Every year before the onset of wet season, tree management departments will complete Tree Risk Assessments (TRAs) and implement appropriate risk mitigation measures to protect tree health and public safety. In recent years, the Government has launched several pilot schemes to explore the use of technology in tree management to enhance its quality and efficiency. This time, I have invited a colleague of the Greening, Landscape and Tree Management Section (GLTMS) of the Development Bureau (DEVB) to talk about tree inspections before wet season. I have also invited Dr WONG Man-sing, Charles, Associate Professor of the Department of Land Surveying and Geo-Informatics of the Hong Kong Polytechnic University (PolyU), to introduce how to apply smart sensing technology to monitor tree stability.

Risk assessments before wet season

According to the “Guidelines on Tree Risk Assessment and Management Arrangement”, every year before the onset of wet season, tree management departments are required to complete TRAs in areas with high pedestrian and traffic flow professionally and systematically and implement appropriate risk mitigation measures based on the assessment results, such as crown pruning and installation of support systems. Dangerous trees with untreatable problems need to be removed as soon as possible to safeguard public safety. Currently, various tree management departments have commenced inspections while the DEVB has also reminded departments to pay closer attention to the trees affected by the typhoons last year and arrange appropriate risk mitigation measures to protect public safety.

Mr TSANG Kwok-on, the GLTMS’s Tree Management Officer, said that tree inspection personnel mainly perform ground inspection to examine and assess various parts of a tree, including the crown, leaves, branches, trunk, roots and the surrounding environment of the tree, etc. Inspection personnel would also employ tools to aid their work, for example, using a plastic mallet to tap the trunk to assess its structural condition and using binoculars to observe the growing conditions of the higher branches and leaves. If necessary, inspection personnel would climb up the tree to inspect the hidden parts from different angles. If decay or other structural problems are found or suspected, they would use a resistograph or sonic tomograph (pictured) to examine the internal structural condition of the tree.

Applying smart sensing technology in tree management

Under adverse weather such as rainstorm or typhoon, it is inevitable that trees would suffer different degrees of damage. A research team formed by DEVB, the PolyU and other tertiary institutions is conducting a 3-year Jockey Club Smart City Tree Management Project to monitor tree stability on a large scale through smart sensing technology and Geographic Information System. The research team will assess the risk of tree failure by monitoring trees’ swaying or tilting condition, thereby strengthening tree risk management.

Dr WONG Man-sing, Charles, Associate Professor of the Department of Land Surveying and Geo-Informatics of the PolyU, said that the sensors are installed at the lower trunks of the trees to assess the trees’ tilting angles and directions, and the data would then be sent to the university’s data centre via network transmission for big data analytics. If it is shown that the tilting angle of the lower trunk of the tree exceeds the threshold, the system would immediately send an alert to the designated parties to undertake timely and appropriate risk mitigation measures.

Large-scale monitoring of tree stability

Dr WONG said that the Smart Sensing Technology pilot scheme started in February last year. An initial trial was conducted in Tai Tong, Yuen Long, to set reference for the design of sensors and monitoring system. Upon fine tuning the system, the research team has installed the second batch of sensors in Wan Chai and Kowloon East districts to test the network transmission performance in urban areas. The whole pilot scheme involves the installation of a total of 8 000 sensors on selected urban trees and all stonewall trees across Hong Kong Island and Kowloon, which will be completed in phases between the end of this year and early next year. The trees are mainly located in areas with high pedestrian and traffic flow such as pavements, slopes and parks.

I understand that we all love trees and trees are very much intertwined with our daily lives. In an urbanised city like Hong Kong, trees let us have a green living environment. While we make our best endeavours to manage tree risks, we also need private property owners and property management companies to conduct proper tree care within their properties. We need your assistance and co-operation in monitoring trees.