Defining ‘adequate’ tree protection: Meeting urban canopy targets requires careful retention of mature trees

Abstract

Canopy cover is increasingly recognised as an important aspect of the urban landscape, and as a result urban forestry plans have been adopted in many cities globally. These are often built around ambitious canopy cover targets, with a goal to keep cities cool and improve their amenity while delivering co-benefits like health improvements, flood reduction and enhanced biodiversity. Canopy targets and tree planting have tended to receive more attention than another critical determinant of canopy expansion: keeping the trees alive once they are planted. In this study, we use a model to examine how varying tree removal rates influences the long-term canopy cover provided by street trees, considering both typical policy timeframes and projections through the end of the century. The model uses allometrics from real urban trees in four cities to project canopy growth under a wide range of removal rates, for a wide range of potential urban forest characteristics. We find that retention of mature and maturing street trees is consistently critical, with removal rates exceeding 3% per annum generally associated with much lower canopy progress (and usually very poor results). The model’s outputs demonstrate how cities can build a clear case for better-integrated urban governance to ensure these challenging threshold values are avoided.