A team of researchers from the University of Gothenburg proposed a practical approach that helps city trees grow taller and develop broader, spreading crowns. The result is a cooler, greener urban microclimate as shade from the crowns combined with leaf transpiration lowers daytime temperatures in densely built areas. The findings appear in Landscape and Urban Planning (LUP).
Researchers explored how the ground cover surrounding tree trunks influences growth for oak, chestnut, and linden trees in Gothenburg and Mölndal. The study evaluated several variables that shape a tree’s impact on its surroundings, including canopy density, overall tree height, and the rate at which leaves release moisture through transpiration. The interplay of these factors determines how effectively a tree can shade streets, sidewalks, and nearby spaces while contributing to evaporative cooling.
Results showed that mature trees aged about 20 to 30 years stood on grass-covered soils about 2.6 meters taller on average than the same trees growing beside asphalt or paving. Moreover, the crowns of trees in grassy areas tended to spread roughly 1.3 meters wider and exhibited about 61% greater density compared with trees in harder ground. Putting grass and soil where there previously was hard surfacing can nearly double a tree’s cooling influence on the surrounding air, especially during heat waves.
The research also revealed how tree species respond to the types of surfaces they border. Chestnuts proved to be the least fussy in terms of tolerance to surface conditions, while grasses generally provided less cooling than either oaks or lime trees. These differences underscore the importance of selecting appropriate species for urban plantings based on the ground cover and surrounding materials.
Earlier work by scientists has highlighted the vast amounts of land across Europe that remain forested yet unaccounted for in planning models. The Gothenburg study contributes to a clearer understanding of how city trees, ground cover choices, and species selection together shape urban cooling, air quality, and comfort for residents.