Predicting tree failure to define roles and guidelines in risk management, a case study in São Paulo / Brazil

Tree failure can pose significant challenges to green-infrastructure planning for potentially jeopardizing ecosystem services provision, infrastructure safety, and citizens’ well-being. The city-wide disturbance caused by the loss of over 2000 trees annually in São Paulo, Brazil, impelled local authorities to collect detailed field-data on tree failure from 2016 to 2018 at the city center, a hotspot of tree failure, and then engage with the academia to support risk management. We aimed at building predictors and defining guidelines to reduce branch, trunk, and root failure based on species, wood status, root collar constrictions, conflicts with overhead cables, pruning methods, and site characteristics of 456 trees using Classification Trees and Bagging. These algorithms commonly used in decision-making yielded up to 70% accuracy, identifying wood status, root collar constrictions, and pruning as the main predictors. Branch failure represents 46% of the dataset. In the absence of wood degradation, branches were the most likely mode of failure. Root failure comes next representing 33% of the dataset, common to trees without wood degradation but with constricted root collars by pavement, compacted soil, or girdling roots. Root failure also dominates in trees with clear signs of wood decay and trunk cavities. Trunk failure only represents 21% of the events, common to trees with wood decay and subject to poor pruning practices. Thus, effective management of trees requires a collaborative approach to collecting data, analyzing, and establishing roles and guidelines. This study points to the role of local authorities in undertaking a detailed assessment of trees’ wood status throughout the city, while the municipality and private companies responsible for their management must adopt appropriate pruning practices. Lastly, those engaged in planting trees must guarantee enough space for the root collar to grow. Neglecting these guidelines can incur the cost of twice as much damage to the city.