The role of planned unit developments in regulating urban tree canopy coverage in Zurich, Switzerland

There is a broad scientific and societal consensus that cities worldwide must become more ecological and climate-resilient. Land-use planning has various instruments for leverage, including conventional zoning at the municipal level and planned unit developments (PUDs) at the neighbourhood level. While they differ in their local characteristics, both instruments are used in many countries. However, their use and efficacy in facilitating sustainable urban transformation (such as the preservation and enhancement of urban trees) have been challenged. This study considered the parts of urban land parcels that are not covered by main buildings as ‘tree area potentials’ (TAPs) and quantitatively compared conventional zones with PUDs in the Canton of Zurich, Switzerland, developed before 2000. We analysed the different ground cover types (such as subsidiary buildings or traffic areas) in the TAP areas and the proportion of each TAP area comprising tree canopy – referred to as ‘realised tree area potential’ (rTAP). In addition, planning documents of PUDs were qualitatively assessed to determine their regulations regarding urban trees and how these regulations related to the rTAP. In general, the assessed PUDs and conventional zones could not be distinguished based on their rTAP values despite PUDs having greater flexibility to regulate open spaces. Exceptions with higher rTAP tended to have clear regulation of trees. Further research is needed to determine why the early PUDs were not more successful in preserving and enhancing trees and how this can be improved in the future.