Investigation results and follow-up work in connection with collapse of Delonix regia in Ho Man Tin on September 16
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After the incident, the LCSD immediately conducted an investigation to find out the cause of the tree collapse. Based on the information collected onsite, the inspection of the collapsed tree at the scene and the laboratory tests on samples, reference to the relevant records of the tree concerned in the past (including tree maintenance, tree inspection, complaints, nearby road excavation work, etc), the LCSD concluded that multiple factors contributed to the tree failure, including:
- Due to a phototropic response, the trunk of the tree concerned leaned towards the vehicular road with most of its crown above the road, and as a result, shifted its centre of gravity away from its root zone;
- The soil covered by paving blocks surrounding the tree pit limited the growing space for the roots below ground, causing the roots to not extend adequately;
- Root tissue analysis revealed that the central portion of the trunk base of the concerned tree was infected by pathogenic fungi, namely Pellinus noxius and Ganoderma multipileum. Pellinus noxius is a white rot fungus that causes brown root rot disease and damages tree roots and its vascular system. Ganoderma multipileum can damage living tissues of trees and cause root decay. When the roots of the tree concerned were damaged and weakened by the fungi, the tension generated by the weight of the leaning trunk broke the roots and caused the tree failure; and
- The location of the fungal infection was deep in the soil and as such could not be observed from outside by visual inspections during previous tree inspections.
A spokesman for the Development Bureau said that the Tree Management Office (TMO) of the Development Bureau agreed with the investigation report submitted by the LCSD after a detailed review.
The spokesman added that, following the request from the TMO, the LCSD has urgently reinspected other trees at Perth Street after the incident and has reported the findings to the TMO. The LCSD reinspected 34 trees at the location and found that three of them had decay inside their trunk bases and had to be removed as soon as possible. To ensure public safety, the LCSD completed the work in early October. Concurrently, the LCSD carried out risk mitigation measures such as crown cleaning and reduction for another 22 trees at Perth Street.
At present, there are over 10 000 Delonix regia growing on roadsides in Hong Kong. Amongst them, the TMO had requested all tree management departments to reinspect more than 4 000 large-sized Delonix regia. The departments have completed their reinspections, have reported the findings to the TMO, and are progressively implementing necessary risk mitigation measures for about 190 of them (including removal of 59 Delonix regia, and pruning, pest and disease control, removal of dead branches, etc for the others). The relevant work is expected to be completed within November.
Various tree management departments are currently reinspecting large roadside trees of other species. The reinspection results are expected to be submitted to the TMO in November.
The Development Bureau set up a task force at the end of September to review the current tree management guidelines, including the methods of tree inspection, follow-up work and implementation efforts, etc, as well as to consider the compatibility of the relatively large roadside trees with the current environment and explore a possible approach for their treatment. The investigation result of the Perth Street incident is an important reference of the task force. The task force endeavours to complete the review and make recommendations by the end of this year as announced earlier.
Ends/Wednesday, October 26, 2022
Issued at HKT 15:05
Issued at HKT 15:05
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