Adopt trees for cleaner cities

Yellow treetops, fallen leaves, empty branches. Those of you thinking of autumn, falling leaves, trees preparing for winter dormancy, are, unfortunately, mistaken. Even in summer, when trees should be the richest green, they now look rather drab. For me, that is a shocking sight. So I asked myself, what is the reason for this? And more importantly, what can I do about it?


Yellow treetops, fallen leaves, empty branches. Those of you thinking of autumn, falling leaves, trees preparing for winter dormancy, are, unfortunately, mistaken. Even in summer, when trees should be the richest green, they now look rather drab. For me, that is a shocking sight. So I asked myself, what is the reason for this? And more importantly, what can I do about it?

For city dwellers like me, trees are often the last everyday connection to nature. In Berlin in particular, I see time and again how much people love their parks and spend their free time in green spaces. Not only do green spaces improve the quality of life for us city dwellers, they also have a significant impact on the urban climate and reduce a city’s CO₂ emissions.

After a brief fact check, it becomes clear why cities need to reduce their CO₂ emissions:

  • More than 50% of the world’s population lives in cities
  • Cities produce more than 75% of global CO₂ emission
  • Cities often feel the impacts of climate change more strongly

In short, cities need to move from being problem creators to problem solvers. The UN has also recognized this. Its eleventh goal for sustainable development is therefore to make cities and communities sustainable. Trees in cities, in particular, can be a solution to this. Trees bind CO₂, produce oxygen, cool their ambient temperature, provide a habitat for insects and birds, filter pollutants from the air, reduce street noise and have a positive effect on our psyche. In short, for a healthy and sustainable life, we need more trees and green spaces in the city. Unfortunately, our trees are anything but well off. The reason for this is the so-called drought stress. More and more urban trees are suffering from a lack of water.

Why are our cities warmer than the surrounding regions?

Especially this summer, cities were noticeably hot. However, if you took a break in nature, you might have noticed a significant drop in temperature compared to the city. This strong temperature difference between city and countryside is called Urban Heating Effect. The difference can be as much as 15° C. It is a typical phenomenon of urban environments. In urban agglomerations, higher air temperatures are observed near the ground compared to rural environments.

What is the reason for this considerable difference? The tightly sealed surfaces in cities absorb solar radiation and thus heat. They also prevent rainwater from seeping into the ground. Consequently, rainwater evaporates very quickly from such surfaces. Moreover, without plants to provide transpiration, no cooling effect occurs. Overall, there are a variety of factors that influence the intensity of the Urban Heating Effect:

  • The size of the city and the urban structure: these include building density, building heights and the proportion of green space. Larger cities tend to have more sealed surfaces.
  • Topographical characteristics and the more general climatic conditions also play a role, i.e. climate zone, weather conditions, atmospheric currents, …. Of course, this can’t really be changed. However, urban planning could take into account the fact that the layout of streets can be adapted to allow natural wind currents through to cool the city.

The Urban Heating Effect means that energy consumption for air conditioning also increases, which in turn leads to higher resource consumption. Since energy is still often generated from fossil fuels, an increase in electricity consumption inevitably leads to increased levels of pollutants and greenhouse gases in the air. In addition, the waste heat from air conditioning systems further heats up ambient temperatures.

The warmer it gets in cities, the more air conditioners are used, but they, in turn, also contribute to rising air temperatures (Copyright: Sergei A. on Unsplash).

For us humans, the heat of the cities has a direct negative impact on our health: dehydration, heat exhaustion, and heart attacks are becoming more frequent. The consequences of heat especially affect the weaker members of our society, such as the elderly and the children. That’s why we need to provide cooling in our cities.

Natural A/C

There is a simple way to cool the city “naturally”: Trees. On the one hand, they provide pleasant shade in the summer. On the other hand, trees already help to prevent the ambient temperature from heating up so much in the first place. The evaporation of water in the leaves creates a cooling effect. In addition, trees convert CO₂ into oxygen. By binding particulate matter, sulfur and nitrogen oxides, the air we breathe in the city is improved. In short: urban trees are real all-rounders!

The amount of CO₂ bound per tree varies greatly. How much CO₂ a tree binds and how quickly it does so depends on many factors. These include the tree species, the age of the tree, its wood density or growth rate. But external factors such as climate, soil quality or water supply also play a role. Therefore, general statements on this question are difficult. However, it can be assumed that a tree stores 10 kilograms of CO₂ per year. This corresponds to about one third of the daily CO₂ emissions of an average German citizen.

Trees have a positive effect not only on our physical condition through the cooling effect and purification of the air, but also on our psyche. Studies show that people living in green streets experience less depression, feel less stress, suffer less from cardiovascular diseases and have an active lifestyle. Another very interesting finding is that urban greenery also correlates with lower crime rates.

So all in all, trees have a positive impact on our everyday lives. However, our “all-rounders” don’t have an easy time thriving in cities: CO₂, particulate matter, radiant heat, construction work, road salt, car tires and ground work damage the root system; dust and air pollutants take their toll on the leaves. Added to this are heavy rains, violent storms blowing through the streets, and nonnative pests.

Copyright: Jan Huber on Unsplash

How do we get more healthy trees into our cities?

The Daniel Schlegel Foundation has already started several tree planting projects, for example trees were planted in Wilmersdorf and in Halensee. It is important that the trees continue to be cared for and, above all, watered after planting. The Daniel Schlegel Foundation ensures this. A team of gardners makes sure that the young trees develop well and healthily.

Of course, it is not possible to simply plant trees haphazardly all over urban areas. But a first step is to take care of the already existing trees so that they get enough water. This can be quite simple: I, for example, have unceremoniously “adopted” a few trees in my street. That is, I water these trees regularly and clean up the trash around the tree.

Are there trees near your house that need some care? Here are some tips for better watering:

  • Watering is best done in the early morning, when the soil is most receptive. Another possibillity is the time after sunset. In any case, do not water in the midday sun, as the water would evaporate immediately.
  • Better to water once a week with ten watering cans per tree than a little every day. It does little good if the water only remains on the surface, as it evaporates quickly. Several cans per tree allow the water to seep down to the roots.
  • When the ground is dry and hard, use a spade or shovel to loosen it up so that the water can reach the roots of the trees better.
  • If you want to provide even more greenery, you can plant small plants into the ground around the tree. Planting around the tree prevents rapid evaporation of water. This way the plants can draw from their water supply for longer spans of time. It is best to use robust and insect-friendly plants. To find out whether this is permitted, contact your local parks department.
  • You can never water a tree too much!

Otherwise, do not lock bicycles to trees, as the bark could be damaged. Dog urine also damages the trees. Berlin has about 433,000 trees, so we have a big task ahead of us. So get out the watering cans and water the trees in your neighborhood!

To summarize, we have found that cities produce large CO2 emissions and suffer from the effects themselves. This is due, for example, to the Urban Heating Effect, which heats up urban environments. The consequences are exhausting for humans, animals and nature. However, they can be reduced by healthy trees. Therefore, adopting trees is a good way to protect the environment and make the urban environment more comfortable.

Ella Straubinger

Ella always has a book with her. She likes to try out new vegan recipes and enjoys travelling in nature - whether on foot, by bike or in a canoe.
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