DDHC Paper No. 20/2016
(for discussion on 28.11.2016)
Southern District Council
District Development and Housing Committee
Enhancement of Vegetated Slopes of Highways Department (HyD)
(Phased Replacement of Senescent Acacia)
Purpose
This paper seeks Southern District Council members’ comments on the
“Enhancement of Vegetated Slopes of HyD – Phased Replacement of Senescent Acacia” (the
Programme) under HyD’s management.
Background
2.
During the past half-century, Hong Kong’s population has grown exponentially
along with the rapid economic and infrastructure development. Many man-made slopes
associated with infrastructure were formed to support development. Acacia confusa (台灣
相 思 ), a pioneer tree species that grows fast and establishes well in hostile slope
environment, was chosen in the former plantation scheme for soil erosion control, slope
stabilization and quick establishment of vegetation cover. At present, the HyD manages
approximately 75 000 Acacia throughout the territory.
3.
Unfortunately, trees, like all living organisms, will eventually die. With an average
life expectancy of approximately 50 to 60 years only, the majority of the Acacia trees planted
in Hong Kong over the past 50 years have now reached their senescent stage. Some local
tree experts even claimed that some Acacia trees in Hong Kong can grow old quickly when
they are only 20 to 30 years old. These overmature trees are more susceptible to drought,
compaction and secondary pathogens when injury occurs.
Their senescence and
deteriorating health problems and declined structural stability have been posing a significant
threat to public safety. According to the report on tree failure cases from the Tree
Management Office in November 2012, Acacia species constituted 35% of all reported tree
failure cases. The Acacia tree failure accidents widely reported by the media include:
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on 11 September 2010, an Acacia located on the slope along Fung Mo Street in
Kowloon City District collapsed, crushed two passing taxis and caused injuries to one of
the drivers; and
on 15 June 2007, a collapsed Acacia blocked and shut down three lanes of Nam Fung
Road in Southern District and caused serious traffic blockage temporarily.
With reference to HyD’s maintenance record, the number of Acacia with poor health and
structure that required removal by the department every year has been increasing steadily.
In order to safeguard the public, there is a need to develop a well-planned replacement
programme.
4.
Moreover, Acacia, being an exotic tree species, has a lower ecological value than
native species. The densely planted Acacia have hindered the natural propagation and
growth of native plants due to light blockage. Acacia also releases detrimental biochemical
(allelopathy) to the understorey plants. In view of this, there is a need to improve the
ecological habitat of highways landscape by replacing the senescent Acacia with suitable
trees, preferably native species of higher ecological value for sustainable development.
Objectives
5.
In order to better safeguard the public and promote the long-term sustainability of
highways landscape, the HyD acknowledged the importance of a well-planned Acacia
replacement programme in Hong Kong by commencing related preparatory works a few
years ago. Also, in March 2015, the Works and Maintenance Committee on Greening of the
Development Bureau (DEVB) requested relevant government departments to formulate
proactive tree management strategies for replacement of senescent Acacia trees for better
safeguarding of public safety and enriching of biodiversity. The Bureau’s directives have
given us further drive in the matter. Subsequently, the programme of ‘Enhancement of
Vegetated Slopes of the HyD - Phased Replacement of Senescent Acacia’ was developed
with the objectives –

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to better safeguard public safety;
to enrich biodiversity; and
to promote the long-term sustainability of highways landscape.
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Stages of the Programme
6.
The Programme would be implemented in three stages:
Stage 1 aims at obtaining expert views from academia, overseas and local tree experts, Tree
Management Office; and community views from District Councils (DCs).
Stage 2 involves carrying out a systematic survey with a scoring system to assess the current
tree health, structure and habitat conditions, and setting the order of work priority. In
determining the schedule for gradually phasing out the senescent Acacia trees, a basket of
factors, such as –

Target rating;
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Failure potential;
Size;
Site conditions; and
Site sensitivity.
Stage 3 includes both the replacement and establishment works. To strike a balance
between public safety and social impacts, a replacement option will be carefully selected for
each chosen site, with due consideration given to traffic impact, visual impact and site
sensitivity. Establishment works will be undertaken to promote healthy growth of new
plants.
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7.
The design for replacement planting is produced in accordance with the principle of
“the Right Trees at the Right Place” promoted by the Greening, Landscape and Tree
Management Section of the DEVB. Factors such as design theme, biodiversity, landscape
character, environmental factors (i.e. soil conditions, microclimate, traffic, spatial factors,
plant characteristics, etc.) are taken into account to determine the appropriate species,
quantity and time for replanting. Native species (such as Pyrus calleryana, Gordonia
axillaris, Bridelia tomentosa, Ilex species, Machilus species, Reevesia thyrsoidea,
Rhaphiolepis indica, Rhodomyrtus tomentosa, etc.) would be extensively used together with
some localized species (such as Bauhinia variegata, Camellia species, Rhododendron
species, Duranta repens, Ligustrum sinense, etc.) so as to establish an urban forest of higher
ecological and aesthetic value, and to promote local district character with seasonal effects.
8.
Since May 2015, we have been soliciting expert views from academia, overseas and
local tree experts as well as the Tree Management Office. We have also been
communicating with the local residents through presenting the proposal to 18 DCs and
distributing leaflets (see Annex) to the stakeholders concerned. Notice boards would be put
up in the replacement areas in advance, to explain to the public the objectives, key
considerations and work arrangements involved.
Advice Sought
9.
The Programme will be implemented in phases and will be commenced in 2016.
The District Council members are invited to offer comments.
Highways Department
Landscape Unit
November 2016
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Annex 1 – Leaflet of the Programme
Footnote*
1.
The principle of appropriate planting promoted by the Greening, Landscape and Tree
Management Section of Development Bureau, including considerations of spatial
factors, design theme, function of planting, plant species, and maintenance requirements,
etc.
http://www.greening.gov.hk/filemanager/content/pdf/knowledge_database/newsletters/G
reening_2015_AUTUMN_ENG.pdf
2.
Native species refer to the plants naturally grow in Hong Kong/ Southern China.
Localized species refer to the plants that wereNative species refer to the plants naturally
grow in Hong Kong/ Southern China. Localized species refer to the plants that were
http://www.greening.gov.hk/filemanager/content/pdf/knowledge_database/Guidelineson
UseofNativeSpecies-AppBversionforwebsite_e.pdf
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