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The EU Environment Council has adopted a restoration regulation that draws attention to the benefits of greenery in urban ecosystems. There is fresh data on the issue: a study has examined the amount and composition of vegetation on residential plots in Helsinki, and their role in local climate impacts such as carbon sequestration and average summer temperatures.

Vegetation lowers temperatures through shading and evaporation, while densely built urban areas retain heat. Summer heat waves are also projected to intensify in the future. 

The study was conducted by Aalto University researchers Paula-Kaisa Leppänen, Antti Kinnunen, Ranja Hautamäki and Seppo Junnila. They and other authors point out that the amount of vegetation on residential plots in Helsinki has decreased by about 15 percentage points between the 1970s and the early 2010s. One of the explanatory factors is plot efficiency and the type of construction. For example, loosely built single-family housing areas take up a lot of space, but they can also accommodate a lot of vegetation, while dense apartment blocks have much less space for vegetation due to parking and other land use requirements. 

However, the study shows that efficiency is not the only explanatory factor: for example, dense residential areas in the early 20th century had more vegetation, especially large trees, than new dense residential areas. This result is significant and reveals that current dense urban development is not able to guarantee green cover in a balanced way. This leads to significantly lower carbon sequestration in new residential areas and higher summer temperatures compared to older areas of the same density. 

, written in collaboration with researchers from the University of Helsinki and Häme University of Applied Sciences, tackles the topical issue of the role of greenery in densifying urban structures and the climate benefits of vegetation. As peak summer temperatures rise due to climate change, the cooling effect of large trees, for example, can only be achieved with local trees. 

The carbon sinks of ground vegetation also play a role in meeting urban climate targets. In addition, vegetation provides other benefits such as supporting stormwater management, biodiversity and habitat amenity. 

In dense urban structures, sufficient space should be reserved for vegetation, with particular attention paid to ground vegetation and large trees. Vegetation must be provided with good conditions for growth and thriving, and space must be ensured for both the canopy above ground and the root system below ground. Vegetation on sites must be taken into account at all levels of urban decision-making, from planning to construction and maintenance.

"A city is built over decades, which means that the solutions we make now will still have an impact on our environment in decades to come," says researcher Paula-Kaisa Leppänen. 

The study is part of the Co-Carbon research project, a multidisciplinary research project funded by the Strategic Research Council. The project will measure and model the carbon sequestration capacity of urban greening and develop practices for the design, implementation and maintenance of carbon-wise urban greening.