Simulation: What the felling of large, old trees means for the future of cities

This might sound like an extreme experiment, but it simulated something that cities actually do: Very often, large healthy trees in urban areas are cut down when they cause a minor problem, or when they get in the way of development. The trees are usually replaced – but mostly with very small saplings that are only a few years old. 

The loss of shade and cooling from that tree is only really ‘replaced’ in thirty years, and that is if the tree survives that long. However, this is not easy for young trees, as cities are becoming increasingly hotter due to global warming and, even more so, due to increasing soil sealing.

Model calculates development of tree crowns after felling

The research team wanted to understand what happens when this kind of ‘replacement’ happens many, many times in a city. “Think about it this way – how many days could you go losing 100 Euros, but finding 20 cents, before you had money problems? This is the same question for trees, except that with our street trees, the scenario is actually happening”, explains Prof. Wolfgang Weisser from the Technical University of Munich (TUM) in Freising-Weihenstephan.

The model developed by TUM in collaboration with RMIT uses realistic growth data and distributions of trees by age in a city. From this, the model calculates how the removal and replacement of trees affects the total tree canopy cover of a city. With heatwaves impacting Europe more and more, it’s become very important to get to around 30% shade to avoid the worst impacts. Can cities get to this number while they are swapping their best trees for saplings?

New plantings fall far short of offsetting the loss of mature trees

The research team found out that canopy cover inevitably declines when cities cut down their large trees and replace them with small new plantings. The decline also occurs when large trees are removed only to a limited extent and the other trees continue to grow. Lead researcher Dr Thami Croeser, who ran the project as a partnership between the Australian university RMIT and TUM, was surprised by how unforgiving the numbers were.

“These big losses take a generation to replace, so they can’t happen often. Even losing just 3% of mature trees annually makes it very difficult to get most cities to safe levels of tree canopy cover before 2050”, Dr. Thami Croeser says.

Too many trees fell for trivial reasons?

Professor Wolfgang W. Weisser, Chair of Terrestrial Ecology at the TUM School of Life Sciences, sees a need for action regarding the liability of tree owners and the acceptance of risks.

“Tree assessors are generally very cautious, as they can be held responsible for any problems that arise later, and therefore recommend felling in cases of doubt. In addition, they are often paid by clients who want trees removed, which inevitably leads to professional conflicts. Architects and urban planners also rarely include existing trees in their plans and act as if there are no trees in a project area."

He adds: "Finally, the approval authorities are too quick to grant permission for felling. All this leads to a high loss of old trees in our cities, which is increasing due to the growing densification of cities. Without better planning by cities on how they want to preserve their old trees and how they can ensure that the trees can grow older, for example by giving them more space, the situation will not change. To this end, it will also be necessary to plan water, electricity, telephone, and sewage lines much better in the future and to bundle the lines rather than laying them haphazardly and uncoordinatedly in the streets."

The study by TUM and RMIT offers important insights into the future of Munich and other cities worldwide. Planting trees is becoming increasingly popular, but protecting existing trees is just as essential.

Further information

You may find the visualizations of the simulations in the article: The importance of old trees for a city (Chair of Terrestrial Ecology at TUM School of Life Sciences).

Original paper: Thami Croesner, Wolfgang Weisser et al.: Defining ‘adequate’ tree protection: Meeting urban canopy targets requires careful retention of mature trees. In: Landscape and Urban Planning, Vol. 264, 2025. doi.org/10.1016/j.landurbplan.2025.105484

Another TUM study (2026): Climate impact of urban vegetation visible in detail for the first time: Urban trees can absorb more CO₂ than cars emit during summer