Green Air Conditioning Systems

On hot summer days, many air conditioners in cities run at full speed. But the most effective cooling devices have been right on our doorstep all the time – tall, deeply rooted and with a dense leaf canopy. Urban trees: they provide shade and noticeably lower the temperature in their surroundings. They are not only natural cooling units, but have also always been places of encounter and respite: from the rural lime tree to the plane tree in ancient Athens. But urban greenery is under pressure. Heat, drought, disease and sealed land prevent trees and bushes from reaching their ecological potential and cause them to die.

Searching for clues in the Upper Rhine Plain

In Karlsruhe and Freiburg, the drought years of 2018 and 2019 made clear how vulnerable young plantings are: “For several years now, around 30 percent of newly planted young trees have been dying. These losses are not only ecologically painful, they are also expensive and high-maintenance, because each tree costs the city several thousand euros and must be planned carefully before it is allowed to take root,” says Dr. Somidh Saha, research group leader at the Institute for Technology Assessment and Systems Analysis (ITAS). These fi gures are more than just statistics: they represent missed investments, gaps in the urban landscape, and people who will have to do without the green cityscape, shades and coolness of trees in the future. In the project “Future-oriented climate change management for urban green spaces (ZUKAMAS), KIT is conducting research together with other partners along the Upper Rhine Plain to determine which tree species, forms of care and planning tools will keep urban trees resilient despite climate change – so that they can continue to provide cooling, create meeting places and make our cities worth living in for future generations. The research area was deliberately chosen: the Upper Rhine Plain is one of the most heavily affected regions in Germany when it comes to heat and drought. “What we are testing here serves as a model for many other cities along the Rhine and Elbe,” says Saha.

High tech meets root systems

A walk through the neighborhood is not enough to assess the health of the trees. Instead, high-tech equipment is used. Sensors measure water uptake, trunk growth and microclimate in real time. Dendroecological analyses provide insights into the reactions of different tree species to historical drought events. Molecular studies show how plant hormones respond to stress and how this affects resilience. At the same time, researchers are collecting data on the perception and acceptance of urban greenery. Which tree species are popular within the population? Where do people want more greenery? And how can new green neighborhoods be prevented from exacerbating social imbalances? “Our philosophy is to take a holistic view of the development of urban trees – from the lab bench to the street,” says project coordinator Saha, explaining the approach.

From record to urban planning

The data obtained is used to develop practical recommendations for cities and municipalities. In Karlsruhe and Freiburg, researchers are working closely with the horticultural authorities to test specifi c options for action during the project period – from targeted irrigation of young trees to recommendations for climate-resilient species. ZUKAMAS aims to provide specific recommendations: plant lists, maintenance plans, irrigation strategies and tools for city administrations for tree felling to be carried out less frequently, in a more targeted manner and for valid reasons. Instead of felling trees across the board – a common reaction because authorities need to protect themselves legally – data-driven assessments could also show whether a tree is actually in severe danger or can still be saved. This saves costs, conserves resources and preserves old trees that are valuable for the urban climate.

Outlook: workshop, tools, impact

Ecologist Saha has a clear vision: “We should aim for at least 30 percent green space in the city.” Whether and how this can be achieved politically depends on resources, administrative capacities and the willingness to plan ahead – precisely where ZUKAMAS wants to provide tools. Over the next few months, the project will evaluate the data it has collected. Saha would also like to hold a final workshop with all relevant stakeholders to discuss findings and explore paths of implementation. Ultimately, it’s not just about planting trees in the short term, but integrating them into the urban infrastructure in the long term – and thus preserving the city’s green air conditioning system: for cooling, quality of life and health.

Further Links:

• Further Information

Images:

• Midjourney
• Amadeus Brahmsiepe / KIT