Why city trees die too soon. And why we urgently need them.

Urban trees provide shade, cool their surroundings, filter pollutants from the air, and create habitats for numerous animals. On hot summer days, the temperature under a large tree can be several degrees lower than in an unshaded area. 

All the more alarming is a development that can be observed in many cities: numerous urban trees are coming under stress, becoming sick or dying significantly earlier than they actually could.

But what is the reason for this?

The answer is complex. It's usually not a single factor, but rather the interplay of several stressors.

Key facts at a glance

• Urban trees cool cities, improve air quality and promote biodiversity.
• Many urban trees are suffering from drought, heat, and soil compaction.
• Sealed surfaces make water absorption more difficult.
• Root space is often severely limited in cities.
• Old trees are especially valuable and almost irreplaceable.
• Citizens, municipalities and companies can work together to protect urban trees.

Why city trees are so important

Trees perform a multitude of functions in cities. They shade streets, squares, and buildings. At the same time, they transpire water through their leaves, thus contributing to... Cooling their surroundings at.

This effect will become increasingly important in the future. Cities often heat up more than their surrounding areas. Asphalt, concrete, and buildings store heat and release it slowly at night.

Furthermore, trees filter fine dust from the air, produce oxygen, and create habitats for birds, insects, and other animals. Studies also show that green spaces and trees... Promoting the well-being of people and improve the quality of life in public spaces. In short: Healthy urban trees make cities more livable.

Why city trees often die earlier than forest trees

A tree in a forest grows under completely different conditions than a tree in a city. In the forest, living soil protects the roots. Rainwater can seep into the ground. Leaves remain and decompose into humus. The soil life ensures a continuous improvement of the soil structure.

City trees, on the other hand, often live under constant stress. Their roots have to contend with compacted soil, sealed surfaces, heat, and water scarcity. Many of these stresses remain invisible to us. You can find more specific information in the article. Increasing problems for city trees.

1. Too little space for the roots

A tree needs space not only above ground, but also below. In cities, root space is often severely restricted. Streets, sidewalks, utility lines, and buildings limit the area in which roots can spread. Often, a large tree has only a small patch of open soil available. This has consequences:

• less water
  
• fewer nutrients
• reduced stability
• limited development

A tree can only be as healthy as its root system.

2. Soil compaction deprives the roots of air.

Many urban soils are highly compacted. Cars, construction sites, heavy machinery, and intensive use squeeze the air out of the soil. This eliminates the pore spaces that should actually store water and oxygen. For trees, this means double the stress.

Firstly, the roots grow less efficiently. Secondly, less oxygen reaches the soil. Even if there were sufficient water, the roots often couldn't absorb it efficiently.

3. The urban heat island

Cities store heat. Asphalt, concrete, and buildings heat up considerably during the day and release the heat again at night. As a result, cities often cool down much less effectively than their surrounding areas. This means additional stress for urban trees.

In many locations, temperatures are consistently elevated during the summer. At the same time, evaporation increases, meaning trees need more water. However, this water is often not sufficiently available.

4. The tree dies of thirst next to the puddle.

This may sound contradictory at first. After heavy rainfall, streets are often flooded. Yet many city trees suffer from drought. The reason lies in the sealing of surfaces. A large portion of the precipitation goes directly into the sewer system. Instead of soaking into the ground, the water is quickly drained away. Often, very little of it remains for the tree. This problem becomes particularly evident during prolonged dry periods.

5. Salt, pollutants and other contaminants

Besides heat and water scarcity, urban trees are exposed to numerous other stresses. These include:

• Road salt in winter
• Pollutants from traffic
• dog urine
• mechanical damage
  
• Construction work in the root area

Each individual factor does not necessarily endanger a tree. However, in combination, they can lead to a continuous decrease in a tree's resilience.

Why old trees are especially valuable

When a tree dies, a new one is often planted. This is important, but it doesn't immediately replace the benefits of an old tree. A large tree stores significantly more carbon, provides more shade, and transpires considerably more water than a young tree. It often takes decades before a newly planted tree can provide similar benefits.

Therefore, protecting existing mature trees is particularly important. Every preserved mature tree is a benefit to the urban climate.

How modern urban tree management works

Many cities are currently developing new strategies to better protect their trees. These include:

• larger tree pits
• permeable surfaces
• improved soil preparation
• Rainwater harvesting
• Sponge city concepts
• climate-resilient tree species

A particularly important realization is that water, soil, and roots must be considered together. A tree doesn't just benefit from additional water; it also needs soil that can retain that water.

The Sponge City Principle

The concept of the sponge city is gaining importance worldwide. The idea is simple: rainwater should ideally stay where it falls. Instead of immediately draining it through the sewer system, attempts are made to store it on-site, allow it to seep into the ground, or retain it temporarily. The following benefit from this:

• city ​​trees
• Green spaces
• the urban climate
• groundwater recharge

The more water remains in the soil, the better trees can survive periods of drought.

What citizens can do for city trees

Protecting urban trees is not solely the responsibility of municipalities. Citizens can also make an important contribution. Possible measures include:

• Take on tree sponsorships
• Water young trees during dry periods
  
• Greening tree bases
  
• Avoid soil compaction
• Support local tree-planting initiatives

Even a few waterings during a heat wave can make a big difference for young urban trees.

Why slow watering often works better

Especially with urban trees, the method of watering is crucial. If water is applied too quickly, some of it runs off the surface or evaporates. A slow release of water gives the soil time to absorb the moisture.

irrigation bags They work on this principle. They release water directly into the root zone over several hours. This allows it to penetrate deeper and be available to the roots for a longer period.

This can significantly improve water supply, especially for young trees and during prolonged dry periods. You can find more information in the article. How can I water trees properly?