Worldwide comparison of the growth and ecosystem services of urban trees influenced by species traits and climate

Urban trees are a defining element of the cityscape. In addition to their aesthetic value, further ecosystem services they provide, such as cooling, shading, air purification and runoff reduction, are currently the subject of research for climate-adapted cities. In times of climate change with frequent drought events and global warming, these are important elements for sustainable urban planning and climate protection in cities. However, the ecosystem services provided are directly related to the tree structures and size dimensions, the tree species, vitality and the site climate. In this study, more than 6,500 tree individuals of 26 common urban tree species in 17 cities worldwide were analyzed. Allometric growth relationships of tree dimensions (trunk diameter, height, crown dimensions, leaf area) were determined and tested for significant differences between species, climate of a city and growth types. A cluster analysis was used to evaluate allometric growth types based on the correlations between heights and crown growth with stem diameter, with which the species can be assigned to different growth types (e.g. Picea glauca as a species with small crown size but steep slope and Quercus robur as a species with large crown size and medium slope). The CityTree model was used to calculate and compare the ecosystem services provided by selected tree species in terms of CO₂ storage, runoff reduction and cooling through shading and transpiration. The results show that species characteristics (growth rate, drought strategy, crown density, light requirements) and location (i.e. urban climate and local site conditions and soil sealing) directly influence growth and ecosystem service provision. Our findings are of relevance for the sustainable planning and management of urban tree populations, particularly under climate change. The presented evaluations provide a simple determination of the expected tree dimensions and ecosystem services for individual tree species in cities.