LCQ21: Tree management

Following is a question by the Hon Chan Han-pan and a written reply by the Acting Secretary for Development, Mr David Lam, in the Legislative Council today (November 8):


     Regarding the Government's tree management work, will the Government inform this Council:

(1) of the number of trees that collapsed or were severely broken due to inclement weather, including typhoons and rainstorms, in each of the past five years, with a tabulated breakdown by species;

(2) as it is learnt that some tree species are undesirable (e.g. Leucaena leucocephala, which will grow rampantly in the suburb and impede the natural succession of native species), whether the Government has drawn up guidelines to exclude such species from its greening plans; if so, of the details and the timetable; if not, the reasons for that;

(3) whether it has considered removing the currently undesirable tree species; if so, of the details and the timetable; if not, the measures in place to ensure that these trees will not pose a danger to the public; and

(4) as it is learnt that the roots of many roadside trees are currently enclosed by concrete, resulting in poor root growth, and that the disproportionate growth of the roots and the tree crown increases risks of failure, whether the authorities have compiled statistics on the current number of such trees in Hong Kong, and whether inspection and maintenance work will be stepped up; if so, of the details and the timetable; if not, the reasons for that?
     The Government is committed to the proper management of trees to ensure healthy tree growth while emphasising the need to safeguard public safety. Regular and proper tree maintenance and systematic tree risk assessment are effective ways to reduce the risk of tree failure. The Tree Management Office of the Development Bureau (DEVB) co-ordinates tree management departments before the onset of wet season every year in conducting tree risk assessment in areas with high pedestrian and vehicular flow according to the "Guidelines for Tree Risk Assessment and Management Arrangement" issued by the DEVB and in taking appropriate mitigation measures, including crown pruning and removal of dead branches. If trees with risks of failure are identified, the departments will remove them as soon as possible.
     During typhoons and extreme weather, government departments will urgently remove fallen trees when feasible for public safety and restore normal community operations as soon as possible. When the Hong Kong Observatory changes the Gale or Storm Signal No. 8 or above to the Strong Wind Signal No. 3, or when the Black Rainstorm Warning Signal is cancelled, the tree management departments will immediately start inspections in various districts and take timely mitigation measures which include removing fallen trees and those that have not fallen but have become unstable due to heavy wind and rain, removing hanging broken branches, stabilising leaning trees back to an upright position and supporting them with cables and supports, and cordoning off trees yet to be handled, to ensure the safety of citizens.
     The reply to several parts of the question raised is as follows:
(1) The species and number of trees fallen or severely broken due to inclement weather, including typhoons and rainstorms, involved in the past five years are set out below:
Year 2019 2020 2021 2022 2023
(as end of October)
Number of trees fallen or severely broken (Note 1)
850 1 040 2 070 1 320 4 790
Major tree species
(Note 2)
Juniperus chinensis 'Kaizuca' 68 Bauhinia x blakeana 72 Leucaena leucocephala 214 Acacia confusa 111 Bauhinia x blakeana 485
Acacia confusa 46 Acacia confusa 58 Acacia confusa 116 Bauhinia x blakeana 76 Acacia confusa 421
Bauhinia x blakeana 44 Spathodea campanulata 48 Bauhinia variegata 94 Hibiscus tiliaceus 61 Hibiscus tiliaceus 316
Hibiscus tiliaceus 38 Bauhinia variegata 48 Plumeria rubra 76 Spathodea campanulata 57 Albizia julibrissin 284
Spathodea campanulata 36 Leucaena leucocephala 43 Bauhinia x blakeana 69 Casuarina equisetifolia 48 Leucaena leucocephala 269
Bauhinia variegata 32 Hibiscus tiliaceus 29 Spathodea campanulata 62 Bauhinia purpurea 41 Lagerstroemia speciosa 181
Leucaena leucocephala 26 Bauhinia purpurea 26 Tabebuia chrysantha 60 Leucaena leucocephala 38 Tabebuia chrysantha 180
Bauhinia purpurea 19 Callistemon viminalis 26 Ficus benjamina 58 Bauhinia variegata 32 Bauhinia variegata 180
Lagerstroemia speciosa 19 Elaeocarpus hainanensis 22 Bauhinia purpurea 53 Macaranga tanarius var. tomentosa 29 Macaranga tanarius var. tomentosa 151
Casuarina equisetifolia 18 Juniperus chinensis 'Kaizuca' 20 Hibiscus tiliaceus 53 Delonix regia 25 Bauhinia spp. 110

(2) and (3) In the selection of tree species, the DEVB promotes the "Right Tree, Right Place" principle, which means the tree species selected for planting should suit the environment while being able to deliver the designed functions and grow healthily in a sustainable manner. On this, the DEVB has formulated the "Street Tree Selection Guide" (Selection Guide) and released guidelines to promote the use of native plant species for departments' reference, with a view to selecting suitable tree species and encouraging the diversity of urban tree species. Leucaena leucocephala mentioned in the question is not included in the Selection Guide. It is an invasive alien species that grows mainly in wasteland or at the edge of sparse woodlands. It is hardy and can hinder the natural succession of native species. Its branches are brittle and can easily break or collapse in strong winds, posing hazard. Government departments remove Leucaena leucocephala during regular vegetation management work to avoid any impact on ecology or public safety.
(4) In September 2022, the DEVB established a task force to review the existing tree management guidelines and related implementation work. Ten improvement measures were proposed in early 2023 which are currently being implemented progressively as scheduled.
     Among the aforementioned measures, the coverage of "Individual Tree Risk Assessment" under the "Tree Basis Assessment" is expanded to conduct risk assessments for more relatively large trees along existing roads, and take timely risk mitigation measures to protect public safety. The DEVB has updated the "Guidelines for Tree Risk Assessment and Management Arrangement" accordingly and has implemented the above measures in the 2023 tree risk assessment and management work cycle.
     However, some trees have become unsuitable for the existing locations due to changes and development in the surrounding environment. These trees do not exhibit imminent risk but may have potential risk in the long run. Hence, the abovementioned measures included a holistic review on the criteria and scoring system for determining suitability and sustainability of existing roadside trees, identifying large roadside trees that may have potential risk in the long run systematically, and devising appropriate measures for different scenarios that may include treatments or removal plan in phases. The relevant study is in progress and it is expected that preliminary recommendations on the scoring system and criteria will be available in 2024, and the overall study would be completed in 2025.
Note 1:Numbers rounded to the nearest 10.
Note 2:The ten species with the highest number of collapse or severe breakage and their numbers.
Ends/Wednesday, November 8, 2023
Issued at HKT 12:45