Trees and other plants are of great importance for large cities, and can play a role which goes far beyond that of visual enrichment alone. Trees filter pollutants, are home to various animal species and offer valuable shade which not only provides short-term refreshment for pedestrians, but at the same time prevents excessive long-term overheating of pavements and house walls. Exceptionally hot such summers such as that experienced in Central Europe in 2015 are expected to occur more frequently, and thus trees will play an even more important role in cities. There are about 86 000 roadside trees in Vienna (those in parks are not included in this number), nearly half of which are between twenty and forty-nine years old, three-quarters are less than fifty years old. The22nd district of Vienna has the most trees, followed by the 2nd and 21st districts.
The most common genus of trees in Vienna is the maple, of which there are nearly 25 000, followed by the lime and the horse-chestnut.
The maple grows to a height of 20 to 30 m, is very heat-tolerant and exceptionally drought-resistant. It is therefore ideally suited for use in warm cities also in the future. Furthermore, maple trees are very wind resistant. This quality plays a role in a city like Vienna, which is known for its atmospheric winds. The maple with its dense crown is a typical tree for boulevards and is therefore often found on Vienna’s famous “Ringstraße”.
There are almost 15 000 lime trees in Vienna, which grow to a lesser height (18 to 25m) than the maple, but tolerate heat and frost equally well. The lime tree is also renowned for reaching an age of over one thousand years.
Horse-chestnut trees (there are about 10 000 of them in Vienna) attract attention particularly in autumn. Their leaves change colour, turning into a beautiful orange-red. This tree grows 25-30 m high and has a domed, dense crown. However, it is sensitive to immissions and salt.