Sorry, your browser doesn’t support embedded videos. Stories Project Stories 13.02.2026 How do urban trees fare? Kaisa Rissanen studies the stress suffered by trees Kuva: Aino Huotari TEXT Sandra Järvenpää Share: The trees have been imported to urban environments to make them more enjoyable and to improve people’s living conditions. However, as the climate becomes warmer and extreme weather conditions more prevalent, are the trees able to provide the desired ecosystem services? What Is It About? Kaisa Rissanen, Postdoctoral Researcher at the Faculty of Agriculture and Forestry, studies, with the funding of the Kone Foundation, stress signals from urban trees as well as their impact on trees and the ecosystem services they provide. As the climate becomes warmer and weather conditions become more extreme, much stock is placed on urban trees: they are expected to bind carbon dioxide and water and cool the air. However, changing environmental conditions also impact the wellbeing of the trees themselves. In her three-year research project, Rissanen is exploring ways of supporting the well-being of trees in cities. When looking at urban trees, one seldom thinks that hardly any of them have chosen to grow there. Trees live primarily in forests, but we humans have imported them to our constructed environment. And in an age of accelerating climate change, we count a great deal on urban trees: Trees are hoped to cool the air, bind stormwater and make urban environments more enjoyable, for example. But how do the trees themselves fare in the cities? In her three-year research project, Kaisa Rissanen studies stress suffered by two tree types, the Silver Birch and the Norway Maple, in urban environments. The research covers a total of 60 individual trees, and Rissanen’s sample distribution is limited to three different urban environments: Streets, parks and urban forests. The research is still incomplete, but Rissanen already has something to say about the Norway Maple: “We found a small indication that maple leaves are smaller on street trees than on trees in forests or parks, which may mean that they are adapting to a smaller volume of water. With smaller leaves, trees evaporate less water. This makes them slightly more prepared for droughts. And with a survival strategy, there is less stress. “For plants, stress means that environmental conditions are outside their optimal capabilities.” What Is the Stress Suffered by Trees? As Rissanen has measured trees in downtown Helsinki, people have come to talk and ask what she is doing. As a result of these discussions, Rissanen started thinking whether there is a better word to describe how trees react. After all, us humans have a clear understanding what stress means: Urgency, anxiety, tension. “For plants, however, it means that environmental conditions are outside their optimal capabilities.” So if the environment is, for example, too warm or too cold, too dry or too wet, and these new conditions impact plants’ ability to function in some way. Various biotic factors, such as fungal diseases and pests, can also cause stress to trees. Rissanen considers stress to be an umbrella term used for various negative factors impacting trees’ ability to function. While researching stress, Rissanen measures various physical phenomena. Binding and evaporation, the size of leaves and amount of water in them. However, the stress signals from these measurements do not yet tell us how a tree experiences stress. “We can measure it but cannot access the tree’s experiences as we are humans. We do not understand plants enough to truly understand their experience – if such a term as experience can be applied to plants.” Photo: Aino Huotari “Thanks to internal combustion engines, the CO2 concentration is much larger in cities, which is favourable for plants.” Cities Also Include Nice Things, Even for Trees However, cities are not just a harmful environment for trees, they also have many favourable factors for trees. “Thanks to internal combustion engines, the CO2 concentration is much larger in cities, which is favourable for plants. And in a colder environment, cities being a slightly warmer place can be a good thing for trees, as it extends the growing season. In addition, cities contain plenty of nutrients.” According to Rissanen, some studies show that trees grow faster in cities than in forests. Could urban trees, in time, develop into a species of their own, such as an Urban Maple? “They could, after an extremely long period. However, we are now studying acclimatization, or the adaptation that occurs during a tree’s lifetime, whereas adaptation refers to the evolutionary process by which natural selection chooses the best.” However, acclimatization indicates that trees are highly adaptable and able to respond to changes in their environment, either through their leaves, the wood, or perhaps their roots. “There is very little research being performed on roots, as they are under all that infrastructure.” Kuva: Aino Huotari “The idea behind this study, too, is that once we have a better understanding of how trees function in different environments and which specific urban environments might be harmful to them, then we can then modify their management, perhaps even the choice of species.” In an Ecosystem, the Stress Experienced by One Organism Impacts Another During the last ten years, research on urban trees has grown exponentially. According to Rissanen, this is because cities will have to respond to climate change and consider ways of adapting infrastructure and the environment to new conditions. “However, previous research has focused on cities that experience heat and drought first, such as Southern Europe.” While Rissanen is a tree physiologist, her studies often involve humans in one way or another. “The idea behind this study, too, is that once we have a better understanding of how trees function in different environments and which specific urban environments might be harmful to them, then we can then modify their management, perhaps even the choice of species. Then, they would produce the ecosystem services that cities desire as efficiently as possible, such as cooling, recreational services for people, or air pollution and carbon binding. Rissanen’s study also involves exploring how the stress suffered by trees impacts the implementation of ecosystem services. The term ecosystem services places focus on what benefits trees offer to humans, but what about others: can the stress of urban trees accumulate to other species living nearby? “That is an interesting question. For certain species, stress can be a good thing, such as for fungi or insects benefiting from dying trees. For other species, such as birds that use tree canopies as cover, trees losing their foliage has a very negative impact. In an ecosystem, the stress suffered by one species always impacts the next one way or another. Photo: Aino Huotari Contact Information Postdoctoral Researcher Kaisa Rissanenkaisa.rissanen@helsinki.fi Ecosystem services describe material and immaterial benefits born from nature. The term is used in particular when the importance of ecosystems and the benefits they provide is sought to be understood holistically and evaluated economically. For example, the forest ecosystem not only produces wood but also sustains biodiversity, stores carbon, and refreshes hikers, among other things. Kuva: Aino Huotari