Objective Green spaces in urban parks contain a large number of trees whose safety hazards are often overlooked. Tree safety risk assessment and accurate grasping the safety status of trees can provide a basis for tree safety management.

Method Trees with DBH ≥ 10 cm in Beijing Zoo were used for the study. Tree health was investigated visually and with simple tools, hierarchical analysis was applied to assess the risk level of tree safety and factor analysis was used to analyze potential impact factors of tree safety risks in Beijing Zoo. Trunk hollow condition was detected by means of PiCUS-3 Sonic Tomograph.

Result (1)There were 6 361 trees with DBH ≥ 10 cm in Beijing Zoo, including 103 very high-risk trees, 229 high-risk trees, 363 medium-risk trees and 5 666 low-risk trees. Those with extremely high and high risk levels were Populus tomentosa, Styphnolobium japonicum, Robinia pseudoacacia, Paulownia fortunei, Salix matsudana ‘Pendula’ with the general characteristics of broken trunks, abnormally toned trunks, severely deflected crowns, infested with pests and diseases, and loose tree bases. (2) The tree trunk (trunk height > 0.3 m) breakage, the base of the tree trunk (trunk height ≤0.3 m) damage and the root abnormalities were the top 3 potential influencing factors for tree safety risks in Beijing Zoo. (3) There was a significant difference in detection of hollow conditions within the trunks of Salix matsudana ‘Pendula’ between PiCUS-3 Sonic Tomograph and simple tools, however, no significant differences were observed between the two methods in tree species such as Populus tomentosa and Paulownia fortunei.

Conclusion In urban zoos, the health status of trees is determined by both intrinsic factors such as tree species, age, and size, as well as external factors including site conditions, abnormal weather, and damage caused by visitors or animals. The safety risk level of trees is determined by a combination of their health status and their planted location. PiCUS-3 Sonic Tomograph can enhance the accuracy of evaluations and can be used selectively. This paper is the first time to carry out tree safety risk assessment in an urban zoo, which is of reference and significance for tree safety risk management and the establishment of a tree safety monitoring mechanism in urban parks.