Every tree betters our lives

Climate-proof for effective cooling

Dr. Roloff has thus devoted decades of work to identifying tree species able to withstand the new climatic conditions that will exist in the cities of the future. “We find such species in locations where heat and drought are already regularly occurring, as well as frosty winters, which trees also have to cope with,” Roloff relates. In addition to their natural habitats, Roloff has cataloged characteristics of these trees like leaf shape and hairiness as an indicator of what species could likely be “climate hardy”.

Victoria Volke, a doctoral student of Forest Science at the Helmholtz Center for Environmental Research (UFZ) in Halle, is studying the characteristics behind the cooling function of trees. “Crown density is an important factor as well as the tree’s capacity to vary its water potential, which in simplified terms is its ability to suction and absorb water from the soil and transports it out to its leaves,” she explains. The long-term objectives of her research are to define the traits that drive cooling function as well as to outline physiological strategies for gauging trees' drought resilience. “This would enable us to select tree species that will be able to exert their cooling function under the climatic conditions of the future,” says Volke. A wide range of characteristics may be relevant for these functions, thus diversity, she notes, is an important consideration in selecting tree species. “If drought stress or pest infestation causes one species to fail at a given location, other species could then compensate for the loss.”
 

Tree adaptability is greatest among trees that grow at a single location. Urban trees, however, are typically grown in nurseries for the first 10 or 20 years. In nurseries, root and branch shoots are clipped several times. They then have a compact root ball when planted at the desired location. “It can take some nursery trees 20 years to grow out of this ball shape,” Roloff says. And that is a long span of time to be particularly vulnerable to drought. “In the best-case scenario, a seedling will develop meter-long roots within a few weeks.” Martin Schreiner, a tree expert at Berlin’s Urban Greenery Protection Authority, says municipalities are doing the right thing when they plant and nurture seedlings directly in their greenspaces and parks. “If after 20 years, three or four healthy trees are left out of 50 seedlings, much has been accomplished with little effort,” he says. He also advocates specific tree nursing steps: “Watering young trees less frequently but with larger amounts of water promotes deeper root growth.” Tree expert Zielisch calls for more greenspaces in cities to be left in an un-landscaped, natural state: “By minimizing human intervention, we create islands where nature can regenerate, where the environment can revitalize itself.” 

Healthy trees, healthy people

Aletta Bonn, Director of the Department of Ecosystem Services at the Helmholtz Center for Environmental Research and of the German Center for Integrative Biodiversity Research (iDiv), is a proponent of greener cities. She researches and develops nature-based solutions to health problems associated with urban living. “Heat buildup, drought, particulate matter, exhaust, noise pollution ... Habitats in cities are subject to many environmental stress factors,” Dr. Bonn explains. Trees improve air quality, absorb street noise, create shade, and provide habitats for birds and insects, which reduces stress and promotes well-being for city dwellers. Trees can even improve our mental health. “We have observed that there is a reduced rate of depression in residential areas with lots of trees planted right in front of the houses compared to resident populations in areas with fewer trees,” she says. Planting trees could be a simple, natural solution for improving the physical and mental health of urban residents. Dr. Bonn’s conclusion: “Every tree betters our lives!”