In Oldenburg, citizens can vote on 22 February on whether the city's tree protection statutes should be retained. In this interview, geographer Thorsten Balke talks about the special importance of trees for the urban climate.
Mr Balke, you are a professor of vegetation ecology and nature conservation. What role do trees play in the city for you?
A very decisive one, especially in view of what we are facing as a result of climate change. By the end of this century, we expect inner-city heatwaves to be up to five degrees Celsius hotter than they are today. Densely built-up and sealed areas in cities such as streets, buildings and squares heat up in summer, forming heat islands and also offering little habitat for insects and birds, for example. Flooding will also occur more frequently due to increasing heavy rainfall events. Urban green spaces and large urban trees in particular help with all these problems: They store water so that when it evaporates, it draws temperature from the air, cooling the environment. Urban trees also provide food and nesting places for animals, regulate the urban water balance, reduce noise and bind particulate matter. They are good for the physical and mental health of city dwellers and also help us to protect the climate by storing carbon dioxide.
Why are old trees in particular worth protecting?
These ecological services that urban trees provide for us are heavily dependent on several factors, for example the size of the crown and the number of leaves per ground area. This is of course considerably higher for an old beech than for a small, young tree. Once a large single tree has been felled, it takes decades to achieve the same leaf area again, even when replanting.
How many trees and green spaces does a city need?
Studies show that for an average cooling effect of one degree, at least 15 per cent of the city area must be covered with trees. With 50 per cent tree cover, a cooling effect of three degrees could even be achieved. Either way, the more urban greenery, the better for the climate, people and nature. It is also important that the trees are distributed as evenly as possible throughout the city.
What is the situation in Oldenburg?
In an international comparison, Oldenburg has a lot of green spaces, accounting for around 66 per cent of the city area, but they are not evenly distributed. In the northern city centre in particular, there are only a few that are publicly accessible. This has also been highlighted by the City of Oldenburg's "Urban Green Masterplan". Above all, not all green spaces consist of trees: According to data from the European Environment Agency, Oldenburg's tree population in 2018, at around 25 per cent of the city's area, was not as large as one might expect by national standards. Measured by this proportion, Oldenburg is in the lower third of the German cities analysed. Oldenburg's tree population is expected to cool the city by around 1.5 degrees in summer. So there is still plenty of room for improvement here.
You differentiate between green and tree areas. Why is that?
Green spaces are still better than sealed surfaces made of concrete, stone or tarmac. But trees have a much stronger cooling effect on their surroundings than grassy areas. I like to do seminar work with my students on the topic of urban greenery on the Dobbenwiese in summer. There you can clearly see in summer how much green spaces without trees can overheat.
To what extent can municipal bylaws help to preserve the local tree population?
From the point of view of environmental protection research, any protection of urban trees is generally better than none. Much then depends on how such bylaws are specifically designed and implemented. To my knowledge, there is still no clear evidence in research as to whether the introduction of tree protection bylaws actually helps to preserve the tree population. However, there is evidence that where tree protection bylaws have been abolished, the tree population has declined. Apart from this, it is important to create additional positive incentives for planting and preserving urban trees on private property - for example through support programmes from which property owners can benefit if they plant additional trees. It can also help to offer low-threshold advice, give away young trees free of charge or place more leaf bins on the streets to make it easier for people to preserve and care for trees.
What role do trees in private gardens play in the urban climate?
They make up a very significant proportion of urban greenery, especially in Oldenburg with its many detached and semi-detached houses. Here, every owner can contribute to the urban climate, but also to flood protection and nature conservation. The basic principle is to seal as little area as possible, to plant shrubs and bushes rather than lawns and to allow a meadow to grow from time to time. Other positive effects are also possible and desirable - for example, if fruit trees are planted and harvested locally. The resulting reduction in average "food miles" - the distance that food travels to reach the end consumer - is a good example of the positive synergy effects of nature-based solutions in the city.
There are also numerous old trees worthy of protection on the grounds of the university's two campuses...
...And the university would do well to preserve its existing trees and plant new trees in appropriate locations for future generations. However, trees in built-up areas can easily come under stress. For example, when construction work is underway, individual trees should be protected from soil compaction and direct damage from vehicles. The university should also pay attention to water management, as most tree species do not like "wet feet". And especially now, in winter, trees need to be protected, for example from road salt.
Interview: Henning Kulbarsch
