City trees

The importance of old trees for a city

This page contains further details on the scientific publication:

Croeser, T., Weisser, W.W., Hurley, J., Rötzer, T., Parhizgar, L., Sun, Q. & Bekessy, S.A. (2025) Defining ‘adequate’ tree protection: Meeting urban canopy targets requires careful retention of mature trees. Landscape and Urban Planning, 264, 105484.

Trees are the most important part of a city’s ‘green infrastructure’. Large trees of certain species, such as the small-leaved lime, have a ‘leaf area index’ (LAI) of more than 4, meaning that the area of all their leaves is more than four times the area beneath the crown. A 110-year-old small-leaved lime has a crown diameter of over 17 metres. This corresponds to a crown projection area of over 200 square metres and a crown volume of over 4,500 cubic metres (ZSK, Rötzer et al. 2021). Within the large crown, sunlight is converted into energy, oxygen is produced and water evaporates. Evaporation cools the surroundings and the reduction in sunlight provides shade. In addition, the leaves filter fine dust from the air, whilst foliage and branches dampen urban noise. When it rains, the crowns catch a great deal of water, which seeps slowly into the ground. The leaves of the trees provide food for many insects, which in turn are eaten by many other animals. Woodpeckers can build their nests in the trunk, goldfinches in the crown, and squirrels can build their nests there too. All these functions of trees depend on their size. For example, a 10-year-old small-leaved lime has a crown diameter of just 2.5 metres and a crown volume of approximately 13.8 cubic metres. A 110-year-old small-leaved lime therefore has more than 330 times the crown volume of the 10-year-old small-leaved lime. The size of trees is also crucial for animals in the city: squirrels build their nests only in crowns that are at least 6 metres high, and great spotted woodpeckers hammer their holes almost exclusively into trees with a diameter at breast height of at least 40 centimetres. A newly planted tree cannot therefore replace the benefits of an old tree. Many newly planted trees are in fact significantly younger than 10 years old and therefore even smaller.

It is therefore not just the number of trees that determines how effective trees are in the city, but how large – and thus how old – they are. Unfortunately, old trees are felled in our cities every day, for a wide variety of reasons. Although city tree protection regulations usually stipulate that a felled old tree must be replaced by a young tree, this means that whilst the number of trees remains the same, a large proportion of the tree’s benefits is lost each time – often over 90% in the case of old trees.

The felling of old trees has major consequences for canopy cover in the city

In our work, we investigated how the management of old trees affects the development of tree canopy cover in a city. The model we developed uses realistic growth data and distributions of tree ages within a city and simulates how the removal and replacement of trees affects the overall tree canopy cover of a city. We have taken a canopy cover of 30% as our target value, a figure that is being aimed for by an increasing number of cities worldwide. We have simulated how canopy cover develops if cities fell 1 per cent of old trees, 2 per cent of old trees, and so on, each year and replace the old trees with young trees. The graphs below show the growth of individual trees (individual coloured lines) and the canopy cover achieved in the city (thick black line) from today until 2050, i.e. over a period of around 25 years.

If only 1 in 100 of the old trees is felled each year – meaning that the old trees are replaced over a period of 100 years – the total crown area increases rapidly because the canopy cover of the old trees is maintained, the trees continue to grow, and the young trees also contribute to overall growth. The target of 30% canopy cover is therefore quickly achieved.

If two or three old trees are felled each year, the target of 30% can be achieved, depending on the composition of the tree population. However, if five out of every 100 old trees are felled each year – meaning that the old tree population is replaced over a period of 20 years – then canopy cover is unlikely to be achieved at all, and if it is, it will not be achieved within the next 25 years.

If one in ten old trees is replaced each year – meaning that the entire stock of old trees is removed over a 10-year period – then the total canopy cover decreases, even if the number of trees remains the same.

Tree protection by-laws that stipulate a 1:1 replacement ratio but do not prioritise the preservation of old trees therefore ultimately fail to achieve their purpose. Their implementation leads to a reduction in canopy volume within the city. It is also of no help if two or three small trees are planted in place of one old tree. As the example of the old small-leaved lime shows, 330 new trees would need to be planted for a felled 110-year-old small-leaved lime, and even then neither the great spotted woodpecker nor the squirrel would be able to make use of these trees. The value of old trees is therefore systematically underestimated.

Trees are often mistreated

A frequently heard argument for tree felling is that urban trees cannot grow old, become diseased and must therefore be felled. Climate change and the hot urban climate are often cited as reasons for this. This ignores the fact that there are many ways to promote tree growth and ensure trees’ water supply. Unfortunately, trees are still planted today in such a way that they will suffer from water shortages if there is not enough rain. Typical mistakes include small tree pits – that is, a very small, unpaved area around the trunk – covering the tree pit with a ‘tree protection grate’, or providing insufficient soil space for the tree’s roots to grow. Other countries, such as Australia, are much more progressive in this regard, as they plan for larger tree pits or utilise so-called passive irrigation, i.e. channelling rainwater that falls onto the street towards the trees. Underground root guidance systems are also used in some countries.

An example of a typical new planting of small trees, here in Jena. Although the tree pits are relatively large, they are covered. The trees in question are black locusts, native to North America, which are problematic from a biological perspective as they spread both within and outside cities and are scarcely utilised by native wildlife.

The main problem in Germany, however, is that even old trees are not treated well. In contrast to Copenhagen, for example, where new ground space is created for old trees (Bühler et al. 2007), the tree pit in Germany often remains small, and neither the tree pit nor the available ground space is expanded during roadworks. On the contrary, roadworks such as resurfacing damage old trees because roots are damaged during the works. Furthermore, utility works—for water, sewage, gas, telephone and internet—often fail to take trees and their roots into account.

Finally, maintenance measures also contribute to the premature death of trees, such as ‘pollarding’ or cutting back trunks or thick branches. This is usually unnecessary, and increases the risk of fungal infection and decay.

In many towns, the felling of old trees is a common occurrence.

Although the method of ‘pollarding’ trees leaves a good portion of the tree standing, it increases the mortality rate of the trees.

How can the situation be improved?

Cities can do much more to protect their old trees. All too often, tree felling is authorised even though the trees are still very healthy. Architects and urban planners rarely incorporate existing trees into their designs. Instead of developing a design that integrates existing trees, they often act as if there are no trees in a project area or as if they can simply be replaced by new ones. The frequently-used German saying ‘building rights override tree rights’ is applicable to the values implicit in development in many countries globally. Common as it is, this perspective is not necessarily right; in fact, it is usually just a sign of poor planning and design.

Roadworks can be systematically used to improve growing conditions for old trees. The right of way held by water suppliers, telecoms and other utility providers does not mean that trees may be damaged, or excluded. In fact, poor planning and coordination during the laying of cables is a major reason why existing trees are damaged and no space is found for new trees. This is a key task for local authorities: to carry out planning processes in a more coordinated manner.

It would also be helpful for tree surveyors and tree owners if legislation were amended, as in Austria, to reduce liability and make it dependent, amongst other things, on the tree’s location. Limiting liability to cases of gross negligence would also be of great assistance to mature trees. Ultimately, tragic incidents in which people outside of forests are injured by falling branches or toppling trees are very rare. Here, too, a comparison with other countries is helpful. For instance, many of the trees planted in Australian cities, including many eucalyptus species, occasionally shed branches when stressed, and the ground beneath the trees is almost always littered with leaves, yet these species remain in frequent use.

References

Bühler, Oliver, Palle Kristoffersen, and Søren Ugilt Larsen. ‘Growth of street trees in Copenhagen with emphasis on the effect of different establishment concepts.’ Arboriculture & Urban Forestry (AUF) 33.5 (2007): 330-337.

Rötzer, T., Reischl, A., Rahman, M., Pretzsch, H., Pauleit, S. (2021): Electronic appendix to the guide on urban trees in Bavaria. Recommendations for action from the project ‘Urban Trees – Growth, Environmental Services and Climate Change’. Centre for Urban Nature and Climate Adaptation, 150 pages.