Forests play an important role in controlling the formation and movement processes of debris flows. They contribute to soil stabilization, regulation of soil water content, and act as robust structures impeding the downstream progression of debris flows. On the positive side, trees, to some extent, can intercept debris flows and effectively mitigate their velocity by increasing flow resistance. On the negative side, trees may suffer damage from debris-flow hazards, characterized by the generation of substantial quantities of wood fragments and consequential ramifications such as river channel blockage, resulting in backwater rise. In extreme cases, this blockage collapse can lead to instantaneous discharge amplification, thereby adversely impacting urban safety and impeding sustainable development. Therefore, in order to grasp the effects of tree characteristics on tree failure modes, the tree failure modes and corresponding parameters, diameters at breast height (DBH) and root-soil plate size, were identified and recorded through the post-event field investigation in Keze Gully, a region prone to debris-flow events in Sichuan, China, respectively. To investigate the impact of spatial variability in tree root distribution on tree failure modes, the root cross-sectional area ratio (RAR), root density (RD), root length density (RLD) and soil detachment rate (SDR) were obtained. The findings indicated that: (1) Tree characteristics reflect the interactions of debris flows and trees, and influence the tree failure modes ultimately. The root distribution characteristics influence the size and shape of the root-soil plate to affect the resistance of trees. (2) Compared to burial and abrasion, stem breakage and overturning are the predominant modes of tree failure in debris-flow hazards. Trees with a smaller DBH primarily experience stem breakage and bending, and trees with a larger DBH mostly experience overturning. (3) The root-soil plate shapes of overturned trees, affected by the root architecture and root growth range, are generally semielliptical or semicircular, and the horizontal and vertical radii increase with DBH, but the correlation between the root-soil plate's breadth-depth ratio and DBH is low. (4) The biomass and RAR decrease with distance. The RAR distribution exhibit the order of upslope direction > downslope direction > lateral direction. The coarse root biomass significantly increases with DBH, but no clear trend in fine root biomass. (5) The roots can significantly enhance the soil erosion resistance, but the erosion resistance of coarse roots is not as significant as that of fine roots. The erosion resistance increases with DBH, and follows the order of upslope direction > downslope direction > lateral direction. The results could provide new insights into the influences of tree and root distribution characteristics on tree failure modes during debris flows.

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 Sa similar to o 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 degrada-tion, 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.