Crown size and growing space requirement of common tree species in urban centres, parks, and forests

•Species-specific crown radius–stem diameter relationships for open grown conditions to estimate development and space requirements for urban trees.•Species grouping by allometric properties was possible.•World wide data set of 22 tree species covering information about tree and crown size.•Discussion deals with using the findings in practice and provides links to ecosystem services of urban trees. Tree crown size determines among others tree's growth, carbon sequestration, shading, filtering of fine air particulates, and risk of wind-breaking. The dependence of crown size on species, resource supply, and tree age complicates an accurate evaluation of a tree's space requirement, and its size-dependent functions and services in urban as well as in forested areas. Based on a world-wide dataset of tree crown measurements of 22 common urban tree species we first derived species-specific crown radius–stem diameter relationships for open grown conditions. By cluster analysis we then assigned the 22 species to 5 crown extension types and developed mean relationships of tree height, crown radius, crown projection area, and crown volume depending on tree diameter for each type. This allometric analysis yielded auxiliary relationships which can be used for estimating the species-specific crown size and dynamics at a given tree dimension. We discuss how the results can support the choice and initial spacing of particular species and the assessment and prognosis of their functions and services.