second World War are now approaching the end of their life cycle and need to be replaced. Urban trees age faster than trees in the ‘wild’. On roads, trees often live only 60 years, whereas in forests or parks the same species can live up to 200 years.
Traditionally, and especially after the Second World War, German cities have relied on fast-growing trees to quickly restore their cityscapes with fresh greenery. Today, future-oriented cities can choose from around 190 tree species that are particularly suitable for planting in urban areas - the so-called future trees.
Street trees in German cities suffer increasingly from heat and drought stress, partly because temperatures in the city are between five and eight degrees higher than in surrounding areas. Since trees evaporate more as a result, but do not receive sufficient water due to ground sealing, they develop drought stress. In addition, urban street soil is extremely dense, which leaves no room for a large root system. Arboriculturists are also alarmed about new pests which have arrived in Germany and neighbouring countries due to climate change.
The following five tree species are the most common in German cities:
Linden trees make up a quarter of street trees in Germany and thus characterise the image of many cities. Maple trees (Ahorn) are also frequently found in cities, with around 15 per cent. They are followed by oaks (Eichen) with nine per cent of Germany's urban trees, plane trees (Plantanen) with six per cent and horse chestnuts (Esskastanien) with four per cent. BACK TO TOP
German cities and their trees
The German capital city Berlin and Karlsruhe (Baden-Württemberg) are the country’s two tree-richest cities. Both cities can boast of having more than 400 street trees (Straßenbäume) per square kilometre. Other cities with more than 300 trees per sq km include the state capitals of Bavaria (München) and North-Rhine Westphalia (Düsseldorf).
Number of street trees (Straßenbäume) per sq km in large German cities
1) Berlin 483
2) Karlsruhe 416
3) München (Munich) 363
4) Düsseldorf 309
5) Essen 298
6) Hamburg 296
7) Gelsenkirchen 267
8) Hannover 227
9) Bremen 218
10) Köln (Cologne) 196
11) Stuttgart 190
12) Leipzig 185
13) Frankfurt 179
14) Dortmund 178
15) Dresden 165
16) Nürnberg 156
(Source: BUND Naturschutz in Bayern)
Berlin (State of Berlin)
Street trees per square km: 483**
Berlin has some 430,000 street trees, of which 35% are lime trees, 20% maple, 9% oak and 6% plane trees. Due to several extreme weather (storm, drought), the number of trees has gone down since 2017.
The numerous tree-lined streets make Berlin a green metropolis. On average, there are around 80 trees on every kilometre of city road today, resulting in a total stock of well over 430,000 street trees. But trees are also an indispensable part of Berlin's green spaces, playgrounds, schoolyards, cemeteries and semi-natural areas.
After 1946, Berlin was able to restore its tree population, which was severely depleted during World War II. By 1946, the number of trees had fallen from around 411,000 in 1939 to around 161,000 in 1946. By the reunification of West and East Berlin in 1990, the city had a stock of around 370,000 street trees.
As a result of the extreme weather events in the autumn of 2017 (heavy rainfall and storms) and the heat and drought of 2018 to 2022, the street tree population has declined again. (Sources: Land Berlin and **BUND, Naturschutz in Bayern) BACK TO TOP
Hamburg (State of Hamburg)
Street trees per square km: 296**
Hamburg has a very large tree population with about 250,000 street trees, 600,000 park trees in addition to trees in private gardens
However, summer heat and dry periods and storms pose a threat to Hamburg's trees. In addition, newly introduced diseases and pests favoured by higher temperatures and a change in precipitation patterns in northern Germany are increasingly endangering more tree species.
The five most common tree species among street trees account for more than 150,000 trees. The dominant species are the lime trees, closely followed by oaks. With more than 100,000 trees, the two species make up 45 per cent of all Hamburg’s street trees. (Sources: Land Hamburg and **BUND, Naturschutz in Bayern) BACK TO TOP
Street trees per square km: 179**
In 2014, The European Arboricultural Council (EAC) awarded Frankfurt the title ‘European City of Trees 2014’. With the award, the association of tree experts from all over Europe recognised Frankfurt's pioneering role in the care of its urban trees and the great importance of trees for a sustainable, liveable city.
The title has been awarded since 2007. Frankfurt was the first German city to receive the award.
In Frankfurt, some 250 kilometres of the street network are flanked by more than 70,000 trees. They provide shade to major throughfares as well as many residential streets. (Sources: Stadt Frankfurt and **BUND, Naturschutz in Bayern) BACK TO TOP
Street trees per square km: 190**
Stuttgart has over 150,000 trees in public spaces and many thousands more on private properties. An important task of the city is to protect and maintain its tree stock.
Urban trees and especially street trees form an important part of Stuttgart’s living environment. In summer, they provide shady, cool places and beautify the streetscape. Their leaves produce oxygen and bind fine dust. They have positive effects on the microclimate, act as dust filters and increase people's physical and psychological well-being. (Sources: Stadt Stuttgart and **BUND, Naturschutz in Bayern) BACK TO TOP
Düsseldorf (North-Rhine Westphalia)
Street trees per square km: 309**
As part of the urban tree concept, tree sites will be newly created or rehabilitated. One million euros will be made available for each of the years 2019 to 2023. The aim is to plant 200 new trees per year. As part of its urban tree concept, the city is planting so-called trees of the future, including the hackberry tree (Celtis australis), the magnolia (Magnolia kobus) and the ironwood tree (Parrotia persica 'Vanessa'). (Sources: Stadt Düsseldorf and **BUND, Naturschutz in Bayern) BACK TO TOP
Street trees per square km: 185**
Urban trees are important elements of Leipzig's urban environment. Whether on streets and squares or in parks and other green spaces - the function of urban trees, regardless of their location, extends far beyond aesthetic and urban planning significance. Urban trees are indispensable as space-forming design elements. In addition, they are largely responsible for the quality of stay in urban spaces and promote our health. They provide shade, absorb sound and improve the urban climate. Thus, they provide cooling in high temperatures and reduce to air pollution through the dust-binding effect of their leaves.
With an (average) annual number of more than 1,000 new and replanted street trees, Leipzig is among the leaders compared to other German cities of its size. (Sources: Stadt Leipzig and **BUND, Naturschutz in Bayern) BACK TO TOP
Dortmund (North-Rhine Westphalia)
Street trees per square km: 178**
Dortmund has identified the following trees as ‘trees of the future’: acers, horse chestnuts, alder trees, rock pear trees, birches, - beech trees, sweet chestnut trees, trumpet trees, hackberry trees, ashes, magnolia trees, plane trees, oaks, poplars, lime trees, elm trees. (Sources: Stadt Dortmund and **BUND, Naturschutz in Bayern) BACK TO TOP
Essen (North-Rhine Westphalia)
Street trees per square km: 298**
A total of around 3.3 million trees grow within the boundaries of the City of Essen, most of them in woodland. However, there are also a large number of trees in streets, on educational properties such as schools and colleges, on green spaces and in cemeteries. (Sources: Stadt Essen and **BUND, Naturschutz in Bayern) BACK TO TOP
Bremen (State of Bremen)
Street trees per square km: 218**
The more than 73,000 street trees in Bremen are made up of 58 different categories and 280 species. A category such as oak, beech or lime consists of several species. The most common type of street trees in Bremen are oaks, lime trees, maple trees, sorbus and beech trees. They make up a good 75 per cent of Bremen's total street tree population. The proportion of deciduous trees is 99 per cent. (Sources: Land Bremen and **BUND, Naturschutz in Bayern) BACK TO TOP
Street trees per square km: 165**
Dresden is a tree-rich city with a great diversity of species. Tree-lined avenues, parks, green spaces and gardens characterise the cityscape. Trees contribute significantly to the quality of life, as they have a positive influence on the urban climate.
There are around 104,000 trees on municipal land. Of these, about 54,000 are street trees, 35,000 trees in parks and green spaces, on playgrounds and schools and about 15,000 trees along water ways.
In addition to the strongly represented typical tree species such as lime, maple and chestnut, the city is host to ginkgo, magnolia, leather pod tree and other rarer tree species. All together almost 140 different types and varieties of trees grow in the city area. (Sources: Stadt Dresden and **BUND, Naturschutz in Bayern) BACK TO TOP
Street trees per square km: 156**
Nürnberg (Nuremberg) has a total of around 270,000 trees on public land, including in woodlands as wells trees on school and kindergarten grounds. Some 80,000 trees grow along street, while park trees account for some 190,000. Mostly represented are: Maple 26%, Lime 21%, Oak 18%, Hornbeam 6%, Robinia 5%, Sycamore 4%, Pine 3%, Birch 3%, Ash 1%, Apple 1%. (Sources: Stadt Nürnberg and **BUND, Naturschutz in Bayern) BACK TO TOP
Duisburg (North-Rhine Westphalia)
Duisburg is a green city. Around 2,500ha, about 10.7% of the total area, consists of woodland. In addition, there are about 50,000 street trees and countless other specimens, some of them quite tall and mature, in parks and private gardens. (Source: Stadt Duisburg) BACK TO TOP
Bonn (North-Rhine Westphalia)
More than 125,000 trees in public parks and gardens, along streets as well as in cemeteries make Bonn an above-average green city. Above all, the approximately 30,000 street trees characterise the cityscape. However, they also suffer more than others from the consequences of climate change. The city invests heavily in tree care and replacement plantings. (Source: Stadt Bonn) BACK TO TOP
Münster (North-Rhine Westphalia)
Münster’s Urban Trees Unit looks after about 143,000 individual trees, of which about 61,000 are street trees, about 75,000 trees on urban sites (public green spaces, playgrounds, schoolyards, etc.) and another 7,000 trees planted as clumps. When selecting the tree species to be planted in Münster, attention is paid to a wide range of varieties and species in order to ensure a great diversity of species. Common tree species are lime (24%), followed by oak (21%), maple (14%), hornbeam (8%) and plane tree (5%). (Source: Stadt Münster) BACK TO TOP
There are approximately 90,000 trees along streets and on public grounds. Altogether, some 50 per cent of Mannheim’s urban area consists of green spaces. (Source: Stadt Mannheim) BACK TO TOP
Street trees per square km: 416**
There are about 137,000 street and park trees in the Karlsruhe urban area. In order to maintain the number of trees in the city area, about 800 to 1,000 trees are replanted and newly planted every year. Due to climate change, so-called future tree species are increasingly being tested in the city area. They are more likely to cope particularly well with current climate conditions. The most widespread tree species are maple, hornbeam, oak, lime and cherry. Chestnut, pine, sycamore, poplar and birch are found in smaller numbers. (Sources: Stadt Karlsruhe and **BUND, Naturschutz in Bayern) BACK TO TOP
Freiburg is shaped by the nearby mountains and forests of the Black Forest. The foothills of the mountains end at the city gates. Therefore, forest trees determine the image of the city. The beech, called the mother of the forest, is the most widespread type of tree. It is popular because it can grow anywhere and creates good growing conditions for many other types of trees.
The fir is another characteristic tree species of Freiburg’s forests and woodlands. Magnificent fir forests gave the Black Forest its name.
The spruce is the best-known coniferous tree but is not native to the Black Forest. The spruce made up only 3-5 percent of the Black Forest. It is an undemanding and fast-growing tree. Therefore, it was particularly suitable for the reforestation of devastated forest areas after the great timber shortage in the 18th century. The spruce was to ensure the wood supply of the national economy. (Source: Stadt Freiburg) BACK TO TOP
In Heidelberg there are now around 50,000 trees in the inner city, not counting the city woodlands and private gardens. In 2005, the number of trees amounted to only 35,000. The majority of urban trees are native species. But they also include exotics such as ginkgo, tulip tree and hemlock, which cope well with Heidelberg's mild climate. The most common species are: maple, cherry, hornbeam, lime, plane, spruce, beech, oak, birch and apple. (Source: Stadt Heidelberg) BACK TO TOP
*Methodology & Sources:
The research was conducted in July and August. Sources include, in addition to municipal parks, recreation and environmental departments, The Lancet journal; Schutzgemeinschaft Deutscher Wald; Deutsche Gartenamtsleiterkonferenz (GALK), Neue Landschaft; The Barcelona Institute for Global Health (ISGlobal); the London School of Hygiene & Tropical Medicine; ** Bund Naturschutz Bayern; ** Bund Naturschutz Nürnberg
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