The Development Bureau announced today the results of an investigation and the follow-up work concerning a tree collapse at Perth Street, Ho Man Tin, on September 16.

According to the Leisure & Cultural Services Department (LCSD) tasked to conduct the investigation, the tree collapse was caused by a fungal infection and other factors.

It made the conclusion based on the information collected onsite, an inspection of the collapsed Delonix regia (flame tree) at the scene, laboratory tests on samples as well as reference to relevant tree records.

The LCSD pointed out that due to a phototropic response, the tree's trunk had shifted its centre of gravity away from its root zone.

With the soil surrounding the tree pit being covered by paving blocks, the roots did not extend adequately.

Additionally, the central portion of the trunk base was infected by Pellinus noxius, a white rot fungus that causes brown root rot disease, and Ganoderma multipileum which can damage living tissues of trees and cause root decay.

When the tree's roots 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.

The location of the fungal infection was deep in the soil and could not be observed from outside by visual inspections during previous tree inspections, the LCSD explained.

The bureau said its Tree Management Office (TMO) agreed with the investigation report after a detailed review.

Upon the TMO’s request, the LCSD urgently re-inspected 34 trees at Perth Street after the incident, and in early October removed three of the trees that were found to have decay inside their trunk bases.

The LCSD also carried out risk mitigation measures such as crown cleaning and reduction for another 22 trees on the same street.

At present, there are over 10,000 Delonix regia growing on roadsides in Hong Kong. The TMO had requested all tree management departments to re-inspect more than 4,000 large-sized Delonix regia.

The departments have completed their re-inspections and are progressively implementing necessary risk mitigation measures for about 190 of the trees inspected, including removing 59 Delonix regia. The work is expected to be completed within November.

The tree management departments are currently re-inspecting large roadside trees of other species and are expected to submit the results in November.