When urban planning becomes tree planning

21.06.2023 When it comes to urban planning, trees often get the short end of the stick. Part of the problem is maps, which tend to oversimplify. In this context, Katharina Scheller decided to focus her master’s thesis on how innovatively designed tree registers can support green urban planning in the long term.

What trees can do

Trees have different effects on urban areas depending on their size, age and root spread.

The temperature underneath a full-grown tree can be up to 8 degrees cooler. Evaporation further increases this cooling effect.

Urban trees offer a basic source of food and provide a habitat for a wide variety of creatures.

Trees are supplied with important nutrients through their symbiotic association with fungus (mycorrhiza). This interaction also enables the exchange of nutrients and information among trees.

What trees need

To thrive, trees need sufficient space both above and below the ground. As a general rule, trees need the same amount of space under the ground as is taken up by their crown.

Soil sealing hinders the absorption of water and nutrients.

Basements and sewers negatively impact root space.

Stressors such as heat, drought and pollutants can reduce tree vitality.

Groups of trees act as an ecosystem, promoting biodiversity. This then increases the vitality of individual trees.

Urban planning with trees: tree registers

Urban trees are recorded as a tree register in georeferenced mapping programs for administrative and planning purposes. Tree registers contain very little information. They simply show where trees are located, and do not provide information about the real dimensions of their living space. Hardly any (visual) information is provided about their ecological requirements and functions.

The symbol’s contours indicate an abstract shape of a tree crown.

Current tree registers show deciduous trees, conifers, fruit trees, street trees and other urban trees.

Information regarding tree species, age and protective status is incomplete on maps and is therefore only of limited significance.

The maps do not show how trees affect their environment or the impact the environment has on them. Yet this information is hugely important, as the conditions of a particular site can have a favourable or negative impact on tree health. In addition, individual trees and groups of trees form complex ecosystems that perform important functions for biodiversity and the urban climate.

How mapping can build bridges

In her master’s thesis, Katharina Scheller puts forward initial ways of improving tree registers. ‘But we’re at the very beginning,’ explains the researcher. ‘Basic research still needs to be carried out in this respect.’ One thing is clear, though – when species-specific requirements, root spread and light and soil requirements are made visible, tree registers can make tree planning easier.

Untersucht wurden verschiedene Möglichkeiten, wie Altersphasen von Bäumen in Karten aussagekräftiger dargestellt werden können. — Exploring different ways of depicting tree life stages on maps in a more meaningful way.

1 Visualising the life stages of trees

Firstly, trees only develop the full extent of their important functions such as cooling, absorption of CO₂ and filtering of pollutants at a certain age. Secondly, the older they get, the more important trees become for biodiversity, as a large number of habitat structures are only formed during the ageing process. Old trees and deadwood provide valuable refuge and habitats for many species in densely built cities.

Ausserdem wurde erprobt, wie sich kartografische Baumdarstellungen mit infografischen Elementen ergänzen lassen. — Investigating how tree mapping can be supplemented by infographics.

2 Representing habitat characteristics

Old trees provide valuable habitats for a wide variety of creatures. Habitat trees are of considerable ecological importance to biodiversity in urban areas. Each of the visual attributes in the example above represents a specific characteristic of a habitat and can be combined at will. However, readability tends to decrease with increasing complexity.

Die Informationen zu den Dimensionen von Bäumen (sowohl ober- als auch unterirdisch) fehlen auf den derzeitigen Baumkartierungen gänzlich. Für Bau- und Gestaltungsprozesse im urbanen Raum sind solche Informationen aber zentral. — Information regarding the dimensions of trees (above and below the ground) is missing from current tree maps entirely. Yet such information is crucial when it comes to construction and design processes in urban spaces.

3 Depicting size

The size of a tree plays a decisive role in what it can do. The bigger the tree, the more of a cooling effect it can have on the surrounding area. In addition, the larger the tree, the more CO₂ it can absorb. Errors are frequently made in urban planning that result in too little space allocated to trees and planting in unfavourable locations. Trees either have not enough space to grow, or are cut down because they take up more space than expected as they grow. With this in mind, it is also important to take into account the underground parts of trees – their roots.